Fischer-Rosanoff convention

In 1906, the Russian-American chemist Martin André Rosanoff, working at that time at New York University, chose glyceraldehyde, a monosaccharide, as the standards for denoting the stereochemistry of carbohydrates and other chiral molecules, a nomenclature system called the Fischer-Rosanoff convention, or Rosanoff convention, or D-L system.
Because Rosanoff didn’t know the absolute configuration of glyceraldehyde, he assigned in a completely arbitrary manner:

  • the D prefix, from the Latin dexter, meaning “right”, to (+)-glyceraldehyde, the dextrorotatory enantiomer, thus assuming that the configuration, in Fischer projections, was that with the hydroxyl group (–OH)  attached to the chiral center on the right side of the molecule;
  • the L prefix, from the Latin laevus, meaning “left”, to (-)-glyceraldehyde, the levorotatory enantiomer, thus assuming that the configuration, in Fischer projections, was that with the hydroxyl group attached to the chiral center on the left  side of the molecule.

Fischer-Rosanoff convention: D- and L-glyceraldehyde as standard for the stereochemistry of chiral molecoles

Although Fischer rejected this nomenclature system, it was universally accepted and used to obtain the relative configurations of the chiral molecules. How? The configuration about a chiral center is related to that of glyceraldehyde by converting its groups to those of the monosaccharide through reactions that occur with retention of configuration, namely, reactions that do not break any of the bonds to the chiral center. This means that the spatial arrangement of the groups around the chiral center in the reagents and products is same. The Fischer convention allows to divide the chiral molecules, such as amino acids and monosaccharides, into two classes, known as the D series and the L series, depending on whether the configuration of the groups around the chiral center is related to that of D-glyceraldehyde or L-glyceraldehyde.

Note: there is no correlation between retention of configuration and sign of the rotatory power: the D-L system does not specify the sign of the rotation of plane-polarized light caused by the chiral molecule, but simply correlates the configuration of the molecule with that of the glyceraldehyde.

CONTENTS

Fischer-Rosanoff convention and carbohydrates

Monosaccharides can be aldoses or ketoses. Aldoses, and ketoses with more than three carbon atoms have at least one chiral center, and, by convention, they belong to the D series or to the L series if the configuration of the chiral carbon farthest from the carbonyl carbon, the carbon with the highest oxidation state, is same as that of D-glyceraldehyde or L-glyceraldehyde, respectively.
In Fischer projections the longest chain of carbon atoms is oriented vertically, and the atoms are numbered so that the carbonyl carbon has the lowest possible number, then, C-1 in aldoses and C-2 in ketoses.
Aldoses, ketoses, carbonyl carbon, and asymmetric center taken as the reference centerNote: in Nature, D-sugars are much more abundant than L-sugars.

If the sign of the rotation of plane-polarized light must be specified in the name, the prefixes (+) or (-) can be employed in addition to the D and L prefixes. For example, fructose, which is levorotatory, can be named D-(-)-fructose, whereas glucose, which is dextrorotatory, can be named D-(+)-glucose.

Fischer-Rosanoff convention and α-amino acids

Amino acids, depending on the position of the amino group (–NH2) with respect to the carboxyl group (–COOH) can be classified as:

  • α-amino acids, in which the amino group is attached to the α-carbon;
  • β-amino acids, in which the amino group is attached to the β-carbon;
  • γ-amino acids, in which the amino group is attached to the γ-carbon;
  • δ -amino acids, in which the amino group is attached to the δ-carbon.

Fischer-Rosanoff convention and alpha-amino acids of the D-series and L-series

α-Amino acids belong to the D series or to the L series if the configuration of the –NH2, –COOH, –R, and H groups attached to the α-carbon, the chiral center, is the same of the hydroxyl, aldehyde (–CHO), and hydroxymethyl (–CH2OH), and H groups of D-glyceraldehyde or L-glyceraldehyde, respectively.
In Fischer projections the molecules are arranged so that the carboxylic group, namely, the carbon with the highest oxidation state, is at the top, and the R group at the bottom.
Among α-amino acids, proteinogenic amino acids, with the exception of glycine whose α-carbon is not chiral, have the L configuration, hence, they are L-α-amino acids.

Note: in Nature, L-α-amino acids are much more abundant than all the other amino acids, which do not participate in protein synthesis.

Relative and absolute configurations

When Rosanoff arbitrarily assigned the D prefix to (+)-glyceraldehyde and the L prefix to (-)-glyceraldehyde, he had 50/50 chance of being correct.
In the early 1950s, a new technique, the x-ray diffraction analysis, made possible to establish the absolute configuration of chiral molecules. In 1951 a Dutch chemist, Johannes Martin Bijvoet established the absolute configuration of sodium rubidium (+)‐tartrate tetrahydrate and, comparing it with glyceraldehyde, demonstrated that Rosanoff’s guess was right. Consequently, the configurations of the chiral compounds obtained by relating them to that of glyceraldehyde were the same as their absolute configurations: hence, the relative configurations became absolute configurations.

Ambiguities of the Fischer-Rosanoff convention

The Fischer-Rosanoff convention gives rise to uncertainties with molecules with more than one chiral center. For example, considering D-(+)-glucose, the D-L system gives information about the configuration of C-2, but no information about the other asymmetric centers, namely, C-3, C-4, and C-5.

Asymmetric centers of D-(+)-Glucose

In these cases, the RS system, developed in 1956 by Robert Sidney Cahn, Christopher Ingold, and Vladimir Prelog, labeling each chiral center, allows to describe accurately the stereochemistry of the molecule. In the case of D-(+)-glucose, the molecule has the (2R,3S,4R,5R)-configuration.
It should also be noted that depending on the chiral center taken as the reference center, the same molecule can belong to both the D and L series.

References

Garrett R.H., Grisham C.M. Biochemistry. 4th Edition. Brooks/Cole, Cengage Learning, 2010

IUPAC. Compendium of Chemical Terminology, 2nd ed. (the “Gold Book”). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). Online version (2019-) created by S. J. Chalk. ISBN 0-9678550-9-8. doi:10.1351/goldbook

Mason S.F. Molecular optical activity and the chiral discriminations. Cambridge University Press, 2009

Moran L.A., Horton H.R., Scrimgeour K.G., Perry M.D. Principles of Biochemistry. 5th Edition. Pearson, 2012

Rosanoff. On Fischer’s classification of stereo-isomers. J Am Chem Soc 1906:28(1);114-121. doi:10.1021/ja01967a014

Fischer projections

In 1891, Hermann Emil Fischer, a German chemist, Nobel Laureate in chemistry in 1902, developed a systematic method for the two-dimensional representation of chiral molecules, the so-called Fischer projections or Fischer projection formulas.
Despite they are two-dimensional structures, Fischer projections  preserve information about the stereochemistry of the molecules and, although not being a representation of how molecules might look in solution, are still widely used by biochemists to define the stereochemistry of amino acids, carbohydrates, nucleic acids, terpenes, steroids, and other molecules of biological interest.

CONTENTS

How to draw Fischer projections

By considering a molecule with a single chiral center, e.g. a carbon atom, for drawing the Fischer projections, the tetrahedral structure is rotated so that two groups point downward, whereas two groups point upwards. Then, you draw a cross, place the chiral center at the center of the cross, and arrange the molecule so that the groups pointing downward, that is, behind the plane of the paper, are attached to the ends of the vertical line, and the groups pointing upwards, that is, out front from the plane of the paper, are attached to the ends of the horizontal line.
How to draw Fischer projections of molecules with one chiral center
For compounds  with more than one chiral center, the same procedure is applied to each asimmetric center.
It is also possible to convert a Fischer projection into a three-dimensional representation, for example using the wedges and dashes of perspective formulas, where the two horizontal bonds are represented by solid wedges, whereas the vertical bonds are represented by dashed lines.

How to manipulate Fischer projection formulas?

Since Fischer projections represent three-dimensional molecules on a two-dimensional sheet of paper, some rules must be respected to avoid changing the configuration.

  • The projections must not be lifted out the plane of the paper, because this causes enantiomer is converted into the other enantiomer.
  • If you rotate the projections in the plane of the paper, you obtain the same enantiomer if you rotate the structures by 180° in either direction, because the vertical groups must lie below  the plane of the paper, whereas the horizontal groups above. Conversely, the rotation by 90° or 270° in either direction causes an enantiomer is converted into the other enantiomer.Rules for manipulating Fischer projection formulas
  • An odd number of exchanges of two groups leads to the other enantiomer.

References

Garrett R.H., Grisham C.M. Biochemistry. 4th Edition. Brooks/Cole, Cengage Learning, 2010

IUPAC. Compendium of Chemical Terminology, 2nd ed. (the “Gold Book”). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). Online version (2019-) created by S. J. Chalk. ISBN 0-9678550-9-8. doi:10.1351/goldbook

Moran L.A., Horton H.R., Scrimgeour K.G., Perry M.D. Principles of Biochemistry. 5th Edition. Pearson, 2012

Voet D. and Voet J.D. Biochemistry. 4th Edition. John Wiley J. & Sons, Inc. 2011

Energy yield of glycogen under aerobic and anaerobic conditions

Glycogen, together with lipids, is a reserve of energy for use when needed. Although quantitatively lower than lipids, glycogen has some metabolic advantages.

  • It is mobilized faster than lipids.
  • It is a storage of glucose used to maintain the blood glucose level during fasting, physical activity, and between meals.
  • It is an energy source used both in aerobic and anaerobic conditions.

By considering the last point, the energy yield of glucose released from glycogen is different under aerobic and anaerobic conditions. The metabolic bases of these differences are analyzed below.

CONTENTS

Energy yield of glycogen stores under anaerobic conditions

Under anaerobic conditions, the oxidation of glucose to lactate via anaerobic glycolysis yields two molecules of ATP.
Below, the yield of ATP from anaerobic oxidation of glucose released during glycogenolysis by the action of glycogen phosphorylase (EC 2.4.1.1), and debranching enzyme (EC 3.2.1.33) is considered.
Note: glycogen phosphorylase releases about 90% of stored glucose in the form of glucose-1-phoshate (G-1-P), and debranching enzyme the remaining 10% in the form of glucose.


Glycogen phosphorylase and the oxidation of G-1-P under anaerobic conditions

Glycogen synthesis from glucose requires 2 ATP for each molecule of glucose.
The release of glucose-1-phosphate by the action of glycogen phosphorylase allows the recovery of one of the 2 molecules of ATP used in the preparatory phase of glycolysis. The anaerobic oxidation of glucose-6-phosphate, produced from glucose-1-phosphate (G-1-P) by the action of phosphoglucomutase (EC 5.4.2.2), yields therefore three molecules of ATP and not two, because:

  • one molecule of ATP, instead of two, is used in the preparatory phase of glycolysis, because hexokinase reaction (EC 2.7.1.1) is bypassed;
  • four molecules of ATP are produced in the payoff phase of glycolysis.

The cost-gain rate is 1/3, namely, there is an energy yield of about 66,7%.
The overall reaction is:

Glycogen(n glucose residues) + 3 ADP + 3 Pi → Glycogen(n-1 glucose residues) + 2 Lactate + 3 ATP

By considering the two molecules of ATP used in the synthesis of glycogen and the anaerobic oxidation of glucose-1-phosphate to lactate, there is a yield of one molecule of ATP for each molecule of glucose stored. The overall reaction is:

Glucose + ADP + Pi → 2 Lactate + ATP

Debranching enzyme and the oxidation of glucose under anaerobic conditions

By considering the glucose released by the action of debranching enzyme, the yield of ATP is zero because:

  • two molecules of ATP are used in the synthesis of glycogen from glucose;
  • debranching enzyme releases glucose, then, two molecules of ATP will be used in the preparatory phase of glycolysis;
  • four molecules of ATP are produced in the payoff phase of glycolysis.

If we now consider the oxidation to lactate of all glucose released from glycogen, there is an energy yield equal to:

1-{[(1/3)*0,9]+[(2/2)*0,1]}=0,60

Then, under anaerobic conditions, there is an energy yield of 60%, hence, glycogen is a good storage form of energy.

Energy yield of glycogen stores under aerobic conditions

Under aerobic conditions, the oxidation of glucose to CO2 and H2O via glycolysis, pyruvate dehydrogenase complex, Krebs cycle, mitochondrial electron transport chain, and oxidative phosphorylation yields about 30 molecules of ATP.
Below, the yield of ATP from aerobic oxidation of glucose released from glycogen by the action of glycogen phosphorylase and debranching enzyme is considered.

Glycogen phosphorylase and the oxidation of G-1-P under aerobic condition

The oxidation of glucose-6-phosphate, produced from glucose-1-phosphate by the action of phosphoglucomutase, to CO2 and H2O yields 31 molecules of ATP, and not 30, because only one molecule of ATP is used in the preparatory phase of glycolysis. The cost-gain rate is 1/31, namely, there is an energy yield of about 97%.
The overall reaction is:

Glycogen(n glucose residues) + 31 ADP + 31 Pi → Glycogen(n-1 glucose residues) + 31 ATP + 6 CO2 + 6 H2O

By considering the two molecules of ATP used in the synthesis of glycogen and the aerobic oxidation of glucose-1-phosphate to CO2 and H2O, there is a yield of 29 molecules of ATP for each molecule of glucose stored.
The overall reaction is:

Glucose + 29 ADP + 30 Pi → 29 ATP + 6 CO2 + 6 H2O

Debranching enzyme and the oxidation of glucose under aerobic condition

By considering the glucose released by the action of debranching enzyme, there is a yield of 30 molecules of ATP, because two molecules of ATP are used in the preparatory phase of glycolysis. The cost-gain rate is 2/30, namely, there is an energy yield of about 93,3%.

If we now consider the oxidation to CO2 and H2O of all glucose released from glycogen, there is an energy yield equal to:

1-{[(1/31)*0,9]+[(2/30)*0,1]}=0,96

Energy yield of glycogen in aerobic and anaerobic conditionsThen, under aerobic conditions, there is an energy yield of 96%, hence, glycogen is a extremely efficient storage form of energy, with a gain of 36% compared to anaerobic conditions.

References

Nelson D.L., Cox M.M. Lehninger. Principles of biochemistry. 6th Edition. W.H. Freeman and Company, 2012

Roach P.J., Depaoli-Roach A.A., Hurley T.D and Tagliabracci V.C. Glycogen and its metabolism: some new developments and old themes. Biochem J 2012;441:763-87. doi:10.1042/BJ20111416

Stipanuk M.H., Caudill M.A. Biochemical, physiological, and molecular aspects of human nutrition. 3rd Edition. Elsevier health sciences, 2012

Voet D. and Voet J.D. Biochemistry. 4th Edition. John Wiley J. & Sons, Inc. 2011

RS system: the priority rules

In 1956, Robert Sidney Cahn, Christopher Ingold and Vladimir Prelog developed a nomenclature system that, based on a few simple rules, allows to assign the absolute configuration of each chirality center in a molecule.
This nomenclature system, called the RS system, or the Cahn-Ingold-Prelog (CIP) system, when added to the IUPAC system of nomenclature, allow to name accurately and unambiguously the chiral molecules, even when there is more than one asymmetric center.
Chiral molecules are, in most cases, able to rotate plane-polarized light when light passes through a solution containing them. In this regard, it should be emphasized that the sign of the rotation of plane-polarized light caused by a chiral compound provides no information concerning RS configuration of its chiral centers.
The Fischer-Rosanoff convention is another way to describe the configuration of chiral molecules. However, compared to RS system, it labels the whole molecule and not each chirality center, and is often ambiguous for molecules with two or more chirality centers.

CONTENTS

The priority rules of the RS system

The RS system assigns a priority sequence to the groups attached to the chirality center and, tracing a curved arrow from the highest priority group to the lowest, labels each chiral center R or S.

First rule

A priority sequence is assigned to the groups based on the atomic number of the atoms directly attached to the chiral center.

  • The atom with the highest atomic number is assigned the highest priority.
  • The atom with the lowest atomic number is assigned the lowest priority.

For example, if an oxygen atom, O, atomic number 8, a carbon atom, C, atomic number 6, a chlorine atom, Cl, atomic number 17, and a bromine atom, Br, atomic number 35 are attached to the chiral center, the order of priority is: Br > Cl > O > C.
For isotopes, the atom with the highest atomic mass is assigned the highest priority.

Second rule

When different groups are attached to the chiral center through identical atoms, the priority sequence is assigned based on the atomic number of the next atoms bound, then moving outward from the chirality center until the first point of difference is reached.
If, for example, –CH3, –CH2CH3 and –CH2OH groups are attached to a chiral center, there are three identical atoms directly attached to the chiral center. Analyzing the next atoms bound, we have:

  • for the methyl group –CH3
H, H, H
  • for the ethyl group –CH2CH3
H, H, C
  • for the hydroxymethyl group –CH2OH
H, H, O

RS system and the sequence rules to assign priorities: the second rule

Because the atomic number of oxygen is higher than that of carbon, that, in turn, is higher than that of hydrogen, the order of priority is –CH2OH > –CH2CH3 > –CH3
The order of priority of some groups is:

–I > –Br > –Cl > –SH > –OR > –OH > –NHR > –NH2 > –COOR > –COOH > –CHO > –CH2OH > –C6H5 > –CH3 > –2H > –1H

Note that the groups attached to a chirality center must not have identical priority ranking, because, in that case, the center cannot be chiral.

Once the priority sequence has been established, the molecule is oriented in space so that the group with the lowest priority is pointed away from the viewer, then behind the chiral center. Now, trace a curved arrow, a circle, from the highest priority group to the lowest.

  • If you move in a clockwise direction, the configuration of chiral center is R, from the Latin rectus, meaning “right”.
  • If you move in a counterclockwise direction, the configuration of chiral center is S, from the Latin sinister, meaning “left”.R configuration of a chiral center

Third rule

This is the third rule of the RS system, by which we can determine the configuration of a chirality center when there are double or triple bonds in the groups attached to the chirality center.
To assign priorities, the atoms engaged in double or triple bonds are considered duplicated and tripled, respectively.

RS system and the sequence rules to assign priorities: the third ruleIn the case of a C=Y double bond, one Y atom is attached to the carbon atom, and one carbon atom is attached to the Y atom.
In the case of a C≡Y triple bond, two Y atoms are attached to the carbon atom, and two carbon atoms are attached to the Y atom.

RS system and multiple chiral centers

When two or more chirality centers are present in a molecule, each center is analyzed separately using the rules previously described.
Consider 2,3-butanediol. The molecule has two chiral centers, carbon 2 and carbon 3, and exists as three stereoisomers: two enantiomers and a meso compound. What is the RS configuration of the chiral centers of the enantiomer shown in figure?

RS configuration of the chiral centers of (2R,3R)-2,3-ButanediolConsider carbon 2. The order of priority of the groups is –OH > –CH2OHCH3 > –CH3 > –H. Rotate the molecule so that the hydrogen, the lowest priority group, is pointed away from the viewer. Tracing a path from –OH, the highest priority group, to –CH3, the lowest priority group, we move in a clockwise direction: the configuration of the carbon 2 is, therefore, R. Applying the same procedure to carbon 3, its configuration is R. Then, the enantiomer shown in figure is (2R,3R)-2,3-butanediol.

Amino acids and gliceraldeide

In the Fischer-Rosanoff convention, all the proteinogenic amino acids are L-amino acids. In the RS system, with the exception of glycine, that is not chiral, and cysteine ​​that, due to the presence of the thiol group, is (R)-cysteine, all the other proteinogenic amino acids are (S)-amino acids.
Threonine and isoleucine have two chirality centers, the α-carbon and a carbon atom on the side chain, and exist as three stereoisomers: two enantiomers and a meso compound. The forms of the two amino acids isolated from proteins are (2S,3R)-threonine and (2S,3S)-isoleucine, in Fischer-Rosanoff convention, L-threonine and L-isoleucine.
In the RS system, L-glyceraldehyde is (S)-glyceraldehyde, and, obviously, D-glyceraldehyde is (R)-glyceraldehyde.

References

Cahn R.S., Ingold C., Prelog V. Specification of molecular chirality. Angew Chem 1966:5(4); 385-415. doi:10.1002/anie.196603851

Garrett R.H., Grisham C.M. Biochemistry. 4th Edition. Brooks/Cole, Cengage Learning, 2010

IUPAC. Compendium of Chemical Terminology, 2nd ed. (the “Gold Book”). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). Online version (2019-) created by S. J. Chalk. ISBN 0-9678550-9-8. doi:10.1351/goldbook

Moran L.A., Horton H.R., Scrimgeour K.G., Perry M.D. Principles of Biochemistry. 5th Edition. Pearson, 2012

Prelog V. and Helmchen G. Basic principles of the CIP‐system and proposals for a revision. Angew Chem 1982:21(8);567-583. doi:10.1002/anie.198205671

Rosanoff M.A. On Fischer’s classification of stereo-isomers. J Am Chem Soc 1906:28(1);114-121. doi:10.1021/ja01967a014

Voet D. and Voet J.D. Biochemistry. 4th Edition. John Wiley J. & Sons, Inc. 2011

Chirality in organic chemistry

The definition of chirality, from the Greek cheir meaning “hand”, is due to Lord Kelvin who enunciated it during the “Baltimore Lectures”, a series of lectures held at Johns Hopkins University in Baltimore, starting from October 1st 1884, and published twenty years later, in 1904, in which the English scientist, among other things, stated: “I call any geometrical figure, or groups of points, chiral, and say it has chirality, if its image in a plane mirror, ideally realized, cannot be brought to coincide with itself“.
Chirality is, therefore, the geometric property of a group of points or atoms in space, or of a solid object, of not being superimposable on its mirror image. These structures, defined as chiral, have the peculiar property of being devoid of symmetry elements of the second kind, namely, a mirror plane, an center of inversion, or a rotation-reflection.
The environment is rich in chiral objects: your hands are the example par excellence, but there are many others, from the shell of a snail to a spiral galaxy. In chemistry, and especially in organic chemistry, chirality is a property of primary importance, because molecules such as carbohydrates, many amino acids, as well as many drugs, are chiral.
Chiral molecules can exist in two forms, mirror images of each other and non-superimposable, namely, there is no combination of rotations or translations on the plane of the sheet that allows their superposition. Such molecules are called enantiomers, from the Greek enántios, meaning “opposite” and meros,meaning “part”.
The most common cause of chirality in a molecule is the presence of a chirality center or chiral center, also called asymmetric center, namely, an atom that bears a set of atoms or functional groups in a spatial arrangement so that the resulting molecule can exist as two enantiomers.
Enantiomers are a kind of stereoisomers, that, in turn, can be defined as isomers having the same number and kind of atoms and bonds, but differing in the spatial orientation of the atoms.

CONTENTS

Enantiomers

Two enantiomers of a chiral molecule, being non-superimposable, are different compounds. How do they differ?
Each pair of enantiomers has identical physical and chemical properties towards achiral properties, such as melting point, boiling point, refractive index, infrared spectrum, the solubility in the same solvent, or the same reaction rate with achiral reagents.
The differences emerge when they interacts with chemical and physical phenomena that have chiral properties.

  • From the chemical point of view, two enantiomers can be distinguished by the way they interact with chiral structures, such as the binding site of a chiral receptor or the active site of a chiral enzyme.
  • From the physical point of view, they differ in their interaction with plane-polarized light, that has chiral properties, namely, they have optical activity.

Chirality and optical activity

The optical activity of materials such as quartz and, more importantly, of organic compounds such as sugars or tartaric acid, was discovered in 1815 by the French scientist Jean-Baptiste Biot.
Chiral molecules can be classified based on the direction in which plane-polarized light is rotated when it passes through a solution containing them.

  • If a solution containing one enantiomer rotates plane-polarized light in a clockwise direction from the observer’s point of view, the molecule is called dextrorotatory or dextrorotary, from the Latin dexter, meaning “right”, and is designated by the prefixes (+)-, or d from dextro-.
  • If a solution containing one enantiomer rotates plane-polarized light in a counterclockwise direction from the observer’s point of view, the molecule is called levorotatory or levorotary, from the Latin laevus, meaning “left”, and is designated by the prefixes (-), or l from laevo-.

Obviously, if we consider a pair of enantiomers, one is dextrorotatory and the other levorotatory.
At present it is not possible to reliably predict the magnitude, direction, or sign of the rotation of plane-polarized light caused by an enantiomer. On the other hand, the optical activity of a molecule provides no information on the spatial arrangement of the chemical groups attached to the chirality center.
Note: a system containing molecules that having the same chirality sense is called enantiomerically pure or enantiopure.

Pasteur and the discovery of enantiomers

In 1848, thirty three years after Biot’s work, studies on the optical activity of molecules led Louis Pasteur, who had been a student of Biot, to note that, following the recrystallization of a concentrated aqueous solution of sodium ammonium tartrate, optically inactive, two kinds of crystals precipitated, that were non-superimposable mirror images of each other. After separating them with tweezers, Pasteur discovered that the solutions obtained by dissolving equimolar amounts of the two kind of crystals were optically active and, interestingly, the rotation angle of plane-polarized light was equal in magnitude but opposite in sign. Because the differences in optical activity were due to the dissolved sodium ammonium tartrate crystals, Pasteur hypothesized that the molecules themselves should be non-superimposable mirror images of each other, like their crystals: they were what we now call enantiomers. And it is Pasteur who first used the term asymmetry to describe this property, then called chirality by Lord Kelvin.

Racemic mixtures

A solution containing an equal amount of each member of a pair of enantiomers is called racemic mixture or racemate. These solutions are optically inactive: there is no net rotation of plane-polarized light since the amount of dextrorotatory and levorotatory molecules is exactly the same.
Unlike what happens in biochemical processes, the chemical synthesis of chiral molecules that does not involve chiral reactants, or that is not followed by methods of separation of enantiomers, inevitably leads to the production of a racemic mixture.
The pharmaceutical chemistry is among the sectors most affected by this. As previously mentioned, two enantiomers are different compounds. Many chiral drugs are synthesized as racemic mixtures, but most often the desired pharmacological activity resides in one enantiomer, called eutomer; the other, called distomer, is less active or inactive. An example is ibuprofen, an arylpropionic acid derivative, and anti-inflammatory drug: only the S enantiomer has the pharmacological activity.

Enantiomers of Ibuprofen

Arylpropionic derivatives are sold as racemic mixtures: a racemase converts the distomer to the eutomer in the liver.
However, it is also possible that the distomer causes harmful effects and must be eliminated from the racemic mixture. A tragic example is thalidomide, a sedative and anti-nausea drug sold as a racemic mixture from the 1950s until 1961, and taken also during pregnancy.

Enantiomers of ThalidomideThe distomer, the S enantiomer, could cause serious birth defects, particularly phocomelia. This is probably the most striking example of the importance of the chiral properties of molecules, which prompted health care organizations to promote the synthesis of drugs, including thalidomide, containing a single enantiomer by the pharmaceutical industry.

Chirality centers

Any tetrahedral atom that bears four different substituents can be a chirality center.
Carbon atom is the classic example, but also other atoms from group IVA of the periodic table, such as the semimetals silicon (Si) and germanium (Ge), have a tetrahedral arrangement and can be chiral centers. Another example is the phosphorus atom in organic phosphate esters that has a tetrahedral arrangement, then, when it binds four different substituents it is a chiral center.
The nitrogen atom of a tertiary amine, an amine in which the nitrogen is bounded to three different groups, is a chiral center. In these compounds, nitrogen is located at the center of a tetrahedron and its four sp3 hybrid orbitals point to the vertices, three of which are occupied by the three substituents, whereas the nonbonding electron pair points towards the fourth.

Nitrogen inversion in a tertiary amineAt room temperature, nitrogen rapidly inverts its configuration. The phenomenon is known as nitrogen inversion, namely, a rapid oscillation of the atom and its ligands, during which nitrogen passes through a planar sp2-hybridized transition state. As a consequence, if the nitrogen atom is the only chiral center of the molecule, there is no optical activity because a racemic mixture exists. The inversion of configuration does not occur only in some cases in which nitrogen is part of a cyclic structure that prevents it. Therefore, the presence of a chiral center could be not sufficient to allow the separation of the respective enantiomers.

Note: in 1874, Jacobus Henricus van ‘t Hoff and Joseph Achille Le Bel, based on the work of Pasteur, first formulated the theory of the tetrahedral carbon atom. For this work van ‘t Hoff received the first Nobel Prize in chemistry in 1901.

Chirality in the absence of a chiral center

Chirality can also occur in the absence of a chiral center, due to the lack of free rotation around a double or a single bond, as in the case of:

  • allene derivatives, organic compounds in which there are two cumulative double bonds, namely, two double bonds localized on the same carbon atom;
  • biphenyl derivatives.

Chirality due to the presence of an axis of chiralityIn this case, chirality is due to the presence of an axis of chirality.

Meso compounds

Meso compounds or meso isomers are stereoisomers with two or more chiral centers that are superimposable on their mirror image, then achiral and, as such, optically inactive. Moreover, they have an internal mirror plane that bisects the molecule, with each half a mirror image of the other. Then, meso compounds can be classified as diastereomers, namely, stereoisomers which are not enantiomers.
For a molecule with n chirality centers, the maximum number of possible stereoisomers is 2n.
Consider 2,3-butanediol. The molecule has two chirality centers, the carbons 2 and 3, so there are 22 = 4 possible stereoisomers, whose structures are depicted in the figure, in the Fischer projections, indicated as A, B, C, D.

Stereoisomers, chirality centers, and meso compounds

Structures A and B are mirror images of each other and non-superimposable, then they are a pair of enantiomers.
Structures C and D are mirror images of each other, but are superimposable. In fact, if we rotate structure C or D of 180 degrees, the two structures are superimposable. Then, they are not a pair of enantiomers: they are the same molecule with opposite orientation. Moreover, they have an internal mirror plane, that bisects the molecule, giving two halves, each a mirror image of the other. Structure C, or D, is therefore a meso compound because it has chiral centers, is superimposable on its mirror image, and has internal mirror plane that divides the molecule into two mirror‐image halves.

References

Capozziello S. and Lattanzi A. Geometrical approach to central molecular chirality: a chirality selection rule. Chirality 2003;15:227-230. doi:10.1002/chir.10191

Garrett R.H., Grisham C.M. Biochemistry. 4th Edition. Brooks/Cole, Cengage Learning, 2010

IUPAC. Compendium of Chemical Terminology, 2nd ed. (the “Gold Book”). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). Online version (2019-) created by S. J. Chalk. ISBN 0-9678550-9-8. doi:10.1351/goldbook

Kelvin WT. Baltimore lectures on molecular dynamics and the wave theory of light. Clay C. J., London: 1904:619. https://archive.org/details/baltimorelecture00kelviala/mode/2

Voet D. and Voet J.D. Biochemistry. 4th Edition. John Wiley J. & Sons, Inc. 2011

Isomerism

The phenomenon that two or more different chemical compounds have the same molecular formula is called isomerism, from the Greek isos meaning “equal”, and meros meaning “part”, a concept and term introduced by the Swedish scientist Jacob Berzelius in 1830.
Isomerism is a consequence of the fact that the atoms of a molecular formula can be arranged in different ways to give compounds, called isomers, that differ in physical and chemical properties.
There are two types of isomerism: structural isomerism and stereoisomerism, which can be divided into further subtypes.

Tree diagram for types of isomerism

CONTENTS

Structural isomerism

In structural isomerism, also called constitutional isomerism, isomers differ from each other in that the constituent atoms are linked in different ways and sequences.
There are several subtypes of structural isomerism: positional, functional group and chain isomerism.

Positional isomers

In positional isomerism, also called position isomerism, isomers have the same functional groups but in different positions on the same carbon chain.
An example is the compound with molecular formula C6H4Br2, of which there are three isomers: 1,2-dibromobenzene, 1,3-dibromobenzene and 1,4-dibromobenzene. These isomers differ in the position of the bromine atoms on the cyclic structure.

Example of position isomers: dibromobenzene

Another example is the compound with molecular formula C3H8O, of which there are two isomers: 1-propanol or n-propyl alcohol, and 2-propanol or isopropyl alcohol. These isomers differ in the position of the hydroxyl group on the carbon chain.

Functional group isomers

Functional group isomerism, also called functional isomerism, occurs when the atoms form different functional groups.
An example the compound with molecular formula C2H6O, of which there are two isomers: dimethyl ether and ethanol or ethyl alcohol, that have different functional groups, an ether group, –O–, and a hydroxyl group, –OH, respectively.

Chain isomers

In chain isomerism, isomers differ in the arrangement of the carbon chains, that may be branched or straight.
An example is the compound with the molecular formula C5H12, of which there are three isomers: n-pentane, 2-methylbutane or isopentane and 2,2-dimethylpropane or neopentane.Example of chain isomerism: n-pentane, 2-methylbutane, and 2,2-dimethylpropane

Stereoisomerism

In stereoisomerism, isomers have the same number and kind of atoms and bonds, but differ in the orientation of the atoms in space. Such isomers are called stereoisomers, from the Greek stereos, meaning “solid”.
There are two subtypes of stereoisomerism, conformational isomerism and configurational isomerism; the latter can be further subdivided into optical isomerism and geometrical isomerism.

Conformational isomerism

In conformational isomerism, the stereoisomers can be interconverted by rotation around one or more single bonds, the σ bonds. These rotations produce different arrangements of atoms in space that are non-superimposable. And the number of possible conformations a molecule can adopted is theoretically unlimited, ranging from the lowest energy structure, the most stable, to the highest energy structure, the less stable. Such isomers are called conformer.
For example, if we consider ethane, C2H4, looking at the molecule from one end down the carbon-carbon bond, using the Newman projection, hydrogen atoms of a methyl group can be, with respect to the hydrogen atoms of the other methyl group, in one of the following conformations.

  • The eclipsed conformation, in which hydrogen atoms of a methyl group are hidden behind those of the other methyl group, then, the angle between carbon-hydrogen bonds on the front and rear carbons, called a dihedral angle, could be 0, 120, 240, 360 degrees. This is the highest energy conformation, then is the less stable.
  • The staggered conformation, in which hydrogen atoms of a methyl group are completely offset from those of the other methyl group, namely, dihedral angles could be 60, 180 or 360 degrees. This is the lowest energy conformation, then the most stable.
  • The skew conformation, corresponding to one of the intermediate conformations between the previous ones.

Newman projections and conformations of ethane

The stability of ethane conformers is due to how the electron pairs of the carbon-hydrogen bonds of the two methyl groups are overlapped:

  • in the staggered conformations they are as far away from each other as possible;
  • in the eclipsed conformations they are as close as possible to each other.

The potential energy barrier between these two conformations is small, about 2.8 kcal/mole (11.7 kJ/mole). At room temperature, the kinetic energy of the molecules is 15-20 kcal/mole (62.7-83.6 kJ/mole), more than enough to allow free rotation around the carbon-carbon bond. As a consequence, it is not possible to isolate any particular conformation of ethane.
Note: the potential energy barrier to rotation around double carbon-carbon bonds is about 63 kcal/mole (264 kJ/mole), corresponding to the energy required to break the π bond. (See geometric isomerism). This value is about three times the kinetic energy of the molecules at room temperature at which, then, free rotation is precluded. Only at temperatures above 300 °C molecules acquire enough thermal energy to break the π bond, allowing free rotation around the remaining σ bond. This allows the trans-isomer to be rearranged to the cis-isomer or vice versa.

Configurational isomerism

In configurational isomerism, the interconversion between the stereoisomers does not occur as a result of rotations around single bonds but involves bond breaking and new bond forming, then it doesn’t occur spontaneously at room temperature.
There are two subtypes of configurational isomerism: optical isomerism and geometrical isomerism.

Optical isomers

Optical isomerism occurs in molecules that have one or more chirality centers or chiral centers, namely, tetrahedral atoms that bear four different ligands. The chiral center can be a carbon, phosphorus, sulfur or nitrogen atom.

Tetrahedral atom that bears to four different ligandsNote: the word chirality derives from the Greek cheiros, meaning “hand”.
Optical isomers lack of a center of symmetry or a plane of symmetry, are mirror image of each other, and cannot be superimposed on one another. Such stereoisomers are called enantiomers, from the Greek enántios, meaning “opposite”, and meros, meaning “part”.
Unlike the other isomers, two enantiomers have identical physical and chemical properties with two exceptions.

  • The direction of rotation of the plane of polarized light, hence the name of optical isomerism.
    If a solution of one enantiomer rotates the plane of polarized light in a clockwise direction, the enantiomer is labeled (+). Conversely, a solution of the other enantiomer rotates the plane of polarized light in a counterclockwise direction by the same angle, and the enantiomer is labeled (-).
  • Although indistinguishable by most techniques, two enantiomers can be distinguished in a chiral environment like the active site of chiral enzymes.

Note: for a molecule with n chiral centers, the maximum number of stereoisomers is equal to 2n.

Geometric isomers

Geometric isomerism, also called cis-trans isomerism, occurs when atoms cannot freely rotate due to a rigid structure such as in:

  • compounds with carbon-carbon, carbon-nitrogen or nitrogen-nitrogen double bonds, where the rigidity is due to the double bond;
  • cyclic compounds, where the rigidity is due to the ring structure.

An example of geometrical isomerism due to the presence of a carbon-carbon double bond is stilbene, C14H12, of which there are two isomers. In one isomer, called cis isomer, the same groups are on the same side of the double bond, whereas in the other, called trans isomer, the same groups are on opposite sides.

Example of cis-trans isomers: trans-stilbene and cis-stilbene

Note: the terms trans and cis are from the Latin trans, meaning “across”, and cis, meaning “on this side of”.
Among the cyclic compounds of carbon, cis-trans isomerism not complicated by the presence of chiral centers occurs in structures with an even number of carbon atoms and substituted in opposite positions, namely, para-substituted. An example is 1,4-dimethylcyclohexane, a cycloalkane, compounds of general formula CnH2n, of which there are two stereoisomers, cis-1,4-dimethylcyclohexane and trans-1,4- dimethylcyclohexane.

Example of geometric isomerism: trans-1,4-dimethylcyclohexane and cis-1,4-dimethylcyclohexane

This kind of stereoisomerism cannot exist if one of the atoms that cannot freely rotate carries two groups the same. Why? For the switching between the trans and cis isomers the groups attached to atoms that cannot freely rotate have to be swapped. If there are two groups the same, the switch leads to the formation of the same molecule.
Note: geometric isomers are a special case of diastereomers or diastereoisomers, that, in turn, are stereoisomers that are not mirror image of each other. The other diastereomers are the meso compounds and non-enantiomeric optical isomers.

References

Graham Solomons, T. W., Fryhle C.B.,  Snyder S.A. Solomons’ organic chemistry. 12th Edition. John Wiley & Sons Incorporated, 2017

Morris D.G. Stereochemistry. Royal Society of Chemistry, 2001. doi:10.1039/9781847551948

North M. Principles and applications of stereochemistry. 1th Edition. CRC Press, 1998

Voet D. and Voet J.D. Biochemistry. 4th Edition. John Wiley J. & Sons, Inc. 2011

Osmotic pressure

In solution, solvent molecules tend to move from a region of higher concentration to one of lower concentration. When two different solutions are separated by a semipermeable membrane, namely, a membrane that allows certain ions or molecules to pass, in this case the solvent molecules, a net flow of solvent molecules from the side with higher concentration to the side with lower concentration will occur. This net flow through the semipermeable membrane produces a pressure called osmotic pressure, indicated as Π, that can be defined as the force that must be applied to prevent the movement of the solvent molecules through a semipermeable membrane.

Osmotic pressure: two different solutions are separated by a semipermeable membrane

Osmotic pressure, together with boiling point elevation, freezing point depression, and vapor pressure lowering, is one of the four colligative properties of solutions, properties that do not dependent of the chemical properties of the solute particles, namely ions, molecules or supramolecular structures, but depend only on the number of solute particles present in solution.
For a solutions of n solutes, the equation that describes osmotic pressure is the sum of the contributions of each solute:

Π = RT(i1c1 + i2c2 + … + incn)

The equation is known as the van ’t Hoff equation, where:

  • T is the absolute temperature in Kelvin;
  • R is the ideal gas constant = 8.314 J/mol K;
  • c is the molar concentration of the solute;
  • i is the van ’t Hoff factor.

CONTENTS

van ’t Hoff factor

The van ‘t Hoff factor is a measure of the degree of dissociation of solutes in solution, and is described by the equation:

i = 1 + α(n-1)

where:

  • α is the degree of dissociation of the solute molecules, equal to the ratio between the moles of the solute molecules that have dissociated and the number of the original moles, and is comprised between 0, for substances that do not ionize or dissociate in solution, and 1, for substances that completely dissociate or ionize;
  • n is the number of ions formed from the dissociation of the solute molecule.

For non ionizable compounds, such as glucose, glycogen or starch, n = 1, and i = 1.
For compounds that completely dissociate, such as strong acids and strong bases or salts, the van ‘t Hoff factor is a whole number greater than one, as α = 1 and n is equal to at least 2. For example, if we consider sodium chloride, NaCl, potassium chloride, KCl, or calcium chloride, CaCl2, in dilute solution:

NaCl → Na+ + Cl
KCl → K+ + Cl
CaCl2 → Ca2+ + 2 Cl

So in the first two cases i = 2, whereas with calcium chloride, i = 3.
Finally, for substances that do not completely ionize, such as weak acids and weak bases, i is not an integer.

The product of the van ’t Hoff factor and the molar concentration of the solute particles, ic, is the osmolarity of the solution, namely, the concentration of the solute particles osmotically active per liter of solution.

Osmotic pressure, osmosis, and plasma membranes

Osmosis can be defined as the net movement or flow of solvent molecules through a semipermeable membrane driven by osmotic pressure differences across the membrane, to try to equal the concentration of the solute on the two sides of the membrane itself.
In biological systems, water is the solvent and plasma membranes are the semipermeable membranes. Plasma membranes allow water molecules to pass, due to protein channels, known as aquaporins, as well as small non-polar molecules that diffuse rapidly across them, whereas they are impermeable to ions and macromolecules. Inside the cell there are macromolecules, such as nucleic acids, proteins, glycogen, and supramolecular aggregates, for example multienzyme complexes, but also ions in a higher concentration than that of the extracellular environment. This causes osmotic pressure to drive water from outside to inside the cell. If this net flow of water toward the inside of the cell is not counterbalanced, cell swells, and plasma membrane is distended until the cell bursts, that is, an osmotic lysis occurs. Under physiological conditions, this does not happen because during evolution several mechanisms have been developed to oppose, and in some cases even exploit, these osmotic forces. Two of these are energy-dependent ion pumps and, in plants, bacteria and fungi, the cell wall.

Energy dependent ion pumps

Ion pumps reduce, at the expense of ATP, the intracellular concentrations of specific ions with respect to their concentrations in the extracellular environment, thereby creating an unequal distribution of the ions across the plasma membrane, namely, an ion gradient. In this way the cell counterbalances the osmotic forces due to the ions and macromolecules trapped inside it. An example of energy-dependent ion pump is Na+/K+ ATPase, which reduces the concentration of Na+ inside the cell relative to the outside.

Cell wall

Plant cells are surrounded by an extracellular matrix, the cell wall, that, being non expandable and positioned next to the plasma membrane, allows cell to resist osmotic forces that would cause its swelling and finally the lysis. How? Inside mature plant cells, the vacuoles are the largest organelles, occupying about 80% of the total cell volume. Large quantities of solutes, for the most part organic and inorganic acids, are accumulated within them and osmotically draw water, causing their swelling. In turn, this causes the tonoplast, the membrane that surrounds the vacuole, to press the plasma membrane against the cell wall, that mechanically opposes to these forces and avoids the osmotic lysis. This osmotic pressure is called turgor pressure, and can reach up 2 MPa, that is, 20 atmospheres, a value about 10 times higher than the air pressure in tires. It is responsible for the rigidity of non woody parts of plants, is involved in plant growth, as well as in:

  • wilting of vegetables, due to its reduction;
  • plants movements, such as:
    • the circadian movements of the leaves;
    • the movements of the leaves of Dionaea muscipula, the Venus flytrap, or of the leaves of the sensitive plants such as Mimosa pudica.

Even in bacteria and fungi, the plasma membrane is surrounded by a cell wall that stably withholds the internal pressure, then preventing osmotic lysis of the cell.

Isotonic, hypotonic, and hypertonic solutions

By comparing the osmotic pressure of two solutions separated by a semipermeable membrane, it is possible to define three types of solutions, briefly described below.

  • The solutions are isotonic when they have the same osmotic pressure.
  • If the solutions have different osmotic pressures, that with the higher osmotic pressure is defined hypertonic with respect to the other.
  • If the solutions have different osmotic pressures, that with the lower osmotic pressure is defined hypotonic with respect to the other.

In biological systems, the cytosol is the reference solution; then, if we place a cell in a:

  • isotonic solution, no net flow of water occurs into or out of the plasma membrane;
  • hypertonic solution, there is a net flow of water out of the cell, therefore the cell loses water and shrinks;
  • hypotonic solution, there is a net flow of water into the cell, the cell swells and can burst, i.e., an osmotic lysis can occur.

In addition to ion pumps and the cell wall, in the course of evolution multicellular organisms have developed another mechanism to oppose the osmotic forces: to surround the cells with an isotonic solution or close to isotonicity that prevents, or at least limits, a net inflow or outflow of water. An example is plasma, that is, the liquid component of blood, which, due to the presence of salts and proteins, primarily albumin in humans, has an osmolarity similar to that of the cytosol.

Osmotic pressure, starch and glycogen

Living organisms store glucose in the form of polymers, glycogen in animals, fungi, bacteria, and starch in photoautotrophs, but not in the free form. In this way they avoid that the osmotic pressure exerted by the carbohydrate stores becomes too high. Indeed, since osmotic pressure, like the other colligative properties, depends only on the number of solute molecules, storing millions of glucose units in the form of a significantly lower number of polysaccharides allows to avoid an excessive pressure. Here are some examples.

  • A gram of polysaccharide, e.g. glycogen or starch, composed of 1000 glucose units has an effect on osmotic pressure lower than that of a milligram of free glucose.
  • In hepatocyte, if the glucose stored in the form of glycogen was present in the free form, its concentration would be about 0.4 M, whereas the polysaccharide concentration of about 0.04 μM, and this would cause a net flow of water inside the cell such as to lead to osmotic lysis.
    Furthermore, even if osmotic lysis could be avoided, there would be problems with the transport of glucose into the cell. In humans, under physiological conditions, the blood glucose concentration ranges from 3.33 to 5.56 mmol/L (60-100 mg/dL); if glucose was present in the free form, its intracellular concentration would be 120 to 72 times greater than that of the blood, and its transport into the hepatocyte would entail a large energy expenditure.

References

Beauzamy L., Nakayama N., and Boudaoud A. Flowers under pressure: ins and outs of turgor regulation in development. Ann Bot 2014;114(7):1517-33. doi:10.1093/aob/mcu187

Berg J.M., Tymoczko J.L., and Stryer L. Biochemistry. 5th Edition. W. H. Freeman and Company, 2002

Garrett R.H., Grisham C.M. Biochemistry. 4th Edition. Brooks/Cole, Cengage Learning, 2010

Heldt H-W. Plant biochemistry – 3th Edition. Elsevier Academic Press, 2005

Michal G., Schomburg D. Biochemical pathways. An atlas of biochemistry and molecular biology. 2nd Edition. John Wiley J. & Sons, Inc. 2012

Moran L.A., Horton H.R., Scrimgeour K.G., Perry M.D. Principles of Biochemistry. 5th Edition. Pearson, 2012

Nelson D.L., Cox M.M. Lehninger. Principles of biochemistry. 6th Edition. W.H. Freeman and Company, 2012

Multifunctional enzymes

Multifunctional enzymes are proteins in which two or more enzymatic activities, that catalyze consecutive steps of a metabolic pathway, are located on the same polypeptide chain. It seems likely they have arisen by gene fusion events, and represent, like the multienzyme complexes, a product of evolution to maximize the catalytic efficiency, providing advantages that you wouldn’t have if such enzymatic activities were present on distinct proteins dissolved in the cytosol.

CONTENTS

What advantages do multifunctional enzymes provide?

Living organisms fight against the natural processes of decay that, if not counteracted, leads to an increasing disorder, until death. At the molecular level, the maintenance of life is made possible by the extraordinary effectiveness achieved by enzymes in accelerating chemical reactions and avoiding side reactions. The rate of ATP turnover in a mammalian cell gives us an idea of the rate at which cellular metabolism proceeds: every 1-2 minutes, the entire ATP pool is turned over, namely, hydrolyzed and restored by phosphorylation. This corresponds to the turnover of about 107 molecules of ATP per second, and, for the human body, to the turnover of  about 1 gram of ATP every minute. Some enzymes have even achieved catalytic perfection, that is, they are so efficient that nearly every collision with their substrates results in catalysis.
Multifunctional enzymesAnd one of the factors limiting the rate of an enzymatic reaction is just the frequency with which substrates and enzymes collide. The simplest way to increase the frequency of collisions would be to increase the concentration of substrates and enzymes. However, due to the large number of different reactions that take place in the cell, this route is not feasible. In other words, there is a limit to the concentration that substrates and enzymes can reach, concentrations that are in the micromolar range for substrates, and even lower for enzymes. Exceptions are glycolytic enzymes in muscle cells and erythrocytes, present in concentrations of the order of 0.1 mM and even higher.
One of the routes taken by evolution to increase the rate at which enzymatic reactions proceed is to select molecular structures, such as multifunctional enzymes and multienzyme complexes, that allow, through the optimization of the spatial organization of the enzymes of a metabolic pathway, to minimize the distance that the product of reaction A must travel to reach the active site that catalyzes the reaction B in the sequence, and so on, thus obtaining the substrate or metabolic channeling of the pathway itself. For some multifunctional enzymes and multienzyme complexes the channeling is obtained through intramolecular channels.
Metabolic channeling  increases the catalytic efficiency, and then the reaction rate, in various ways, briefly described below.

  • It minimizes the diffusion of substrates in the bulk solvent, then their dilution; this allows to obtain high local concentrations, even when their concentration in the cell is low, thus increasing the frequency of enzyme-substrate collisions.
  • It minimizes the time required by substrates to diffuse from one active site to the next.
  • It minimizes the probability of side reactions.
  • It minimizes the probability that labile intermediates are degraded.

Multifunctional enzymes offer advantages with regard to the regulation of their synthesis, too: being encoded by a single gene, it is possible to coordinate the synthesis of all the enzymatic activities.
Finally, like multienzyme complexes, multifunctional enzymes allow the coordinated control of their catalytic activities. And, because the enzyme that catalyzes the committed step of the sequence often catalyzes the first reaction, this prevents the synthesis of unneeded molecules, which would be produced if the control point were downstream of the first reaction, as well as a waste of energy and the removal of metabolites from other metabolic pathways.

Examples of multifunctional enzymes

Like multienzyme complexes, multifunctional enzymes, too, are very common and involved in many metabolic pathways, both anabolic and catabolic.
Here are some examples.

Acetyl-CoA carboxylase

Acetyl-CoA carboxylase or ACC (EC 6.4.1.2), a biotin-dependent carboxylase, is composed of two enzymes, a biotin carboxylase (EC 6.3.4.14) and a carboxyltransferase, plus a biotin carboxyl-carrier protein or BCCP. ACC catalyzes the synthesis of malonyl-CoA by the carboxylation of acetyl-CoA. The reaction, which is the committed step of fatty acid synthesis, proceeds in two steps. In the first step, biotin carboxylase catalyzes, at the expense of ATP, the carboxylation of a nitrogen atom of biotin, that acts as a carbon dioxide (CO2) carrier, while the source of CO2 is bicarbonate ion. In the second step, carboxyltransferase catalyzes the transfer of the carboxyl group from carboxybiotin to acetyl-CoA to form malonyl-CoA. Malonyl-CoA is the donor of an activated two carbon unit to fatty acid synthase (EC 2.3.1.85) during fatty acid elongation.
In mammals and birds, acetyl-CoA carboxylase is a multifunctional enzyme, as biotin carboxylase activity and carboxyltransferase activity, plus BCCP, are located on the same polypeptide chain.
Conversely, in bacteria it is a multienzyme complex made up of three distinct polypeptide chains, namely, the two enzymes plus BCCP.
Both forms are present in higher plants.

Type I fatty acid synthase

Fatty acid synthase or FAS catalyzes the synthesis of palmitic acid using malonyl-CoA, the product of the reaction catalyzed by acetyl-CoA carboxylase, as a donor of two-carbon units.
There are two types of FAS.
In animals and fungi, it is a multifunctional enzyme, and is called type I. In animals it is an homodimer, and each polypeptide chain contains all seven enzymatic activities plus acyl carrier protein or ACP. In yeast and fungi FAS consists of two multifunctional subunits, called α and β, arranged in an α6β6 heterododecameric structure.
In most prokaryotes and in plants, fatty acid synthase, called type II, it is not a multifunctional enzyme but a multienzyme complex, being composed of distinct enzymes plus ACP.

PRA-isomerase:IGP synthase

The synthesis of the amino acid tryptophan from chorismate involves several steps, briefly described below.
In the first step, glutamine donates a nitrogen to the indole ring of chorismate, that is converted to anthranilate, and glutamine to glutamate; the reaction is catalyzed by anthranilate synthase (EC 4.1.3.27). Anthranilate is phosphoribosylated to form N-(5’-phosphoribosyl)-anthranilate or PRA, in a reaction catalyzed by anthranilate phosphoribosyltransferase (EC 2.4.2.18); in the reaction 5-phosphoribosyl-1-pyrophosphate or PRPP acts as a donor of a 5-phosphoribosyl group. In the next step, catalyzed by PRA isomerase (EC 5.3.1.24), PRA is isomerized to enol-1-o-carboxyphenylamino-1-deoxyribulose phosphate or CdRP. CdRP is converted to indole-3-glycerol phosphate or IGP, in a reaction catalyzed by indole-3-glycerol phosphate synthase or IGP synthase (EC 4.1.1.48). Finally, tryptophan synthase (EC 4.2.1.20) catalyzes the last two steps of the pathway: the conversion of IGP to indole, a hydrolysis, and the reaction of indole with a serine to form tryptophan.
In E. coli, PRA isomerase and IGP synthase are located on a single polypeptide chain, which is therefore a bifunctional enzyme. In other microorganisms, such as Bacillus subtilis, Salmonella typhimurium and Pseudomonas putida, the two catalytic activities located on distinct polypeptide chains.
Conversely, tryptophan synthase is a classic example of a multienzyme complex, and one of the best characterized examples of metabolic channeling.

Glutamine-PRPP amidotransferase

Glutamine-PRPP amidotransferase or GPATase (EC 2.4.2.14) catalyzes the first of ten steps leading to de novo synthesis of purine nucleotides, namely, the formation of 5-phosphoribosylamine through the transfer of the glutamine amide nitrogen to PRPP. Note that glutamine acts as a nitrogen donor.
The reaction proceeds in two steps, which take place on different active sites, an N-terminal active site and a C-terminal active site. In the first step, the N-terminal active site catalyzes the hydrolysis of glutamine amide nitrogen to form glutamate and ammonia. In the second step, catalyzed by the C-terminal active site, which has phosphoribosyltransferase activity, the released ammonia is attached at the C-1 of PRPP to form 5-phosphoribosylamine. In this step the inversion of configuration about the C-1 position of the ribose, from α to β, occurs, then establishing the anomeric form of the future nucleotide.
There are three control points that cooperate in the regulation of de novo synthesis of purine nucleotides, and the reaction catalyzed by glutamine-PRPP amidotransferase, the first committed step of the pathway, is the first control point.
Like in bacterial carbamoyl phosphate synthetase complex (EC 6.3.4.16), the active sites of this multifunctional enzyme are connected through an intramolecular channel. However, this channel is shorter, being about 20 Å long, and lined by conserved nonpolar amino acids, then, it is highly hydrophobic. Lacking hydrogen-bonding groups, it does not impede the diffusion of the ammonia to the other active site.

CAD

The de novo synthesis of pyrimidine nucleotides occurs through a series of enzymatic reactions that, unlike de novo synthesis of purine nucleotides, begins with the formation of the pyrimidine ring, which is then bound to ribose 5-phosphate. The first three steps of the pathway are catalyzed sequentially by carbamoyl phosphate synthetase, aspartate transcarbamoylase (EC 2.1.3.2), and dihydroorotase (EC 3.5.2.3), and are common to all species.
In the first step, carbamoyl phosphate synthetase, which has two enzymatic activities, namely, a glutamine-dependent amidotransferase and a synthase, catalyzes the synthesis of carbamoyl phosphate from glutamine, bicarbonate ion and ATP. In the second step, which is the committed step of the metabolic pathway and is catalyzed by aspartate transcarbamoylase, carbamoyl phosphate reacts with aspartate to form N-carbamoyl aspartate. Finally, dihydroorotase, catalyzing the removal of H2O from N-carbamoyl aspartate, leads to the closure of the pyrimidine ring to form of L-dihydroorotate.
In eukaryotes, particularly in mammals, in Drosophila and Dictyostelium, a genus of amoebae, the three enzymatic activities are located on a single polypeptide chain, encoded by a gene derived from a gene fusion event occurred at least 100 million years ago. The multifunctional enzyme, known by the acronym CAD, is a homomultimer of three subunits or more.
Conversely, in prokaryotes, the three enzymes are distinct, and carbamoyl phosphate synthase is an example of a multienzyme complex.
In yeasts the dihydroorotase is present on a distinct protein.
Studies on enzyme activity have revealed the existence of a substrate channeling, more effective in yeast protein, with respect to the first two steps, than in that of mammals.

References

Alberts B., Johnson A., Lewis J., Morgan D., Raff M., Roberts K., Walter P. Molecular Biology of the Cell. 6th Edition. Garland Science, Taylor & Francis Group, 2015

Eriksen T.A., Kadziola A., Bentsen A-K., Harlow K.W. & Larsen S. Structural basis for the function of Bacillus subtilis phosphoribosyl-pyrophosphate synthetase. Nat Struct Biol 2000:7;303-308. doi:10.1038/74069

Hyde C.C., Ahmed S.A., Padlan E.A., Miles E.W., and Davies D.R. Three-dimensional structure of the tryptophan synthase multienzyme complex from Salmonella typhimurium. J Biol Chem 1988;263(33):17857-17871. doi:10.1016/S0021-9258(19)77913-7

Hyde C.C., Miles E.W. The tryptophan synthase multienzyme complex: exploring structure-function relationships with X-ray crystallography and mutagenesis. Nat Biotechnol 1990:8;27-32. doi:10.1038/nbt0190-27

Michal G., Schomburg D. Biochemical pathways. An atlas of biochemistry and molecular biology. 2nd Edition. John Wiley J. & Sons, Inc. 2012

Muchmore C.R.A., Krahn J.M, Smith J.L., Kim J.H., Zalkin H. Crystal structure of glutamine phosphoribosylpyrophosphate amidotransferase from Escherichia coli. Protein Sci 1998:7;39-51. doi:10.1002/pro.5560070104

Nelson D.L., Cox M.M. Lehninger. Principles of biochemistry. 6th Edition. W.H. Freeman and Company, 2012

Yon-Kahn J., Hervé G. Molecular and cellular enzymology. Springer, 2009

Multienzyme complexes

Multienzyme complexes are discrete and stable structures composed of enzymes associated noncovalently that catalyze two or more sequential steps of a metabolic pathway.
They can be considered a step forward in the evolution of catalytic efficiency as they provide advantages that individual enzymes, even those that have achieved catalytic perfection, would not have alone.

CONTENTS

What advantages do multienzyme complexes provide?

During evolution, some enzymes evolved to reach a virtual catalytic perfection, namely, for such enzymes nearly every collision with their substrate results in catalysis. Examples are:

  • fumarase (EC 4.2.1.2), which catalyzes the seventh reaction of the citric acid cycle, the reversible hydration/dehydration of fumarate’s double bond to form malate;
  • acetylcholinesterase (EC 3.1.1.7), which catalyses the hydrolysis of acetylcholine, a neurotransmitter, to choline and acetic acid, which in turn dissociates to form an hydrogen ion and acetate;
  • superoxide dismutase (EC 1.15.1.1), which catalyzes the conversion, and then the inactivation, of the highly reactive superoxide radical, (O2.-), to hydrogen peroxide (H2O2) and water;
  • catalase (EC 1.11.1.6), which catalyzes the degradation of H2O2 to water and oxygen.

Then, the rate at which an enzymatic reaction proceeds is partly determined by the frequency with which enzymes and their substrates collide. Hence, a simple way to increase it is to increase the concentrations of enzymes and substrates. However, their concentrations cannot be high because of the enormous number of different reactions that occur within the cell. And in fact in the cells most reactants are present in micromolar concentrations (10-6 M), whereas most enzymes are present in much lower concentrations.
So, evolution has taken different routes to increase the reaction rate, one of which has been to optimize the spatial organization of enzymes with the formation of multienzyme complexes and multifunctional enzymes, that is, structures that allow minimizing the distance that the product of a reaction must travel to reach the active site that catalyzes the subsequent step in the sequence, being active sites close to each other. In other words, what happens is the substrate or metabolic channeling, that can also occur through intramolecular channels connecting the active sites, as in the case of, among the multienzyme complexes, tryptophan synthase complex (EC 4.2.1.20), whose tunnel was the first to be discovered, and bacterial carbamoyl phosphate synthase complex (EC 6.3.4.16).
Metabolic channeling can increase the reaction rate, but more generally, the catalytic efficiency, in several ways, briefly described below.

  • The diffusion of substrates and products in the bulk solvent is minimized, then their dilution and decrease of concentration, too. This leads to the production of high local concentrations, even when their intracellular concentration is low. In turn this leads to an increase in the frequency of enzyme-substrate collisions.
  • The time required by substrates to diffuse between successive active sites is minimized.
  • The probability of side reactions is minimized.
  • Chemically labile intermediates are protected from degradation by the solvent.

Another metabolic advantage of multienzyme complexes, similarly to what happens with multifunctional enzymes, is that they allow to control coordinately the catalytic activity of the enzymes that compose them. And taking into account that the enzyme that catalyzes the first reaction of a pathway is often the regulatory enzyme, it is possible to avoid:

  • the synthesis of unneeded intermediates, which would be produced if the sequence of reactions were regulated downstream of the first reaction;
  • the removal of metabolites from other pathways as well as a waste of energy.

Examples of multienzyme complexes

From what was said above, it is not surprising that, especially in eukaryotes, the multienzyme complexes, like multifunctional enzymes, are common and involved in different metabolic pathways, both anabolic and catabolic, whereas there are few enzymes that diffuse freely in solution. Below are some examples.

2-Ketoacid dehydrogenase family

A classic example of multienzyme complexes are the three complexes belonging to the 2-ketoacid dehydrogenase family, also called 2-oxoacid dehydrogenase family, namely:

  • the pyruvate dehydrogenase complex (PDC);
  • the branched-chain α-keto acid dehydrogenase complex (BCKDH);
  • the α-ketoglutarate dehydrogenase complex, also called 2-oxoglutarate dehydrogenase (OGDH).

These complexes are similar both from structural and functional points of view.
For example, PDC is composed of multiple copies of three different enzymes:

  • pyruvate dehydrogenase or E1 (EC 1.2.4.1);
  • dihydrolipoyl transacetylase or E2 ;(EC 2.3.1.12);
  • dihydrolipoyl dehydrogenase or E3 (EC 1.8.1.4).

Then, PDC, both in prokaryotes and eukaryotes, has the basic E1-E2-E3 structure, a structure also found in the other two complexes. Moreover, within a given species:

  • dihydrolipoyl dehydrogenase is identical;
  • pyruvate dehydrogenase and dihydrolipoyl transacetylase are homologous.

And, although these enzymes are specific for their substrates, they use the same cofactors, namely, coenzyme A, NAD, thiamine pyrophosphate, FAD, and lipoamide.
In order to differentiate them, for the pyruvate dehydrogenase complex, the α-ketoglutarate dehydrogenase complex, and the branched-chain α-ketoacid dehydrogenase complex, they are indicated, respectively:

  • E1p, E1o and E1b (EC 1.2.4.4);
  • E3p, E3o, and E3b (EC 1.8.1.4).

Note: the eukaryotic PDC is the largest multienzyme complex known, larger than a ribosome, and can be visualized with the electron microscope.

The pyruvate dehydrogenase complex is the bridge between glycolysis and the citric acid cycle, and catalyzes the irreversible oxidative decarboxylation of pyruvate, an α-keto acid. During the reactions the carboxyl group of pyruvate is released as carbon dioxide (CO2) and the resulting acetyl group is transferred to coenzyme A to form acetyl-coenzyme A. Furthermore, two electrons are released and transferred to NAD+.
Even during the reactions catalyzed by the α-ketoglutarate dehydrogenase complex and the branched-chain α-keto acid dehydrogenase complex, respectively, the fourth reaction of the citric acid cycle, the oxidation of α-ketoglutarate to succinyl-CoA, and the oxidation of α-ketoacids deriving from the catabolism of the branched-chain amino acids valine, leucine and isoleucine, it occurs:

  • the release of the carboxyl group of the α-keto acid as CO2;
  • the transfer of the resulting acyl group to coenzyme A to form the acyl-CoA derivatives;
  • the reduction of NAD+ to NADH.

The remarkable similarity between protein structures, required cofactors and reaction mechanisms undoubtedly reflect a common evolutionary origin.

What are keto acids?

Keto acids or oxoacids are organic compounds containing two functional groups: a carboxyl acid group and a ketone group. Depending on the position of the ketone group, alpha-keto acids, beta-keto acids and gamma-keto acids can be identified.

  • Alpha-keto acids or 2-oxoacids have the ketone group at position α (2) from the carboxylic acid group, that is, adjacent to it. These compounds are important in biology, being involved in glycolysis, like pyruvic acid, the simplest α-keto acid, and in the citric acid cycle, like oxalacetic acid and α-ketoglutaric acid.
  • Beta-ketoacids or 3-oxoacids have the ketone group is at position β (3) from the carboxylic acid group. An example is acetoacetic acid, the simplest β-ketoacid, and one of the three ketone bodies, together with acetone and β-hydroxybutyric acid, produced by the hepatocyte in presence of an excess of acetyl-CoA, such as during fasting or low-carbohydrate diets.
  • Gamma-keto acids or 4-oxoacids have the ketone group is in position γ (4) from the carboxylic acid group. An example is levulinic acid, the simplest beta-keto acid, deriving from the catabolism of cellulose.Multienzyme complexes, the 2-ketoacid dehydrogenase family, and ketoacids

Tryptophan synthase complex

The tryptophan synthase complex is one of the best-studied examples of substrate channeling. Present in bacteria and plants, but not in animals, in bacteria it is composed of two α and two β subunits associated as αβ dimers, which are considered the functional unit of the complex, in turn associated to form an αββα tetramer.
The complex catalyzes the final two steps of the synthesis of tryptophan. In the first step, indole-3-glycerol phosphate undergoes an aldol cleavage, catalyzed by a lyase (EC 4.1.2.8) present on the α subunits, to yield indole and a molecule of glyceraldehyde 3-phosphate. Indole then reaches the active site of the β subunit via a about 30 Å long hydrophobic tunnel that, in each αβ dimers, connects the two active sites. In the second step, in the presence of pyridoxal 5-phosphate, a condensation between indole and a serine forms tryptophan.

Acetyl-CoA carboxylase

Acetyl-CoA carboxylase (ACC) (EC 6.4.1.2), a member of the biotin-dependent carboxylase family, catalyzes the committed step of de novo fatty acid synthesis, namely, the carboxylation of acetyl-CoA to malonyl-CoA, which, in turn, serves as a donor of two-carbon units for the elongation process leading to the synthesis of palmitic acid, catalyzed by fatty acid synthase (EC 2.3.1.85).
In bacteria, ACC is an multienzyme complex composed of two enzymes, biotin carboxylase (EC 6.3.4.14) and a carboxytransferase, plus a biotin carboxyl-carrier protein or BCCP.
Conversely, in mammals and birds, it is a multifunctional enzyme, as the two enzymatic activities, and BCCP, are present on the same polypeptide chain.
In higher plants both forms are present.

Carbamoyl phosphate synthetase complex

Another well-characterized example of substrate channeling is the bacterial carbamoyl phosphate synthetase complex, which catalyzes the synthesis of carbamoyl phosphate, needed for pyrimidine and arginine synthesis. The complex has a about 100 Å long tunnel that connects the three active sites.
The first active site catalyzes the release of the amide nitrogen of glutamine as ammonium ion, that enters the tunnel and reaches the second active site where, at the expense of ATP, is combined with bicarbonate to yield carbamate, that, in the last active site, is phosphorylated to carbamoyl phosphate.

References

Alberts B., Johnson A., Lewis J., Morgan D., Raff M., Roberts K., Walter P. Molecular biology of the cell. 6th Edition. Garland Science, Taylor & Francis Group, 2015

Garrett R.H., Grisham C.M. Biochemistry. 4th Edition. Brooks/Cole, Cengage Learning, 2010

Hilario E.,  Caulkins B.G., Huang Y-M. M., You W., Chang C-E. A., Mueller L.J., Dunn M.F., and Fan L. Visualizing the tunnel in tryptophan synthase with crystallography: insights into a selective filter for accommodating indole and rejecting water. Biochim Biophys Acta 2016;1864(3):268-279. doi:10.1016/j.bbapap.2015.12.006

Hyde C.C., Ahmed S.A., Padlan E.A., Miles E.W., and Davies D.R. Three-dimensional structure of the tryptophan synthase multienzyme complex from Salmonella typhimurium. J Biol Chem 1988;263(33):17857-17871. doi:10.1016/S0021-9258(19)77913-7

Hyde C.C., Miles E.W. The tryptophan synthase multienzyme complex: exploring structure-function relationships with X-ray crystallography and mutagenesis. Nat Biotechnol 1990:8;27-32. doi:10.1038/nbt0190-27

Koolman J., Roehm K-H. Color atlas of Biochemistry. 2nd Edition. Thieme, 2005

Michal G., Schomburg D. Biochemical pathways. An atlas of biochemistry and molecular biology. 2nd Edition. John Wiley J. & Sons, Inc. 2012

Moran L.A., Horton H.R., Scrimgeour K.G., Perry M.D. Principles of Biochemistry. 5th Edition. Pearson, 2012

Nelson D.L., Cox M.M. Lehninger. Principles of biochemistry. 6th Edition. W.H. Freeman and Company, 2012

Perham R.N., Jones D.D., Chauhan H.J., Howard MJ. Substrate channeling in 2-oxo acid dehydrogenase multienzyme complexes. Biochem Soc Trans 2002;30(2):47-51. doi:10.1042/bst0300047

Rodwell V.W., Bender D.A., Botham K.M., Kennelly P.J., Weil P.A. Harper’s illustrated biochemistry. 30th Edition. McGraw-Hill Education, 2015

Voet D. and Voet J.D. Biochemistry. 4th Edition. John Wiley J. & Sons, Inc. 2011

Yon-Kahn J., Hervé G. Molecular and cellular enzymology. Springer, 2009

Welch G. R., Easterby J.S. Metabolic channeling versus free diffusion: transition-time analysis. Trends Biochem Sci 1994;19(5):193-197. doi:10.1016/0968-0004(94)90019-1

Zhou Z.H., McCarthy D.B., O’Connor C.M., Reed L.J., and J.K. Stoops. The remarkable structural and functional organization of the eukaryotic pyruvate dehydrogenase complexes. Proc Natl Acad Sci USA 2001;98(26):14802-14807. doi:10.1073/pnas.011597698

Pyruvate dehydrogenase complex: location, functions, enzymes, and regulation

The pyruvate dehydrogenase complex (PDC) is a mitochondrial multienzyme complex composed of three different enzymes:

  • pyruvate dehydrogenase or E1 (EC 1.2.4.1);
  • dihydrolipoyl transacetylase or dihydrolipoamide acetyltransferase or E2 (EC 2.3.1.12);
  • dihydrolipoyl dehydrogenase or dihydrolipoamide dehydrogenase or E3 (EC 1.8.1.4).

Each of these enzymes is present in multiple copies whose number, and then the size of the complex itself, varies from species to species, with molecular masses ranging from 4×106 to 1×107 daltons.
The multienzyme complex contains additional subunits:

  • five different coenzymes;
  • in plants, fungi, and among animals, aves and mammals, two enzymes with regulatory properties: a Mg2+-dependent pyruvate dehydrogenase kinase (EC 2.7.1.99) and a Ca2+-activated pyruvate dehydrogenase phosphatase(EC 3.1.3.43);
  • in eukaryotes, a binding protein called E3BP.

The pyruvate dehydrogenase complex catalyzes, through five sequential reactions, the oxidative decarboxylation of pyruvate, an α-keto acid, to form a carbon dioxide molecules (CO2) and the acetyl group of acetyl-coenzyme A or acetyl-CoA, with the release of two electrons, carried by NAD.

Oxidative decarboxylation of pyruvate to acetyl-CoA catalyzed by pyruvate dehydrogenase complex
PDC Reaction

The overall reaction is essentially irreversible, with a ΔG°’ of -8.0 kcal/mol (-33.4 kJ/mol), and requires the intervention of the three enzymes, whose activities are sequentially coordinated. During the reactions, the intermediate products remain bound to the enzymes and, at the end of the reaction sequence, the multienzyme complex is ready for the next cycle;.

The five reactions catalyzed by pyruvate dehydrogenase complex
The Five Reactions Catalyzed by the PDC

Note: the pyruvate dehydrogenase complex catalyzes the same reactions through similar mechanisms in all organisms.

CONTENTS

The coenzymes of the pyruvate dehydrogenase complex

Five coenzymes are used in the pyruvate dehydrogenase complex reactions: thiamine pyrophosphate or TPP, flavin adenine dinucleotide or FAD, coenzyme A or CoA, nicotinamide adenine dinucleotide or NAD, and lipoic acid.

  • Thiamine pyrophosphate is the active form of thiamine or vitamin B1. TPP is the coenzyme of pyruvate dehydrogenase, to which it is strictly bound through noncovalent interactions. It is involved in the transfer of hydroxyethyl or “activated aldehyde” groups.
  • Flavin adenine dinucleotide is one the active forms of riboflavin or vitamin B2; the other is flavin mononucleotide (FMN).
    Skeletal formula of the oxidized and reduced form of flavin adenine dinucleotide, the active form of vitamin B2
    Reduced and Oxidized Form of Flavin Adenine Dinucleotide

    FAD is the coenzyme of dihydrolipoyl dehydrogenase, to which it is strictly bound. Like NAD, it participates in electron transfer, or hydride ion (:H or H+ + 2e) transfer.

  • Coenzyme A consists of a β-mercaptoethylamine group connected to pantothenic acid or vitamin B5 through an amide linkage, which, in turn, is bonded to 3′-phosphoadenosine moiety, through a pyrophosphate bridge.
    CoA is involved in the reaction catalyzed by dihydrolipoyl transacetylase, and acts as a carrier of acyl groups.

    Skeletal formula of coenzyme A and acetyl-coenzyme A
    Coenzyme A and Acetyl-coA

    The β-mercaptoethylamine moiety terminates with a sulfhydryl group (–SH), a reactive thiol crucial for the role played by the coenzyme, because the acyl groups bonded to it through a thioester bond have a high standard free energy of hydrolysis. This provides acyl groups with a high transfer potential, equal to -31.5 kcal/mol (-7.5 kJ/mol), slightly more exergonic [1 kJ/mol (0.2 kcal/mol )] than that for the hydrolysis of ATP to ADP and Pi. Thioesters have therefore a high transfer potential of the acyl group and can donate it to a variety of molecules, that is, such acyl group can be considered as an activated group ready for a transfer. It is also possible to state that the formation of the thioester bond allows to conserve a portion of the free energy derived from the oxidation of the metabolic fuel. It should be noted that coenzyme A is also abbreviated as CoA-SH to emphasize the role played by the thiol group.
    Note: in the thioester bond a sulfur atom sits in the position where an oxygen atom is in the ester bond.

    Ester bond and thioester bond
    Ester and Thioester Bonds
  • Nicotinamide adenine dinucleotide can be synthesized from tryptophan, an essential amino acid, or from niacin or vitamin B3 or vitamin PP, from Pellagra-Preventing, the source of the nicotinamide moiety.
Skeletal formula of the oxidated and reduced form of nicotinamide adenine dinucleotide
Reduced and Oxidized Form of Nicotinamide Adenine Dinucleotide Phosphate

NAD is involved in the reaction catalyzed dihydrolipoyl dehydrogenase, and, like FAD, participates in electron transfer, or hydride transfer.

  • Unlike the other coenzymes of the pyruvate dehydrogenase complex, lipoic acid does not derive, directly or indirectly, from vitamins and/or essential amino acids, that is, from building blocks that cannot be synthesized de novo by the organism and must be supplied by the diet.
    It is the coenzyme of dihydrolipoyl transacetylase, to which it is covalently bound, through an amide linkage, to the ɛ-amino group of a lysine residue to form a lipoyl-lysine or lipoamide, the so-called lipoyllysyl arm. It couples electron transfer to acyl group transfer.

    Lipoamide, the functional form of lipoic acid, the coenzyme of dihydrolipoyl transacetylase
    Lipoamide or Lipoyl-lysine

    Lipoic acid has two thiol groups that can undergo a reversible intramolecular oxidation to form a disulfide bridge (-S-S-), a reaction analogous to that between two cysteine (Cys) residues of a protein.
    Because the disulfide bridge (note: a cyclic disulfide) is capable of undergoing redox reactions, during the reactions catalyzed by the pyruvate dehydrogenase complex, it is first reduced to dihydrolipoamide, a dithiol or the reduced form of the prosthetic group, and then, reoxidized to the cyclic form.

Note
Many enzymes require small non protein components, called cofactors, for their catalytic activity. Cofactors can be metal ions or small organic or metalloorganic molecules, and are classified as coenzymes and prosthetic groups.
A prosthetic group is cofactor that binds tightly to an enzyme by non-covalent or covalent bond, namely, it is permanently bound to the protein.
A coenzyme is cofactor that is not permanently bound to the enzyme.

Where is the pyruvate dehydrogenase complex located?

In eukaryotes, the pyruvate dehydrogenase complex, like the enzymes for citric acid cycle and oxidation of fatty acids, is located in the mitochondrion, where is associated with the surface of the inner membrane facing the matrix.
In prokaryotes, it is located in the cytosol.

Functions of the pyruvate dehydrogenase complex

The main functions of the pyruvate dehydrogenase complex are to produce acetyl-CoA and NADH.

  • The acetyl group linked to coenzyme A, an activated acetate, depending on the metabolic conditions within the cell and/or the cell type, can be:

oxidized to two carbon dioxide molecules via the citric acid cycle reactions to harvest a portion of the potential energy stored in the form of ATP or GTP;
utilized for the synthesis of fatty acids, cholesterol, steroids, isoprenoids, ketone bodies and acetylcholine.

It is therefore possible to state that, depending on the metabolic conditions and/or cell type, the pyruvate dehydrogenase complex commits carbon intermediates from amino acid and glucose catabolism to:

citric acid cycle, and then to the production of energy, e.g. in skeletal muscle in aerobic conditions, and, always, in cardiac muscle;
synthesis of lipids and acetylcholine.

  • In aerobic organisms, NADH can be oxidized to NAD+ via hydride ion transfer to the mitochondrial electron transport chain that, in turn, carries the two electrons to molecular oxygen (O2), allowing the production of 2.5 ATP molecules per pair of electrons.Note: in anaerobic organisms there are electron acceptors alternative to oxygen, such as sulfate or nitrate.

Conceptually, the pyruvate dehydrogenase complex is the bridge between glycolysis and the citric acid cycle. However, due to the irreversibility of the overall reaction catalyzed by the multienzyme complex, it acts as an one-way bridge: pyruvate can be decarboxylated, oxidized and the remaining acetyl unit linked to the CoA, but it is not possible to carry out the opposite reaction, namely, to convert acetyl-CoA into pyruvate.
The irreversibility of this reaction, and the absence of alternative pathways, explain why it is not possible to use acetyl-CoA, and therefore fatty acids, as a substrate for gluconeogenesis (see below).

Other sources of acetyl-CoA

Other than pyruvate, the acetyl group of acetyl-CoA can derive from the oxidation of fatty acids and the catabolism of many amino acids. However, regardless of its origin, acetyl-CoA represents an entry compound for new carbon units into the citric acid cycle. And, it is also possible to state that the acetyl group of acetyl-CoA represents the form in which most of the carbon enters the cycle.

Sources of pyruvate

Pyruvate can derive from different cytosolic sources.

  • Under physiological conditions, in most cells it derives mainly from glycolysis: the oxidation of a glucose molecule yields two pyruvate molecules.
  • Lactate can be oxidized to pyruvate in the reaction catalyzed by lactate dehydrogenase (EC 1.1.1.27). In fact, isoenzymes in which the H subunit predominates, such as LDH1 or H4, a homopolymer of H subunits found in cardiac muscle, a tissue with completely aerobic metabolism, preferentially catalyze lactate oxidation.
    Lactate oxidation can occurs in hepatocytes, too, during gluconeogenesis, favored by the low NADH/NAD+ ratio in the cytosol.
    It should be noted that lactate is a metabolite deriving from the catabolism of glucose, and therefore from carbohydrate catabolism.
  • Another source is malate, by the reaction catalyzed by the cytosolic malic enzyme (EC 1.1.1.40), which is implicated in the exchange of intermediates of the citric acid cycle, such as oxaloacetate, malate and citrate, between the cytosol and the mitochondrial matrix.

Malate + NADP+ → Pyruvate + CO2 + NADPH + H+

  • Finally, the carbon skeleton of six amino acids, namely, alanine, cysteine, glycine, serine, threonine and tryptophan, can be partly or completely converted into pyruvate.

In turn, pyruvate is a metabolic intermediate which can be metabolized by different pathways, both anabolic and catabolic. Examples are gluconeogenesis, the synthesis of lipids, and the oxidative metabolism. In addition, it can have an anaplerotic function, playing a role in the maintenance of the metabolic flow through the acid cycle citric.

Mitochondrial pyruvate transport

In eukaryotes, glycolysis occurs in the cytosol whereas all the subsequent steps of the aerobic metabolism, that is, the reactions catalyzed by the pyruvate dehydrogenase complex, the citric acid cycle, the electron transport chain, and the oxidative phosphorylation occur in the mitochondria.
Similarly to most other metabolites and anions, the transport of pyruvate across the outer mitochondrial membrane is probably mediated by a relatively non-specific, voltage-dependent anion channel. Conversely, its transport across the inner mitochondrial membrane occurs through a specific transporter made up of two proteins named MPC1 and MPC2, acronym of mitochondrial pyruvate carrier, which form a hetero-oligomeric complex in the membrane.

Structure of the pyruvate dehydrogenase complex

Although the pyruvate dehydrogenase complex is composed of multiple copies of three different enzymes and catalyzes the same reactions by similar mechanisms in all the organisms in which it is present, it has a very different quaternary structure.
The structure of E. coli multienzyme complex, which has a weight of ∼4,600 kD and a diameter of ∼300 Å, was the first to be characterized, thanks to the work of Lester Reed. In this complex, 24 units of dihydrolipoyl transacetylase form a structure with cubic symmetry, namely, the enzymes, associated as trimers, are placed at the corners of a cube. Dimers of pyruvate dehydrogenase are associated with dihydrolipoyl transacetylase core, at the center of each edge of the cube, for a total of 24 units. Finally, dimers of dihydrolipoyl dehydrogenase are located at the center of each of the six faces of the cube, for a total of 12 units. Note that the entire complex is composed of 60 units. A similar structure with cubic symmetry is also found in most other Gram-negative bacteria.
In some Gram-positive bacteria and in eukaryotes, the pyruvate dehydrogenase complex has a dodecahedral form, namely, that of a regular polyhedron with 20 vertices, 12 pentagonal faces, and 30 edges, with icosahedral symmetry, also called I symmetry. Considering for example the multienzyme complex present in mitochondria, it is the largest known multienzyme complex, with a weight of ∼10,000 kD and a diameter of ∼500 Å, so more than 5 times the size of a ribosome. Noteworthy, it can be visualized with the electron microscope. The complex is composed of a dodecahedral core, with a diameter of about 25 nm, formed, as in Gram-negative bacteria, by dihydrolipoyl transacetylase, but consisting of 20 trimers of the enzyme,  for a total of 60 units, located at the vertices of the structure. The core is surrounded by 30 units of pyruvate dehydrogenase, one centered on each edge, and 12 units of dihydrolipoyl dehydrogenase, one centered on each face. The entire complex is therefore composed of 102 units.

The quaternary structure of the complex is further complicated, as mentioned previously, by the presence of three additional subunits: a pyruvate dehydrogenase kinase, a pyruvate dehydrogenase phosphatase, and the E3-binding protein.
Kinase and phosphatase are bound to the dihydrolipoyl transacetylase core.
E3BP is bound to each of the 12 pentagonal faces, and therefore is present in about 12 copies. It is required to bind dihydrolipoyl dehydrogenase to the core of dihydrolipoyl transacetylase, as demonstrated by the fact that its partial proteolysis decreases the binding ability of the dehydrogenase. In E3-binding protein it is possible to identify a C-terminal domain, that has no catalytic activity, and a lipoamide-containing domain, similar to that of dihydrolipoyl transacetylase, capable of accepting an acetyl group, too. However, the removal of this domain does not cause any reduction of the catalytic activity of the multienzyme complex.

Structure of pyruvate dehydrogenase or E1

Pyruvate dehydrogenase of eukaryotes and some Gram-positive bacteria is composed of two different polypeptide chains, called α and β, associated to form a 2-fold symmetric α2β2 heterotetramer. Conversely, in E. coli and other Gram-negative bacteria the two subunits are fused to form a single polypeptide chain, and the enzyme is a homodimer.
The enzyme has two active sites.
Considering the heterotetrameric structure of Bacillus stearothermophilus pyruvate dehydrogenase, a Gram-positive bacteria, each thiamine pyrophosphate binds between the N-terminal domains of an α and a β subunit, at the end of a ∼21 Å deep funnel-shaped channel leading to the active site, with its reactive group, the thiazole ring (see fig. 3), closest to the channel entrance. At the entrance this channel there are also two conserved loops, essential both for the catalytic activity of the enzyme and for its regulation. The X-ray analysis of B. stearothermophilus enzyme, when it binds both TPP and the peripheral subunit-binding domain (PSBD) of dihydrolipoyl transacetylase, which binds to the C-terminal domain of the β subunits, has revealed that, in addition to a heterotetramer with a very tight structure, the two active sites have a different structure, in particular regarding the arrangement of the two conserved loops. In fact, in one enzyme subunit, in the presence of the activated form of thiamine pyrophosphate, the inner loop is ordered in a way that it blocks the entrance to the active site, whereas the loop at the entrance of the other active site is disordered and does not block the entrance. This explains, from a structural point of view, the observed differences in the rate of substrate binding exhibited by the two active sites. A similar arrangement and asymmetry have been observed in all thiamine pyrophosphate-dependent enzymes of which the structure has been solved.
In addition to TPP and a magnesium ion (Mg2+), located in each of the two active sites, a third Mg2+ is located at the centre of the tetramer, within a ∼20 Å deep solvent-filled tunnel that connects the two active sites. The tunnel is largely lined by 10 conserved amino acid residues from all four subunits, in particular glutamate (Glu) and aspartate (Asp), six and four, respectively, plus other acidic residues around the TPP aminopyrimidine ring. And it should be underlined the absence of basic residues to neutralize them. Similar tunnels have been found in all thiamine pyrophosphate-dependent enzymes with known crystalline structure, with dimeric or tetrameric structure, for example in transketolase, an enzyme of the pentose phosphate pathway.

Note: B. stearothermophilus belongs to the phylum Firmicutes and is recently renamed Geobacillus stearothermophilus.

What is the function of the acidic tunnel?

Through mutagenesis experiments on B. stearothermophilus pyruvate dehydrogenase, the tunnel has been shown to play a role in the catalytic mechanism.
The change of some of the aforementioned acidic residues to neutral amino acids does not alter, compared with the wild-type pyruvate dehydrogenase:

  • the efficiency of incorporation of the modified enzyme into the multienzyme complex;
  • the structure of active sites;
  • the quaternary structure of the enzyme.

However, rate of decarboxylation is reduced by over 70% compared to the wild-type enzyme, as well as, once the multienzyme complex is assembled with the mutant pyruvate dehydrogenase, the PDC activity, which is reduced by over 85% compared to the wild-type complex. But how does this occur?
Because the distance between the substituted amino acids and the active sites is ≥7 Å, that is, these amino acids are remote from the active sites of pyruvate dehydrogenase, they cannot directly influence its catalytic activity. Then, the catalytic mechanism described below was proposed.
Considering the apoenzyme, thiamine pyrophosphate binds fast and strongly to the first active site, is activated, and the active site is closed, thus protecting the zwitterionic thiazolium from the external environment.
Conversely, in the second active site TPP binds, but is not activated, and the active site remains in an open conformation.
In the first active site, pyruvate reacts with the thiazolium C-2, and thiamine pyrophosphate of the second active site, which is a general acid, donates a proton to the first site. The result is a decarboxylation in the first site and the activation of the coenzyme in the second site, which is then closed.
It should be noted that while the activation of the first thiamine pyrophosphate is the result of the binding to the active site, the activation of the second coenzyme, and therefore of the second active site, is coupled to the decarboxylation of pyruvate in the first active site. Or, from another point of view, while an active site requires a general acid, the other requires a general base.
Protons are needed for the catalytic activity, and their transport between the two active sites occurs via the acidic tunnel. They are reversibly shuttled along a chain of donor-acceptor groups provided by glutamate and aspartate residues and the entrained water, that act as a proton wire.
It seems, therefore, that unlike many other enzymes, in which the communication between the active sites occurs through conformational changes and subunit rearrangements, in pyruvate dehydrogenase and in the other TPP-dependent enzymes, the proton wire is the molecular basis of such communication.
At this point, the holoenzyme has been formed and the active sites are in a dynamic equilibrium, each exchanging between the dormant and the activated state. This seems to be the state in which the enzyme is found in vivo at the start of each catalytic cycle.
A consequence of such a mechanism is that, as the catalytic cycles occur, the two active sites are out of phase with each other, namely, when an active site requires a general acid, the other requires a general base, and vice versa.
Finally, it should be noted that this mechanism allows the switching of the loops that close the active sites so as to:

  • coordinate substrate uptake and product release;
  • explain the asymmetry existing between the two active sites.

Note: an apoenzyme is an enzyme that lacks the association of its cofactors. Conversely, an holoenzyme is an apoenzyme together with its cofactors. The apoenzyme is a catalytically inactive enzyme, whereas the holoenzyme is a catalytically active enzyme.

Structure of dihydrolipoyl transacetylase or E2

Three functionally distinct domains can be identify in the structure of dihydrolipoyl transacetylase: an N-terminal lipoyl domain, a peripheral subunit-binding domain, and a C-terminal catalytic domain or acyltransferase domain. These domains are connected by 20- to 40 amino acid residues rich in alanine and proline, hydrophobic amino acids that are interspersed with charged residues. These linkers are highly flexible and largely extended, that allows the three domains to kept away from each other.

Domains of Dihydrolipoyl Transacetylase
E2 Domains

Note: flexible linkers are present in E3BP, too.

  • The N-terminal lipoyl domain is composed of ∼80 amino acid residues, and is so called because it binds lipoic acid. The number of these domains depend on the species, ranging from one to three. For example, there is one domain in B. stearothermophilus and in yeasts, two in Streptococcus faecalis and in mammals, and three in Azotobacter vinelandii and E. coli.
    The link between the ɛ-amino group of a lysine residue and lipoic acid leads to the formation of a flexible arm, the lipoyl-lysine, which has a maximum extended length of ∼14 Å. Adding the polypeptide segment which connects the N-terminal domain to the adjacent domain, whose length is greater than 140 Å, the resulting flexible tether is able to swings the lipoyl group between the active sites of pyruvate dehydrogenase and dihydrolipoyl dehydrogenase, as well as to interact with neighboring dihydrolipoyl transacetylases of the core.
    It should be noted that the number of these tethers is 3 x 24 = 72 in E. coli, whereas in mammals 2 x 60 = 120, based on the number of N-terminal domains and the units of dihydrolipoyl transacetylase.
    One pyruvate dehydrogenase can therefore acetylate numerous dihydrolipoyl transacetylases, and one dihydrolipoyl dehydrogenase can reoxidize many dihydrolipoamide groups.
    Moreover, it also occurs:

an interchange of the acetyl groups between the lipoyl groups of the dihydrolipoyl transacetylase core;
the exchange of both acetyl groups and disulfides between the tethered arms.

  • PSBD is composed of ∼35 amino acid residues arranged to form a globular structure that binds to both pyruvate dehydrogenase and dihydrolipoyl dehydrogenase, that is, it holds the multienzyme complex together.
  • The C-terminal catalytic domain, which, of course, contains the active site, is composed of ∼250 amino acid residues arranged to form a hollow cage-like structure containing channels large enough to allow substrates and products to diffuse in and out. For example, CoA ad lipoamide, the two substrate of dihydrolipoyl transacetylase, bind, in their extended conformation, at the opposite ends of a channel located at the interface between each pair of subunits in each trimers.

Structure of dihydrolipoyl dehydrogenase or E3

The structure of dihydrolipoyl dehydrogenase was deduced from studies of the enzyme in several microorganisms. It has a homodimeric structure, with each ∼470 amino acid residue chain folded into four domains, from the N-terminal to the C-terminal end: a FAD-binding domain, a NAD+-binding domain, a central domain, and an interface domain. All domains participate in the formation of the active site.
FAD is almost completely hidden inside the protein because, unlike thiol or NADH, it is easily oxidizable and must therefore be protected from the surrounding solution, namely, from O2. In fact, in the absence of the nicotinamide coenzyme, the phenol side chain of a tyrosine residue (Tyr), for example Tyr181 in the Gram-negative bacteria Pseudomonas putida, covers the NAD+-binding pocket so as to protect FADH2 from the contact with the surrounding solution.
Conversely, when NAD+ is located in the active site, the phenol side chain of the aforementioned tyrosine residue is interposed between the nicotinamide ring and the flavin ring.
In the active site of the enzyme’s oxidized form is also present a redox-active disulphide bridge. It forms between two cysteine residues located in a highly conserved segment of the polypeptide chain, e.g., Cys43 and Cys48 in P. putida, and is located on opposite side of the flavin ring with respect to the nicotinamide ring. The disulphide bridge links consecutive turns in a segment of a distorted α-helix, and, noteworthy, in the absence of such distortion, Cα atoms of the two cysteine residues would be too distant to allow the disulfide bridge to form.
Dihydrolipoyl dehydrogenase has therefore two electron acceptors: FAD and the redox-active disulphide bridge.
Note: the heterocyclic rings of NAD and FAD are parallel and in contact through van der Waals interactions; S48 is also in contact through van der Waals interactions with the flavin ring, on the opposite side of it from the NAD ring.

Reaction of pyruvate dehydrogenase or E1

In the reaction sequence catalyzed by components of the pyruvate dehydrogenase complex, pyruvate dehydrogenase catalyzes the first two steps, namely:

  • the decarboxylation of pyruvate to form CO2 and the hydroxyethyl-TPP intermediate;
  • the reductive acetylation of the lipoyl group of dihydrolipoyl transacetylase.

The first reaction is essentially identical to pyruvate decarboxylase reaction (EC 4.1.1.1), which carries out a non-oxidative decarboxylation in glucose fermentation to ethanol. What differs is the fate of the hydroxyethyl group bound to thiamine pyrophosphate that, in the reaction catalyzed by pyruvate dehydrogenase is transferred to the next enzyme in the sequence, dihydrolipoyl transacetylase, whereas in the reaction catalyzed by pyruvate decarboxylase is converted into acetaldehyde.

Catalytic mechanism of pyruvate dehydrogenase or E1

In thiamine pyrophosphate-dependent enzymes, the thiazolium ring is the active center, but only as dipolar carbanion or ylid, namely, as a dipolar ion, or zwitterion (German for “hybrid ion”), with positive charge on the N-3 and negative charge on C-2. Conversely, the positively charged thiazolium ring, that is, positive charged nitrogen and no charge on C-2, can be defined as “dormant” or inactive form.
The reaction begins with the nucleophilic attack by C-2 carbanion to the carbonyl carbon of pyruvate, which has the oxidation state of an aldehyde, and leads to the formation of a covalent bond between coenzyme and pyruvate.
Then, the cleavage of C-1–C-2 bond of pyruvate occurs. This leads to the release of the carboxyl group, namely, of the C-1 as CO2, while the remaining carbon atoms, C-2 and C-3, stay bound to the thiamine pyrophosphate as hydroxyethyl group. The cleavage of the C-1–C-2 bond, and therefore the decarboxylation of pyruvate, is favored by the fact that the negative charge on the C-2 carbon, that is unstable, is stabilized by the presence in the thiazolium ring of the positively charged N-3, a imine nitrogen (C=N+), that is, due to the presence of an electrophilic or electron deficient structure that acts as an electron sink or electron trap, in which the carbanion electrons can be delocalized by resonance.
At this point, the intermediate stabilized by resonance can be protonated to form hydroxyethyl-TPP.
Note: this first reaction catalyzed by pyruvate dehydrogenase is that in which the private dehydrogenase complex exercises its substratum specificity; furthermore, it is the slowest of the five reactions, hence limiting the rate of the overall reaction.

Catalytic mechanism of pyruvate dehydrogenase, one of the components of the pyruvate dehydrogenase complex
Catalytic mechanism of pyruvate dehydrogenase

The enzyme then catalyzes the oxidation of the hydroxyethyl group to an acetyl group, and its transfer on lipoyllysyl arm of dihydrolipoyl transacetylase. The reaction begins with the formation of a carbanion on the hydroxylic carbon of the hydroxyethyl-TPP, by the removal of the carbon-linked proton by an enzyme base.
The carbanion carries out a nucleophilic attack on the lipoamide disulfide, with the formation of a high-energy acetyl-thioester bond with one of the two -SH groups. In this reaction, the oxidation of the hydroxyethyl group to an acetyl group occurs with the concomitant reduction of the lipoamide disulfide bond: the two electrons removed from the hydroxyethyl group are used to reduce the disulfide. This reaction is therefore a reductive acetylation accompanied by the regeneration of the active form of pyruvate dehydrogenase, namely, the enzyme with the thiazolium C-2 in the deprotonated form, the ylid or dipolar carbanion form.

Note that the energy derived from the oxidation of the hydroxyethyl group to an acetyl group drives the formation of the thioester bond between the acetyl group and coenzyme A.

Note: as previously said, the lipoyllysyl arm, arranged in an extended conformation in the channel where TPP is also found, allows the transfer of hydroxyethyl from hydroxyethyl-TPP to CoA, that is, it can move from the active site of pyruvate dehydrogenase to the active sites of dihydrolipoyl transacetylase, and then of dihydrolipoyl dehydrogenase.

A deeper look on thiamine pyrophosphate

Thiamine pyrophosphate molecule consists of three chemical moieties, from which its chemistry and enzymology depend: a thiazolium ring, a 4-aminopyrimidine ring, and the diphosphate side chain (see fig. 3).
The diphosphate side chain binds the cofactor to the enzyme via the formation of electrostatic bonds between the negative charges carried by its phosphoryl groups and the positive charges carried by Ca2+ and Mg2+ ions, in turn, bound to highly conserved sequences, GlyAspGly (GDG) and GlyAspGly-X26-AsnAsn (GDG-X26-NN), respectively.
The thiazolium ring plays a central role in catalysis, due to its ability to form the C-2 carbanion, that is, a nucleophilic center on the C-2 atom.
Note: as mentioned previously in this article, once bound to the enzyme, thiamine pyrophosphate locates in the active site so that the thiazolium ring is positioned close to the channel entrance leading to the active site.
The aminopyrimidine ring has a dual function:

  • it anchors the coenzyme holding it in place;
  • it has a specific catalytic role, participating in acid/base catalysis, as evidenced by studies with thiamine pyrophosphate analogs in which each of the three nitrogen atoms of the ring were replaced in turn. These studies demonstrated that the N-1’ atom and the N-4’-amino group are required, whereas the other nitrogen atom of the ring, the N-3’ atom, is required to a lesser extent.

How is the dipolar carbanion of thiamine pyrophosphate formed?

Three tautomeric forms of the aminopyrimidine ring can be identified in the enzyme-bound coenzyme not involved in the reaction:

  • the canonical 4’-aminopyrimidine tautomer;
  • the N-1 protonated form, that is, 4-aminopyrimidinium ion;
  • the 1’,4’-iminopyrimidine tautomer.

It seems that the 1′,4′-imino tautomer is the tautomer that undergoes deprotonation, before the entry of the substrate into the active site. The C-2 of the thiazolium ring is “much more acidic than most =C-H groups found in other molecules” (see also in the article on the pentose phosphate pathway). The higher acidity, i.e., the fact that the C-2 proton is easily dissociable, is due to the presence of the quaternary nitrogen on the thiazolium ring, a positively charged nitrogen atom able to electrostatically stabilize the resulting carbanion. In the deprotonation reaction, the amino group of the aminopyrimidine ring seems to play an essential role: it acts as a base and is suitably positioned to accept the proton. However, in the 4’-aminopyrimidine tautomer one of its protons sterically collides with the C-2 proton; in addition, its pK is too low to carry out the deprotonation efficiently. A mechanism was therefore proposed, in which the side chain of a conserved glutamate residue, for example βGlu59 in B. stearothermophilus, or Glu51 in Saccharomyces uvarum (brewer’s yeast) pyruvate decarboxylase, donates a proton to the aminopyrimidine, converting it to its 1′,4′-iminopyrimidine tautomer that, accepting the C-2 proton, returns to the canonical 4′-aminopyrimidinic form and allows the formation of the carbanion.

Note: carbanion formation on C-2 is a consequence of an intramolecular proton transfer.

Deprotonation of thiamine pyrophosphate and closure of pyruvate dehydrogenase active site

The loss of the C-2 proton of the thiazolium ring leads, from a positively charged ring, to a dipolar ion, or zwitterion. This change in state of charge triggers a conformational change in one of the two conserved loops at the entrance of the active site channel, specifically, the inner of these loops, that, in turn, leads to the closure of the channel to the surrounding water environment. In this closed conformation the thiazolium carbanion is protected against electrophiles.
To sum up: the deprotonation of thiamine pyrophosphate leads to the closure of the active site and the protection of the newly formed dipolar carbanion, that is, TPP-dependent enzymes would be only active in closed conformation.
Conversely, in the other active site, thiamine pyrophosphate is not in the ylid form, the channel is open, and the site is inactive.

Reaction of dihydrolipoyl transacetylase or E2

In the reaction sequence catalyzed by components of the pyruvate dehydrogenase complex, dihydrolipoyl transacetylase catalyzes the third step, namely, the transfer of the acetyl group from acetyl-dihydrolipoamide to CoA to form acetyl-CoA and dihydrolipoamide, the fully reduced form of lipoamide, the dithiol.
It should be noted that the acetyl group, initially bound by ester linkage to one of the –SH group of lipoamide is next bound to the –SH group of coenzyme A, again by ester bond, hence the term transesterification.

Catalytic mechanism of dihydrolipoyl transacetylase or E2

During the reaction, the sulfhydryl group of coenzyme A carries out a nucleophilic attack on the carbonyl carbon of the acetyl group of acetyl dihydrolipoamide-dihydrolipoyl transacetylase to form a transient tetrahedral intermediate, that “decomposes” to dihydrolipoamide-dihydrolipoyl transacetylase and acetyl-CoA.

Catalytic mechanism of dihydrolipoyl transacetylase, one of the components of the pyruvate dehydrogenase complex
E2: Catalytic Mechanism

As previously said, the mobility of the lipoyllysyl arm plays a central role in the reaction mechanism.

Reaction of dihydrolipoyl dehydrogenase or E3

In the reaction sequence catalyzed by components of the pyruvate dehydrogenase complex, dihydrolipoyl dehydrogenase catalyzes the fourth and fifth steps.
The enzyme catalyzes electron transfers needed to regenerate the disulfide bridge of the lipoyl group of dihydrolipoyl transacetylase, that is, to regenerate the oxidized form of the prosthetic group, and thus completing the catalytic cycle of the transacetylase.
The reaction has a ping-pong catalytic mechanism: it occurs in two successive half-reaction, in which each of the two substrates, NAD+ and dihydrolipoamide, reacts in the absence of the other. Moreover during the first half-reaction, the release of the first product and the formation of an enzyme intermediate complex occur before the second substrate binds, while the enzyme underogoes a structural change, whereas in the second half-reaction the release of the second product and the return of the enzyme to its starting state, again, via a structural change, occur.
Considering the ping-pong kinetic mechanism of dihydrolipoyl dehydrogenase:

  • in the first half-reaction the oxidation of dihydrolipoamide to lipoamide occurs;
  • in the second half-reaction the reduction of NAD+ to NADH occurs.

Catalytic mechanism of dihydrolipoyl dehydrogenase or E3

Below, the reaction mechanism of P. putida dihydrolipoyl dehydrogenase is described.
In the first half-reaction, the oxidized dihydrolipoyl dehydrogenase (E), i.e., the enzyme with the disulfide bridge between Cys43 and Cys48, binds dihydrolipoamide (LH2) to form the enzyme-dihydrolipoamide complex (E●LH2). At this point, a sulfur atom of dihydrolipoamide carries out a nucleophilic attack on the sulfur of Cys43, to form the disulfide bridge lipoamide-Cys43 (E-S-S-L), while the sulfur of Cys48 is released as a thiolate ion (S48).
The proton on the second thiol group of lipoamide is then abstracted by histidine (Hys) 451, that acts as a general acid-base catalyst, leading to the formation of a second thiolate ion, this time on the lipoamide (E-S-S-L ●S), that, through a nucleophilic attack, displaces the sulfur of Cys43, S43, aided in this by general acid catalysis by Hys451 which donates a proton to S43. The catalytic action of Hys451 is essential, as demonstrated by mutagenesis studies in which its substitution with a glutamine residue causes the enzyme to retain ∼ 0.4% of the wild-type catalytic activity.
The thiolate anion S48 then contacts, through non-covalent interactions, the flavin ring near 4a position, i.e., an electron pair of S48, which acts as electron donor, is partially transferred to the oxidized flavin ring, which, in turn, is the electron acceptor. The resulting structure is called charge-transfer complex.
Meanwhile, the phenolic side chain of the Tyr181 continues to hinder access to the flavin ring, thus protecting it from oxidation by O2.

 Oxidation of Dihydrolipoamide by Dihydrolipoyl Dehydrogenase
Dihydrolipoamide Oxidation via E3

To sum up, what occurs is an interchange reaction of disulfide bridges leading to the formation of the oxidized form of lipoamide, the first product, which is released, and the reduced form of the dihydrolipoyl dehydrogenase.

The second half-reaction involves the reduction of NAD+ to NADH + H+ by electron transfer from the reactive disulfide of the enzyme via FAD.

Reduced dihydrolipoyl dehydrogenase (E3) is reoxidized by NAD+
Oxidation of Reduced E3

It begins with the entry of NAD+ into the active site and its binding to form the EH2●NAD+ complex. It should be noted that the entry of the coenzyme causes the phenolic side chain of the Tyr181 to be pushed aside by the nicotinamide ring.
Following the collapse of the charge-transfer complex, a covalent bond is formed between the flavin atom C-4a and S48, to which the extraction of a proton from S43 by the flavin atom N-5 is accompanied, with the formation of the corresponding thiolate anion, S43.
S43carries out a nucleophilic attack on S48, leading to the formation of the redox-active disulphide bridge between Cys43 and Cys48, followed by the breakdown of the covalent bond between S48 and the flavin atom C-4a to form reduced FADH anion, FADH, with negative charge on atom N-1. It should be noted that dihydrolipoyl dehydrogenase is in the oxidized form (E).
FADH has a transient existence because the proton bound to its N-5 is instantly transferred, as hydride ion, to the nicotinamide atom C-4, that is juxtaposed to flavin atom N-5. This leads to the formation of FAD and of the second product of the reaction, NADH, which is released.
To sum up, what occurs is that the electrons removed from the hydroxyethyl group, which derives from pyruvate, pass, via FAD, to NAD+. The catalytic cycle of dihydrolipoyl dehydrogenase is therefore completed, being the enzyme and its coenzymes in their oxidized form. At this point, the catalytic cycle of the entire pyruvate dehydrogenase complex is completed, too, and the complex is ready for a new reaction cycle.

Note: unlike the thiazolium ring of thiamine pyrophosphate, FAD does not acts as electron trap or electron sink, but rather as an electron conduit between the redox-active disulphide, in its reduced form, and NAD+.

Note: the catalytic mechanism of dihydrolipoyl dehydrogenase has been determined in analogy with that of glutathione reductase (EC 1.8.1.7), at 33% identical and whose structure is more extensively characterized. It should, however, be noted that although the two enzymes catalyze similar reactions, these usually occur in opposite direction:

  • dihydrolipoyl dehydrogenase uses NAD+ to oxidize two –SH groups to a disulfide (–S–S–);
  • glutathione reductase uses NADPH to reduce a –S–S– to two thiol groups.

Nevertheless, their active sites are closely superimposable.

Regulation of the pyruvate dehydrogenase complex

In mammals, the regulation of the activity of the pyruvate dehydrogenase complex is essential, both in the fed and fasted states. In fact, the multienzyme complex plays a central role in metabolism because, catalyzing the irreversible oxidative decarboxylation of pyruvate, represents the entry point of the carbon flux from all carbohydrate sources as well as from ∼50% of carbon skeletons of glucogenic amino acids, that, as a whole, correspond to ∼60% of the daily calorie intake, into:

  • the citric acid cycle, and therefore to the full oxidation to CO2;
  • the synthesis of lipids (fed state) and acetylcholine (see above).

The importance of the regulation of the conversion of pyruvate into acetyl-CoA is also underlined by the fact that mammals, although able to produce glucose from pyruvate, cannot synthesize it from acetyl-CoA, because of the irreversibility of pyruvate dehydrogenase reaction and the absence of alternative pathways. Then, the inhibition of the activity of the complex allows to spare glucose and the amino acids that can be converted into pyruvate, such as alanine, when other fuels, for example acetyl-CoA from fatty acid oxidation, are available.
This explains why the activity of the complex is carefully regulated by:

  • feedback inhibition;
  • nucleotides;
  • covalent modifications, namely, phosphorylation and dephosphorylation of specific target proteins.

Regulation of the pyruvate dehydrogenase complex by feedback inhibition and energy status of the cell

The activity of the dephosphorylated form of the pyruvate dehydrogenase complex is regulated by feedback inhibition.
Acetyl-CoA and NADH allosterically inhibit the enzymes that catalyze their formation, dihydrolipoyl transacetylase and dihydrolipoyl dehydrogenase, respectively.
In addition, CoA and acetyl-CoA, as well as NAD+ and NADH, compete for binding sites on E2 and E3, respectively, that catalyze reversible reactions. This means that, in the presence of high ratios of [Acetyl-CoA]/[CoA] and [NADH]/ [NAD+], the reactions of transacetylation and dehydrogenation work in reverse; therefore, dihydrolipoyl transacetylase cannot accept the hydroxyethyl group from TPP because it is maintained in the acetylated form. This cause thiamine pyrophosphate to remain bound to pyruvate dehydrogenase in its hydroxyethyl form, which, in turn, decreases the rate of pyruvate decarboxylation. Hence, high ratios of [Acetyl-CoA]/[CoA] and [NADH]/[NAD+] indirectly influence pyruvate dehydrogenase activity.

Regulation of pyruvate dehydrogenase complex activity by feedback inhibition
PDC Activiy: Regulation by Feedback Inhibition

Acetyl-CoA and NADH are also produced by fatty acid oxidation, which takes place, like the reactions of the pyruvate dehydrogenase complex, within the mitochondrion. This means that the cell, by regulating the activity of the multienzyme complex, preserves carbohydrate stores when fatty acids are available for energy. For example, during the fasted state, liver, skeletal muscle and many other organs and tissues rely primarily on fatty acid oxidation for energy. Conversely, the activity of the multienzyme complex is increased in the fed state, when many different types of cells and tissues mainly use glucose as a fuel.
More generally, when the production of NADH and/or acetil-CoA exceeds the capacity of the cell to use them for ATP production, the activity of the pyruvate dehydrogenase complex is inhibited. The same is true when there is no need for additional ATP to be produced. Infact, the activity of multienzyme complex is also sensitive to the energy charge of the cell. Through allosteric mechanisms, high ATP levels inhibit the activity of the pyruvate dehydrogenase component of the complex, whereas high ADP levels, that signs that the energy charge of the cell may become low, activate it, thus committing the carbon skeleton of carbohydrates and some amino acids to energy production.

Note: in the skeletal muscle, the activity of the pyruvate dehydrogenase complex increases with increased aerobic activity, resulting in a in greater dependence on glucose as a fuel source.

Regulation of the pyruvate dehydrogenase complex by phosphorylation/dephosphorylation

Unlike prokaryotes, in mammals the activity of the pyruvate dehydrogenase complex is also regulated by covalent modifications, i.e., phosphorylation and dephosphorylation of three specific serine residues of the α subunit of pyruvate dehydrogenase, the enzyme that catalyzes the first, irreversible step of the overall reaction sequence.
Note: as mammalian pyruvate dehydrogenase is an heterotetramer, there are six potential phosphorylation sites.

Regulation of pyruvate dehydrogenase complex activity by covalent modifications
PDC Activity: Regulation by Covalent Modifications

Phosphorylation, which inactivates pyruvate dehydrogenase, and then blocks the overall reaction sequence, is catalyzed by pyruvate dehydrogenase kinase. Two of the aforementioned serine residues are located on the more C-terminal loop, at the entrance of the substrate channel leading to the respective active site, and the phosphorylation of only one of them inactivates the pyruvate dehydrogenase, hence demonstrating the out of phase coupling between its active sites.
Conversely, in the dephosphorylated state the complex is active. Dephosphorylation is catalyzed by a specific protein phosphatase, the pyruvate dehydrogenase phosphatase.
The activities of pyruvate dehydrogenase kinase and pyruvate dehydrogenase phosphatase are in turn subject to allosteric regulation by several modulators.

Regulation of pyruvate dehydrogenase kinase

The activity of pyruvate dehydrogenase kinase depends on the ratios of [NADH]/[NAD+], [acetyl-CoA]/[CoA], and [ATP]/[ADP] , as well as on the pyruvate concentration, in the mitochondrial matrix (see fig. 19).

  • High ratios of [NADH]/[NAD+] and [acetyl-CoA]/[CoA], as during the oxidation of fatty acids and ketone bodies, activate the kinase, pyruvate dehydrogenase is phosphorylated, and the pyruvate dehydrogenase complex is inhibited. This allows tissues, such as cardiac muscle, to preserve glucose when fatty acids and/or ketone bodies are utilized for energy, because acetyl-CoA synthesis from pyruvate, and hence from carbohydrates (and some amino acids) is turned off.
    Conversely, when the concentrations of NAD+ and coenzyme A are high the activity of the kinase is inhibited and the multienzyme complex is active.
    Therefore, acetyl-CoA and NADH, two of the three end products of the reactions catalyzed by the pyruvate dehydrogenase complex, allosterically control their synthesis by regulating directly and indirectly, by regulating the activity of pyruvate dehydrogenase kinase, the activity of the complex.
  • A high ratio of [ATP]/[ADP] activates the kinase, and then inhibits the pyruvate dehydrogenase complex.
    Note: unlike many other kinases, such as those involved in the control of glycogen metabolism, pyruvate dehydrogenase kinase is not regulated by cAMP levels, but by molecules that signal changes in energy status of the cell and in the availability of biosynthetic intermediates: ATP and NADH, and acetyl-CoA, respectively.
  • Pyruvate allosterically inhibits pyruvate dehydrogenase kinase.
    When its levels are high, it binds to kinase and inactivates it, pyruvate dehydrogenase is not phosphorylated, and the pyruvate dehydrogenase complex remains active.
  • Pyruvate dehydrogenase kinase is also activated by interaction with dihydrolipoyl transacetylase in its acetylated form, i.e. when acetyl-dihydrolipoamide is present.

Other activators of the kinase is potassium and magnesium ions.

Regulation of pyruvate dehydrogenase phosphatase

The activity of pyruvate dehydrogenase phosphatase depends on the ratios of [NADH]/[NAD+] and [acetyl-CoA]/[CoA], as well as on [Ca2+], in the mitochondrial matrix (see fig. 19).

  • Low ratios of [NADH]/[NAD+] and [acetyl-CoA]/[CoA] activate the phosphatase, pyruvate dehydrogenase is dephosphorylated, and the pyruvate dehydrogenase complex is activated.
    Conversely, when the aforesaid ratios are high, phosphatase activity is reduced, kinase activity is increased, and the multienzyme complex is inhibited.
  • Calcium ion activates pyruvate dehydrogenase phosphatase.
    Ca2+ is an important second messenger that signals the cell requires more energy. Therefore, when its levels are high, as in cardiac muscle cells after epinephrine stimulation or in skeletal muscle cells during the muscular contraction, the phosphatase is active, the complex is dephosphorylated, and then active.
  • Insulin, too, is involved in the control of the activity of the pyruvate dehydrogenase complex through the activation of pyruvate dehydrogenase phosphatase. The hormone, in response to increases in blood glucose, stimulates glycogen synthesis and the synthesis of acetyl-CoA, a precursor in the synthesis of lipids.

Fasting and subsequent refeeding, too, affect the activity of the multienzyme complex.
In tissues such as skeletal muscle, cardiac muscle or kidney, fasting significantly decreases the activity of the complex, whereas refeeding reverses the inhibition of fasting.
In the brain, however, these variations are not observed because the activity of pyruvate dehydrogenase complex is essential for ATP production.

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The pentose phosphate pathway

The pentose phosphate pathway, also called the phosphogluconate pathway, is a metabolic pathway, common to all living organisms, for the oxidation of glucose alternative to glycolysis, from which it branches downstream of glucose 6-phosphate synthesis, and whose main functions are the production, in variable ratios, of NADPH, a reduced coenzyme, and ribose 5-phosphate, a five-carbon phosphorylated sugar, namely, a pentose phosphate, hence the name pentose phosphate pathway.

Steps of the pentose phosphate pathway, involved enzymes, intermediates, and products
Pentose Phosphate Pathway

In addition to the production of NADPH and ribose 5-phosphate, this pathway has other functions, both anabolic and catabolic.

  • In yeasts and many bacteria it is involved in the catabolism of the five carbon sugars ribose, xylose and arabinose.
    In humans too, it is involved in catabolism of the aforementioned pentoses and of the less common sugars with three, four and seven carbon atoms derived from diet, as well as of:

pentoses derived from the catabolism of structural carbohydrates;
ribose 5-phosphate derived from nucleotide catabolism.

  • In photosynthetic organisms it contributes to carbon dioxide (CO2) fixation during the Calvin cycle.
  • In addition to ribose 5-phosphate, it also provides other intermediates for various biosynthetic processes, such as:

erythrose 4-phosphate, used for the synthesis of phenylalanine, tryptophan, and tyrosine, the three aromatic amino acids;
ribulose 5-phosphate, used for riboflavin synthesis;
sedoheptulose 7-phosphate which, in Gram-negative bacteria, is used for the synthesis of heptose units in the lipopolysaccharide layer of the outer membrane.

The phosphogluconate pathway, branching from glycolysis, is also called the hexose monophosphate shunt.

It has been estimated that more than 10% of glucose is shuttled through this metabolic pathway that, noteworthy, although it oxidizes the monosaccharide, does not involve any direct production or consumption of ATP.

CONTENTS

Elucidation of the pentose phosphate pathway

The first evidence of the existence of the phosphogluconate pathway was obtained in the 1930s by the studies of Otto Warburg, Nobel Prize in Physiology or Medicine in 1931, who discovered NADP during studies on the oxidation of glucose 6-phosphate to 6-phosphogluconate.
Further indications came from the observation that glucose continued to be metabolized in tissues even in the presence of glycolysis inhibitors, such as fluoride and iodoacetate ions, inhibitors of enolase (EC 4.2.1.11) and glyceraldehyde 3-phosphate dehydrogenase (EC 1.2.1.12), respectively.
However, the pathway was fully elucidated only in 1950s thanks to the work of several researchers and primarily of Efraim Racker, Fritz Lipmann, Nobel Prize in Physiology or Medicine in 1953 thanks to the discovery of coenzyme A, Bernard Horecker and Frank Dickens.

Functions of NADPH and ribose 5-phosphate

NADPH is needed for reductive biosynthesis, such as the synthesis of fatty acids, cholesterol, steroid hormones and of two non-essential amino acids, proline and tyrosine, from glutamate and phenylalanine, respectively, as well as for the reduction of oxidized glutathione. In such reactions the reduced coenzyme acts as an electron donor, or rather as a donor of a hydride ion (:H), namely, a proton and two electrons.

Skeletal formula of the reduced and oxidized form of nicotinamide adenine dinucleotide phosphate or NADPH
Reduced and Oxidized Form of Nicotinamide Adenine Dinucleotide Phosphate

Note: In vertebrates, about half of the NADPH necessary for the reductive steps of fatty acid synthesis derives from the pentose phosphate pathway, and the rest from the malic enzyme (EC 1.1.1.40) reaction.

Malate + NADP+ ↔ Pyruvate + NADPH + H+ + HCO3

Ribose 5-phosphate is used for the synthesis of nucleotides and nucleic acids, DNA and RNA, of ATP, coenzymes such as coenzyme A, NAD, NADP and FAD, and of the essential amino acids tryptophan and histidine.
Ribose 5-phosphate is not used as such; it is activated to  5-phosphoribosyl 1-pyrophosphate (PRPP), in the reaction catalyzed by ribose phosphate pyrophosphokinase or PRPP synthase (EC 2.7.6.1).

Ribose 5-phosphate + ATP → 5-Phosphoribosyl 1-pyrophosphate + AMP

Where does the pentose phosphate pathway occur?

In animal cells and bacteria, the hexose monophosphate shunt, as well as glycolysis, fatty acid synthesis, and most of the reactions of gluconeogenesis, occurs in the cytosol. And, considering glycolysis, gluconeogenesis and the pentose phosphate pathway we can state that these three metabolic pathways are interconnected through several shared enzymes and/or intermediates.
In plant cells the phosphogluconate pathway occurs in plastids, and its intermediates can reach the cytosol through membrane pores of these organelles.

In humans, the level of expression of the enzymes of the pathway varies widely from tissue to tissue.
Relatively high levels are found in the liver, adrenal cortex, testicles and ovaries, thyroid, mammary glands during lactation, and in red blood cells. In all these sites, constant supply of NADPH is required to support reductive biosynthesis and/or to counteract the effects of reactive oxygen species (ROS) on sensitive cellular structures, such as DNA, membrane lipids, and proteins by the reduction of oxidized glutathione (GSSG) to reduced glutathione (GSH), in the reaction catalyzed by glutathione reductase (EC 1.8.1.7).

GSSG + NADPH + H+ → 2 GSH + NADP+

Note: Glutathione is a tripeptide, namely, γ-glutamyl-cysteinyl-glycine, that in the reduced state contains, in the cysteine residue, a sulfhydryl group (-SH), hence the abbreviation GSH. It is the major intracellular antioxidant in erythrocytes, as in most other cells.

The defense against ROS effects is particularly important in cells such as red blood cells and the cells of the cornea and crystalline lens that are directly exposed to oxygen.
High levels of the of the phosphogluconate pathway enzymes  are also present in rapidly dividing cells such as enterocytes, skin cells, bone marrow cells, those of the early embryo and, in pathological conditions, cancer cells. Indeed, these cell types require a constant supply of ribose 5-phosphate for nucleic acid synthesis.
Conversely, these enzymes are present in very low levels in skeletal muscle, in which the pentose phosphate pathway is virtually absent and glucose 6-phosphate is primarily used for energy production via glycolysis and the citric acid cycle.

The two phases of the pentose phosphate pathway

Conceptually, the hexose monophosphate shunt can be viewed as consisting of two phases.

  • In the first phase, the oxidative phase, glucose 6-phosphate, a six-carbon phosphorylated sugar, is converted to ribulose 5-phosphate, a five-carbon phosphorylated sugar, with the concomitant formation of two molecules of NADPH and the release of C-1 of glucose as CO2.
  • In the second phase, the nonoxidative phase, several phosphorylated carbohydrates are produced, whose fate depends on the relative needs for NADPH, ribose 5-phosphate, and ATP of the cell.

Steps of the oxidative phase of the pentose phosphate pathway

The oxidative phase of the phosphogluconate pathway consists of three steps, two irreversible oxidations, the first and third reactions, and a hydrolysis.

Oxidative phase of the pentose phosphate pathway
Oxidative Phase of the Pentose Phosphate Pathway

Below, the reaction mechanisms of the involved enzymes are explained and, with regard to glucose 6-phosphate dehydrogenase or G6PD (EC 1.1.1.49), the regulation of the enzymatic activity too.

Oxidation of glucose 6-phosphate to 6-phosphoglucono-δ-lactone

In the first step of the oxidative phase, glucose 6-phosphate dehydrogenase catalyzes the oxidation of glucose 6-phosphate to 6-phosphoglucono-δ-lactone, an intramolecular ester, via the transfer of a hydride ion from carbon 1of glucose 6-phosphate to NADP+, that acts as oxidizing agent.

Glucose 6-phosphate + NADP+ → 6-Phosphoglucono-δ-lactone + NADPH + H+

Note: This reaction yields the first molecule of NADPH of the pentose phosphate pathway.
The reaction catalyzed by glucose 6-phosphate dehydrogenase is unique to the pathway. And, similarly to what happens in most metabolic pathways, also in this case the first reaction unique to  the pathway, generally known as a committed step, is an essentially irreversible step, with a ΔG in the liver of -17.6 kJ/mol (-4.21 kcal/mol), and is highly allosterically regulated.  And the enzyme is indeed the major control point for the flow of metabolites through the pathway.
In humans, the highest levels of G6PD are found in neutrophils and macrophages, phagocytic cells in which, during inflammation, NADPH is used for to produce superoxide radicals (O2-.) from molecular oxygen in the reaction catalyzed by NADPH oxidase (EC 1.6.3.1).

2 O2 + NADPH → 2 O2-. + NADP+ + H+

In turn, superoxide radicals can be used for the synthesis for defensive purposes, namely, to kill phagocytized microorganisms, of other ROS but also of reactive nitrogen species (RNS), such as:

  • hydrogen peroxide (H2O2), in the reaction catalyzed by superoxide dismutase or SOD (EC 1.15.1.1)

2 O2-. + 2 H+ → H2O2 + O2

  • peroxynitrite (O=N–O–O), in the reaction with nitric oxide (•NO)

O2. + •NO → O=N–O–O

  • hydroperoxide radical (HOO•)

O2-. + H+ → HOO•

Catalytic mechanism  of glucose 6-phosphate dehydrogenase

The catalytic mechanism of the enzyme has been studied in great detail in the microorganism Leuconostoc mesenteroides, whose glucose 6-phosphate dehydrogenase has the peculiar characteristic of being able to use NAD+ and/or NADP+ as coenzyme.

Catalytic mechanism of glucose 6-phosphate dehydrogenase, enzyme of the pentose phosphate pathway
Catalytic Mechanism of Glucose 6-Phosphate Dehydrogenas

The enzyme does not require metal ions for its activity; one of the amino acids in the active site acts as a general base being able to abstract a hydride ion from the hydroxyl group bound to C1 of glucose 6-phosphate.
In the bacterial enzyme this is carried out by the atom Nɛ2 of the imidazole ring of a histidine side chain. This nitrogen atom has a lone pair of electrons able to make a nucleophilic attack. This causes glucose 6-phosphate, a cyclic hemiacetal with carbon 1 in the aldehyde oxidation state, to be oxidized to a cyclic ester, namely, a lactone. This allows the transfer of an hydride ion from C1 of glucose to C4 of the nicotinamide ring of NADP+ to form NADPH.
Because such histidine is conserved in many of the glucose 6-phosphate dehydrogenases sequenced, it is likely that this catalytic mechanism can be generalized to all glucose 6-phosphate dehydrogenases.

Regulation of glucose 6-phosphate dehydrogenase activity

Glucose 6-phosphate dehydrogenase is the major control point of carbon flow through the pentose phosphate pathway, and then the major control point for the rate of NADPH synthesis.
In humans, the enzyme exists in two forms: the inactive monomeric form, and the active form that exists in a dimer-tetramer equilibrium.
One of the main modulators of its activity is the cytosolic NADP+/NADPH ratio. High levels of NADPH inhibit enzyme activity, because NADPH is a potent competitive inhibitor of G6PD, whereas NADP+ is required for the catalytic activity and for the maintenance of the active conformation. In fact, the binding of the oxidized coenzyme to a specific site close to the dimer interface, but distant from the active site, is required to maintain its dimeric conformation.

Regulation of glucose 6-phosphata dehydrogenase activity
Regulation of G6PD Activity

Under most metabolic conditions the NADP+/NADPH ratio is low, less NADP+ is available to bind to the enzyme, and hence enzyme activity is reduced, regardless of gene expression levels. Under these conditions the oxidative phase is virtually inactive.
Conversely, in cells in which metabolic pathways and/or reactions using NADPH are particularly active, the reduction of cytosolic NADPH concentration, and hence the increase in NADP+ concentration occurs. This leads to an increase in glucose 6-phosphate dehydrogenase activity, and to the activation of the oxidative phase of the hexose monophosphate pathway.
Therefore it is possible to state that the fate of glucose 6-phosphate, an intermediate common to both glycolysis and the phosphogluconate pathway, also depends on the current needs for NADPH.
A second mechanism for the regulation of glucose 6-phosphate dehydrogenase activity calls into question the accumulation of acyl-CoAs, intermediates in fatty acid synthesis. These molecules, by binding to the dimeric form of the enzyme, lead to dissociation into the constitutive monomers, and then to the loss of the catalytic activity.
Insulin up-regulates the expression of the genes for glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase. Therefore, in the well-fed state, the hormone increases carbon flow through the pentose phosphate pathway and then the production of NADPH.
Note: Insulin also promotes the synthesis of fatty acids.

Hydrolysis of 6-phosphoglucono-δ-lactone to 6-phosphogluconate

In the second step of the oxidative phase 6-phosphoglucono-δ-lactone is hydrolyzed to 6-phosphogluconate, a linear molecule.
6-Phosphoglucono-δ-lactone is hydrolytically unstable and undergoes a nonenzymatic ring-opening, a reaction that occurs at a significant rate. However, in the cell this ring-opening reaction, an hydrolysis, is accelerated by the catalytic action of 6-phosphogluconolactonase (EC 3.1.1.31).

6-Phosphoglucono-δ-lactone + H2O → 6-Phosphogluconate + H+

Oxidative decarboxylation of 6-phosphogluconate to ribulose 5-phosphate

In the third step of the oxidative phase, 6-phosphogluconate undergoes an oxidative decarboxylation to form ribulose 5-phosphate, a keto pentose, CO2, and a molecule of NADPH. The reaction is catalyzed by 6-phosphogluconate dehydrogenase (EC 1.1.1.44), enzyme that requires the presence of magnesium ions, Mg2+.

 6-Phosphogluconate + NADP+ → Ribulose 5-phosphate + NADPH + CO2

Note: This reaction yields the second molecule of NADPH of the pentose phosphate pathway.

Catalytic mechanism of 6-phosphogluconate dehydrogenase

The catalytic mechanism of the enzyme is similar to that of isocitrate dehydrogenase (EC 1.1.1.41), an enzyme of the citric acid cycle. It consists of an acid-base catalysis proceeding through a three step mechanism in which two strictly conserved residues, a lysine (Lys), and a glutamate (Glu), are involved; in humans, Lys185 and the Glu192. Lysine acts as acid/base group, whereas glutamate as an acid.

Catalytic mechanism of 6-phosphogluconate dehydrogenase, enzyme of the pentose phosphate pathway
Catalytic Mechanism of 6-Phosphogluconate Dehydrogenase

In the first step, the oxidative step, 6-phosphogluconate is oxidized to a β-keto acid, the 3-keto-6-phosphogluconate.
In this step the ε-amino group of the aforementioned lysine acts as a general base, as a nucleophile, abstracting a proton from the hydroxyl group bound to C-3. Then, the transfer of a hydride ion from C-3 to C-4 of the nicotinamide ring of the NADP+occurs. This leads to the formation of the 3-keto intermediate and a molecule of NADPH that leaves the active site.
In the second step, the decarboxylation step, 3-keto-6-phosphogluconate, that is very susceptible to decarboxylation, is converted to the cis-1,2-enediol of ribulose 5-phosphate, a high energy intermediate. In this step the aforementioned lysine acts as a general acid donating an H+ at the C-3 carbonyl oxygen, and the C-1 of glucose 6-phosphate is lost as CO2.
Finally,  6-phosphogluconate dehydrogenase catalyzes a stereospecific keto-enol conversion leading to the formation of ribulose 5-phosphate. In this step, the aforementioned glutamic acid residue acts as a general acid donating an H+ to the C-1 of cis-1,2-enediol intermediate, while the ε-amino group of the lysine accepts a proton from the hydroxyl group bound to the C-2. The result is the formation of ribulose 5-phosphate.

Note: An enediol is an organic compound containing two carbon atoms linked by a double bond and an hydroxyl group (-OH) bound to both carbon atoms. The enediol can have cis or trans configuration. For example, in the plant world many polyphenols possess enediol structures.

Therefore, the oxidative phase of the pentose phosphate pathway ends with the production of ribulose 5-phosphate, namely, the substrate for the reactions of the non-oxidative phase.
The overall equation of the oxidative phase is:

3 Glucose 6-phosphate + 6 NADP+ + H2O → 6 NADPH + 6 H+ + 3 CO2 + 3 Ribulose 5-phosphate

Steps of the nonoxidative phase of the pentose phosphate pathway

The nonoxidative phase of the pathway consists of five steps, all freely reversible, in which a series of interconversions of phosphorylated sugars occurs.
This phase begins with two reactions: the isomerization and epimerization of ribulose 5-phosphate to form ribose 5-phosphate and xylulose 5-phosphate, respectively.

Note: Enzymatic isomerizations and epimerizations play an important role in carbohydrate metabolism.
Epimerases (EC 5.1), a subclass of Isomerases (EC 5.), catalyze the configurational reversal at an asymmetric carbon atom, usually by a deprotonation/protonation mechanism.
In isomerization reactions, the interchange of chemical groups occurs between carbon atoms.

Isomerization of ribulose 5-phosphate to ribose 5-phosphate

In the isomerization reaction, ribulose 5-phosphate, a ketose, is converted to the corresponding aldose, ribose 5-phosphate. This reaction is catalyzed by phosphopentose isomerase or ribose 5-phosphate isomerase (EC 5.3.1.6).

Ribulose 5-phosphate ⇄ Ribose 5-phosphate

Catalytic mechanism of phosphopentose isomerase or ribose 5-phosphate isomerase

The catalytic mechanism of the enzyme is similar to that of phosphohexose isomerase (EC 5.3.1.9), an enzyme of the glycolytic pathway, and leads to the formation of the high energy intermediate cis-1,2-enediol of ribulose 5-phosphate. The formation of the cis-1,2-enediol intermediate occurs via a proton-transfer mechanism common to the aldose-ketose isomerizations.
The proposed catalytic mechanism for phosphopentose isomerase from E. coli, in the direction of ribulose 5-phosphate formation from ribose 5-phosphate, as in the Calvin cycle of photosynthesis, is described below.

Catalytic mechanism of phosphopentose isomerase, enzyme of the pentose phosphate pathway
Catalytic Mechanism of Phosphopentose Isomerase

In the first step, the furanose ring of the substrate is opened, opening induced by the interaction with an aspartic acid residue (Asp81) that accepts a proton from the hydroxyl group bound to C-1, whereas it is likely that water is the proton donor.
Note: The opening of the furanose ring is quite rare in solution (<0.5%).
Once the chain is opened, a glutamic acid residue (Glu103) acts as a general base, as a nucleophile, abstracting a proton bound to the C-2, whereas Asp81 donates a proton. As a result, cis-1,2-enediol intermediate is produced.
Finally, the protonated Glu103 acts as a general acid and donates an H+ at C-1 of the cis-1,2-enediol intermediate, while Asp81 acts as a general base accepting a proton from the hydroxyl group bound to C-2. The result is the formation of ribulose 5-phosphate.

During the synthesis of ribose 5-phosphate from ribulose 5-phosphate phosphopentose isomerase works in reverse.

Epimerization of ribulose 5-phosphate to xylulose 5-phosphate

The other metabolic fate of ribulose 5-phosphate in the pentose phosphate pathway is to be epimerized to xylulose 5-phosphate, a ketose like ribulose 5-phosphate, in the reaction catalyzed by phosphopentose epimerase (EC 5.1.3.1).

Ribulose 5-Phosphate ⇄ Xylulose 5-Phosphate

Note: Xylulose 5-phosphate is a regulatory molecule that inhibits gluconeogenesis and stimulates glycolysis by controlling the levels of fructose 2,6-bisphosphate in the liver.

Catalytic mechanism of phosphopentose epimerase

Also this reaction, like those catalyzed by 6-phosphogluconate dehydrogenase and ribose 5-phosphate isomerase, proceeds through the formation of an enediol intermediate, but with the double bond between C-2 and C-3 and not between C-1 and C-2.
During the reaction an amino acid residue present in the active site of the enzyme acts as a general base, as a nucleophile, and abstracts a proton bound to the C-3, leading to the formation of the cis-2,3-enediol intermediate. Then, an acidic amino acid residue donates a proton to C-3, but from the opposite side, hence, with an inversion at C-3 to form xylulose 5-phosphate.

Catalytic mechanism of phosphopentose epimerase, enzyme of the pentose phosphate pathway
Catalytic Mechanism of Phosphopentose Epimerase

To this point, the hexose monophosphate shunt has generated for each molecule of glucose 6-phosphate metabolized:

  • a pool of three pentose 5-phosphates, namely, ribulose 5-phosphate, ribose 5-phosphate and xylulose 5-phosphate, that coexist at equilibrium;
  • 2 molecules of NADPH.

In the following three steps, from the sixth to the eighth, transketolase (EC 2.2.1.1) and transaldolase (EC 2.2.1.2), two enzymes unique to the pentose phosphate pathway, catalyze a series of rearrangements of the carbon skeletons leading to the formation of three-, four-, six-, and seven carbon units, that can be used for various metabolic purposes, depending on the needs of the cell.
Analyzing the flow of metabolites through the different metabolic pathways, the concerted action of transketolase and transaldolase allows the interaction of the pentose phosphate pathway, in particular of its non-oxidative phase, with glycolysis, and gluconeogenesis, as well as with the pathways leading to the formation of numerous vitamins, coenzymes and nucleic acid precursors.

Transketolase: step 6 and 8

Transketolase is the rate-limiting enzyme of the non-oxidative phase of the pentose phosphate pathway, and the first enzyme that acts downstream of ribose 5-phosphate isomerase and phosphopentose epimerase.
Discovered independently in 1953 by Horecker and Racker, and named by Racker, it catalyzes in the sixth and eighth steps, the transfer of a two carbon unit from a ketose, the donor substrate, namely, xylulose 5-phosphate, sedoheptulose 7-phosphate or fructose 6-phosphate, to an aldose, the acceptor substrate, ribose 5-phosphate, glyceraldehyde 3-phosphate or erythrose 4-phosphate.

The general reaction, and the step 6 and 8 of the pentose phosphate pathway catalyzed by transketolase
Reactions Catalyzed by Transketolase

Taking as an example the forward reactions, in the sixth step, the ketose donor is xylulose 5-phosphate, whereas the aldose acceptor is ribose 5-phosphate, to form glyceraldehyde 3-phosphate, the remaining three-carbon fragment from xylulose 5-phosphate, and sedoheptulose 7-phosphate, a seven-carbon sugar that will be used in the next step, the seventh.
In the eighth step, the ketose donor is xylulose 5-phosphate, whereas the aldose acceptor is erythrose 4-phosphate, to form another glyceraldehyde 3-phosphate and a fructose 6-phosphate.

It should be noted that three of the four products of the reactions catalyzed by this enzyme, two molecules of glyceraldehyde 3-phosphate and one of fructose 6-phosphate, are also intermediates of glycolysis.

Transketolase and thiamine pyrophosphate

Transketolase is an enzyme that requires thiamine pyrophosphate (TPP) as a cofactor.
Thiamine pyrophosphate is the biologically active form of thiamin or vitamin B1, and is tightly bound to the enzyme.

Skeletal formula of thiamine pyrophosphate, the active form of vitamin B1
Thiamine Pyrophosphate

Other enzymes that require TPP as a cofactor are:

  • pyruvate decarboxylase (EC 4.1.1.1), that is involved in alcoholic fermentation;
  • pyruvate dehydrogenase or E1 (EC 1.2.4.1) of the pyruvate dehydrogenase complex;
  • alpha-keto acid dehydrogenase or E1 component (EC 1.2.4.4) of the branched-chain alpha-ketoacid dehydrogenase complex;
  • alpha-ketoglutarate dehydrogenase or E1 component (EC 1.2.4.2) of the alpha-ketoglutarate dehydrogenase complex, an enzyme of the citric acid cycle.

Thiamine pyrophosphate is involved in the transfer of activated aldehyde intermediates by stabilizing the two-carbon carbanion formed during the reaction.

Catalytic mechanism of transketolase

The carbon atom between the sulfur and nitrogen atoms of the thiazolium ring of thiamine pyrophosphate, namely, the C-2 atom, is much more acidic than most =CH groups found in other molecules because of adjacent positively charged nitrogen atom that electrostatically stabilizes the carbanion resulting from dissociation of the proton. This causes the C-2 proton to be easily dissociable to form a carbanion, i.e. a carbon atom with a negative charge. Such proton abstraction is catalyzed by transketolase.
The carbanion attacks the carbonyl carbon of the substrate, in the step 6, xylulose 5-phosphate or, in the reverse reaction, sedoheptulose 7-phosphate, whereas in the step 8, xylulose 5-phosphate or, in the reverse reaction, fructose 6-phosphate.
Taking as an example the forward reaction of step 6, the covalent adduct between thiamine pyrophosphate and xylulose 5-phosphate undergoes fragmentation, via the cleavage of the C2-C3 bond of xylulose 5-phosphate, to form glyceraldehyde 3-phosphate, that is released, and a two carbon unit, a negatively charged hydroxyethyl group, that remains bound to C-2 of the thiazolium ring.

Catalytic mechanism of transketolase, enzyme of the pentose phosphate pathway
Catalytic Mechanism of Transketolase

The negative charge on the hydroxyethyl intermediate, that is, the carbanion intermediate, is stabilized by the thiazolium ring of thiamine pyrophosphate because of the positively charged nitrogen atom that acts as an electron trap or electron sink. Therefore, thiazolium ring provides an electron deficient or electrophilic structure that can delocalize by resonance the carbanion electrons.
Then, the condensation occurs between the hydroxyethyl group and the ribose 5-phosphate, the acceptor aldehyde substrate, via carbanion attack on the aldehyde carbon of ribose 5-phosphate, to form a covalent adduct bound to thiamine pyrophosphate.
Finally, the cleavage of the adduct leads to the release of sedoheptulose 7-phosphate, and regenerates the TPP carbanion.

Note: In addition to xylulose 5-phosphate, sedoheptulose 7-phosphate and fructose 6-phosphate, transketolase can use as substrates other 2-keto sugars in a similar way, as well as a variety of different aldose phosphates.

Carbanions and carbocations

A carbanion is a species containing a negatively charged, trivalent carbon.
It is an highly reactive reaction intermediate, resulting from the heterolytic cleavage of a bond between a carbon atom and another atom or group.
The carbanions, having an unshared electron pair, are strong nucleophiles and bases, and attack a proton or an electrophilic center, like a polarized or positively charged center, to form a covalent bond. Due to their reactivity, and with few exceptions, they are transient intermediates in organic reactions, like free radicals and carbocations.
A carbocation is a species containing a positively charged, trivalent carbon. Like free radicals, carbocations are species characterized by an electron deficiency, having not eight but only six electrons in their valence shell. Free radicals have seven electrons in their valence shell. Because of this electronic deficiency, free radicals and carbocations are strong electrophiles, and, like carbanions, are highly reactive reaction intermediates. During the reactions they accept electrons, one the free radicals, two the carbocations, to achieve the stable octet configuration.

The cleavage of a covalent bond between two carbon atom, and more generally between atom A and B, can take place via two different mechanisms: homolytic or heterolytic cleavage.

  • In homolytic bond cleavage, each atom takes one of the two electrons holding the atoms together to form two species with an odd number of electrons, namely, with one unpaired electron, without charge, called free radicals.
  • In heterolytic bond cleavage, two charged species, namely, a cation and an anion, are produced, because of one atom retains both bonding electrons.
Homolytic and heterolytic cleavage of a covalent bond
Homolysis and Heterolysis

Note: Heterolytic cleavage is more common than homolytic cleavage.

Transaldolase: step 7

Discovered in 1953 by Horecker and Smyrniotis in the brewer’s yeast, assigned to the species Saccharomyces cerevisiae, it catalyzes, in the seventh step of the pentose phosphate pathway, the transfer of a three carbon unit from a donor substrate, sedoheptulose 7-phosphate, to an acceptor substrate, glyceraldehyde 3-phosphate, to form fructose 6-phosphate and erythrose 4-phosphate.

Sedoheptulose 7-phosphate + Glyceraldehyde 3-phosphate ⇄ Fructose 6-phosphate + Erythrose 4-phosphate

Note: Like in transketolase catalyzed reactions, the carbon unit donor is a ketose while the acceptor is an aldose.
In the reverse reaction, the donor substrate is fructose 6-phosphate, while the acceptor substrate  is erythrose 4-phosphate.

Catalytic mechanism of transaldolase

Unlike transketolase, transaldolase does not require a cofactor for activity.
The reaction occurs in two step, an aldol cleavage and an aldol condensation.  Below, the catalytic mechanism of E. coli transaldolase B is analyzed, taking as an example the forward reaction leading to erythrose 4-phosphate and fructose 6-phosphate synthesis.
In the first step an ε-amino group of a lysine residue (Lys132) in the active site, after a proton transfer to a glutamic acid residue (Glu96) mediated by a water molecule, performs a nucleophilic attack on the carbonyl carbon of sedoheptulose 7-phosphate, that is, on the C-2 atom. The result is the formation of a carbinolamine with sedoheptulose 7-phosphate.

Catalytic mechanism of transaldolase, enzyme of the pentose phosphate pathway
Catalytic Mechanism of Transaldolase

In the second step, the removal of a water molecule from carbinolamine leads to the formation of an enzyme-bound imine or Schiff base intermediate; this step, too, involves the transfer of a proton from Glu96 to the “catalytic” water molecule.
Note: This enzyme-substrate covalent intermediate is quite similar to that formed in the reaction catalyzed by aldolase (EC 4.1.2.13) in the fourth step of glycolysis.
In the next step, the carboxylic group of an aspartic acid residue (Asp17) extracts a proton from the hydroxyl group bound to C-4, leading to the cleavage of the C–C bond between C-3 and C-4. This reaction is an aldol cleavage and releases the first product, erythrose 4-phosphate, an aldose, whereas a three-carbon carbanion remains bound to the enzyme and is stabilized by resonance, like in transketolase catalyzed reactions. In fact, like the nitrogen atom in the thiazolium ring of thiamine pyrophosphate, the nitrogen atom with a positive charge of the Schiff base acts as an electron trap stabilizing the negative charge carried by the carbanion.
Once the acceptor substrate glyceraldehyde 3-phosphate is in the active site, the carbanion performs a nucleophilic attack on the carbonyl carbon of glyceraldehyde 3-phosphate to form, by aldol condensation, a new C–C bond and an enzyme-bound ketose.
Then, the hydrolysis of the Schiff base releases fructose 6-phosphate, a ketose and the second product of the reaction. At this point, a new reaction cycle can start.

Finally, as seen previously, in the eighth step of the pentose phosphate pathway, transketolase catalyzes the synthesis of fructose 6-phosphate and glyceraldehyde 3-phosphate from erythrose 4-phosphate and xylulose 5-phosphate.

Regulation of the pentose phosphate pathway

Glucose 6-phosphate is a metabolite that can enter glycolysis or the pentose phosphate pathway depending on the cell’s need for ATP, NADPH and ribose 5-phosphate.
In case of increased need for ATP, glucose 6-phosphate is mostly channelled into glycolysis.
Conversely, if the need for NADPH and/or ribose 5-phosphate increases, most of the phosphorylated sugar is channeled into the pentose phosphate pathway.
From the molecular point of view, the fate of glucose 6-phosphate depends, to a large extent, on the relative activities of the enzymes that metabolize it in glycolysis, namely, phosphofructokinase 1 (PFK-1) (EC 2.7.1.11), and in the hexose monophosphate shunt, namely, glucose 6-phosphate dehydrogenase, activities that are highly regulated.

Note: In the glycolytic pathway, glucose 6-phosphate is a substrate of phosphohexose isomerase that catalyzes the reversible isomerization to fructose 6-phosphate, which, in turn, is a  substrate of phosphofructokinase 1.

PFK-1 is inhibited when ATP and/or citrate concentrations increase, namely, when the energy charge of the cell is high, whereas it is activated when AMP and/or fructose 2,6-bisphosphate concentrations increase, namely, when the energy charge of the cell is low. Thus, when the energy charge of the cell is high, the carbon flow, and therefore the flow of glucose 6-phosphate through the glycolytic pathway decreases.
Glucose 6-phosphate dehydrogenase is inhibited by NADPH and acyl-CoAs, intermediates in fatty acid biosynthesis. Thus, when the cytosolic levels of NADPH increases, the flow of glucose 6-phosphate through the pentose phosphate pathway is inhibited, whereas if NADPH levels drop, the inhibition disappears, the pathway switches on again, and NADPH and ribose 5-phosphate are synthesized.

However, even when glucose 6-phosphate dehydrogenase is active, the cell is still able to respond to the relative needs of NADPH, ribose 5-phosphate and ATP, regulating accordingly the carbon flow through the phosphogluconate pathway. And, depending on the cell’s need for ATP, NADPH and ribose 5-phosphate, some reactions of glycolysis, gluconeogenesis, and the pentose phosphate pathway can be combined in novel ways to emphasize the synthesis of needed metabolites, also exploiting the fact that the non-oxidative phase  of the hexose monophosphate shunt is essentially controlled by the availability of the substrates.
The four principal possibilities are described below.

The need for NADPH is much greater than that for ribose 5-phosphate and ATP

When much more NADPH than ribose 5-phosphate is needed, and there is no need for additional ATP to be produced, namely, the energy charge of the cell is high, glucose 6-phosphate enters the pentose phosphate pathway and is completely oxidized to CO2. Such metabolic conditions are found, for example, in the adipose tissue during fatty acid synthesis.
In the oxidative phase of the pathway, two molecules of NADPH are produced for each molecule of glucose 6-phosphate oxidized to ribulose 5-phosphate. Through a combination of the reactions of the non-oxidative phase and of some reactions of gluconeogenesis, namely, those catalyzed by triose phosphate isomerase (EC 5.3.1.1), aldolase (EC 4.1.2.13), phosphohexose isomerase (EC 5.3.1.9), and fructose 1,6-bisphosphatase (EC 3.1.3.11), six molecules of ribulose 5-phosphate are converted into five molecules of glucose 6-phosphate. Thus, it is possible to state that the reactions of the non-oxidative phase allow the reactions of the oxidative phase to proceed.
Three groups of reactions can be identify.

  • In the first group there are the reactions catalyzed by the enzymes of the oxidative phase, leading to the formation of two molecules of NADPH and one molecule of ribulose 5-phosphate.

6 Glucose 6-phosphate + 12 NADP+ + 6 H20 → 6 Ribulose 5-phosphate + 6 CO2 + 12 NADPH + 12 H+

  • In the second group there are the reactions catalyzed by the enzymes phosphopentose epimerase, ribose 5-phosphate isomerase, transketolase and transaldolase, namely, those of the non-oxidative phase of the pathway, that lead to the conversion of ribulose 5-phosphate to fructose 6-phosphate and glyceraldehyde 3-phosphate.

6 Ribulose 5-phosphate → 4 Fructose 6-phosphate + 2 Glyceraldehyde 3-phosphate

  • Finally, fructose 6-phosphate and glyceraldehyde 3-phosphate can be recycled to glucose 6-phosphate via some reactions of gluconeogenesis, so that the cycle can begin again.

4 Fructose 6-phosphate + 2 Glyceraldehyde 3-phosphate + H2O → 5 Glucose 6-phosphate + Pi

The sum of the last two reactions shows that six molecules of ribulose 5-phosphate are converted to five molecules of glucose 6-phosphate.

6 Ribulose 5-phosphate+ H2O → 5 Glucose 6-phosphate + Pi

The sum of the reactions of the first, second and third group gives the overall reaction:

Glucose 6-phosphate + 12 NADP+ + 7 H20 → 6 CO2 + 12 NADPH + 12 H+ + Pi

Therefore, one molecule of glucose 6-phosphate, via six cycles of the pentose phosphate pathway coupled with some reactions of gluconeogenesis, is converted to six molecules of CO2, with the concomitant production of 12 molecules of NADPH, and without net production of ribose-5-phosphate.

The need for NADPH and ATP is much greater than that for ribose 5-phosphate

When much more NADPH than ribose 5-phosphate is needed, and the energy charge of the cell is low, that is, there is a need for ATP, ribulose 5-phosphate formed in the oxidative phase is converted to fructose 6-phosphate and glyceraldehyde 3-phosphate through the reactions of the non-oxidative phase. These two intermediates, through the reactions of glycolysis, are oxidized to pyruvate with concomitant ATP production.
The net reaction is:

3 Glucose 6-phosphate + 6 NADP+ + 5 NAD+ + 5 Pi + 8 ADP → 5 Pyruvate + 3 CO2 + 6 NADPH + 5 NADH + 8 ATP + 2 H2O + 8 H+

If the cell requires more ATP, the pyruvate produced can be oxidized through the citric acid cycle.
Conversely, if there is no need for additional ATP to be produced, the carbon skeleton of pyruvate can be used as a building block in several biosynthetic pathways.

Note: As in the previous case, there is no net production of ribose 5-phosphate.

The need for ribose 5-phosphate is much greater than that for NADPH

When much more ribose 5-phosphate than NADPH is needed, as in rapidly dividing cells in which there is a high rate of synthesis of nucleotides, precursors of DNA, the reactions of the oxidative phase of the pentose phosphate pathway are bypassed, and there is no synthesis of NADPH. Conversely, because the reactions of the non-oxidative phase are easily reversible, the drop in ribose 5-phosphate levels, due to its rapid use, stimulates its synthesis.
What happens is that, through the glycolytic pathway, most of the glucose 6-phosphate is converted to fructose 6-phosphate and glyceraldehyde 3-phosphate. Then, transaldolase and transketolase lead to the synthesis of ribose 5-phosphate and xylulose 5-phosphate. Xylulose 5-phosphate, through the reactions catalyzed by phosphopentose epimerase and ribose 5-phosphate isomerase, is converted to ribose 5-phosphate.
The net reaction is:

6 Glucose 6-phosphate + ATP → 6 Ribose 5-phosphate + ADP + H+

Under this metabolic conditions therefore, what happens is an interplay between reactions of glycolysis and of the non-oxidative phase of the phosphogluconate pathway, with the latter in the direction of ribose 5-phosphate synthesis.
It should be noted no metabolites return to glycolysis.

The needs for ribose 5-phosphate and NADPH are balanced

If one molecule of ribose 5-phosphate and two molecules of NADPH per molecule of glucose 6-phosphate metabolized satisfy the metabolic needs of the cell, the reactions that predominate are those of the oxidative phase and that catalyzed by ribose 5-phosphate isomerase.
The net reaction is:

Glucose 6-phosphate + 2 NADP+ + H2O → Ribose 5-phosphate + 2 NADPH + 2 H+ + CO2

Under this metabolic conditions, too, no metabolites return to glycolysis.

References

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Cosgrove M.S., Naylor C., Paludan S., Adams M.J., and Richard Levy H. On the mechanism of the reaction catalyzed by glucose 6-phosphate dehydrogenase. Biochemistry 1998;37(9);2759-2767. doi:10.1021/bi972069y

Garrett R.H., Grisham C.M. Biochemistry. 4th Edition. Brooks/Cole, Cengage Learning, 2010

Hanau S., Montin K., Cervellati C., Magnani M., and Dallocchio F. 6-Phosphogluconate dehydrogenase mechanism: evidence for allosteric modulation by substrate. J Biol Chem 2010;285(28):21366-21371. doi:10.1074/jbc.M110.105601

Harvey R.A., Ferrier D.R. Lippincott’s illustrated reviews: biochemistry. 5th Edition. Lippincott Williams & Wilkins, 2011

Horecker B.L. The pentose phosphate pathway. J Biol Chem 2002;277(50):47965-47971. doi:10.1074/jbc.X200007200

Jelakovic S., Kopriva S., Süss K-H and Schulz G.E. Structure and catalytic mechanism of the cytosolic D-ribulose-5-phosphate 3-epimerase from rice. J Mol Biol 2003;326:127-135. doi:10.1016/S0022-2836(02)01374-8

Michal G., Schomburg D. Biochemical pathways. An atlas of biochemistry and molecular biology. 2nd Edition. John Wiley J. & Sons, Inc. 2012

Nelson D.L., M. M. Cox M.M. Lehninger. Principles of biochemistry. 6th Edition. W.H. Freeman and Company, 2012

Patra K.C. and Hay N. The pentose phosphate pathway and cancer. Trends Biochem Sci 2014;39(8):347-354. doi:10.1016/j.tibs.2014.06.005

Rosenthal M.D., Glew R.H. Medical Biochemistry – Human Metabolism in Health and Disease. John Wiley J. & Sons, Inc., 2009

Kochetov G.A., Solovjeva O.N. Structure and functioning mechanism of transketolase. Biochim Biophys Acta 2014;1844(9):1608-1618. doi:10.1016/j.bbapap.2014.06.003

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Voet D. and Voet J.D. Biochemistry. 4th Edition. John Wiley J. & Sons, Inc. 2011

Wang J. and Yang W. Concerted proton transfer mechanism of Clostridium thermocellum ribose-5-phosphate isomerase. J Phys Chem B 2013;117:9354-9361. doi:10.1021/jp404948c

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The glycolytic pathway: steps, products, and regulation

Glycolysis, from Greek word glykys, meaning “sweet”, and lysis, meaning “dissolution or breakdown”, can be defined as the sequence of enzymatic reactions that, in the cytosol, also in the absence of oxygen, leads to the conversion of one molecule of glucose, a six carbon sugar, to two molecules of pyruvate, a three carbon compound, with the concomitant production of two molecules of ATP, the universal energy currency in biological systems.

Steps of glycolysis, involved enzymes, and intermediates
The Glycolytic Pathway

Glycolysis, which evolved before a substantial amount of oxygen had accumulated in the atmosphere, is the metabolic pathway with the largest flux of carbon in most living cells, and is present in almost all organisms.
This pathway, not requiring oxygen, played a crucial role in metabolic processes during the first 2 billion years of evolution of life, and probably represents the most ancient biological mechanism for extracting energy from organic molecules when oxygen availability is low. Moreover, it is a source of precursors for aerobic catabolism and for various biosynthetic processes.
Note: glycolysis is also known as the Embden-Meyerhof pathway, named after Gustav Embden and Otto Meyerhof, the two researchers who elucidated the entire pathway in the muscle.

CONTENTS

Glycolysis: the first metabolic pathway to be elucidated

The development of biochemistry has gone hand in hand with the elucidation of glucose metabolism, especially glycolysis, the first metabolic pathway to have been elucidated.
Though the elucidation of this metabolic pathway was worked out in the ‘40 of the last century, the key discovery about glucose metabolism was made in 1897, quite by accident, following a problem arose a year earlier, when a German chemist, M. Hahn, in attempting to obtain and preserve cell-free protein extracts of yeast, encountered difficulties in its conservation. A colleague, Hans Buchner, remembering a method for preserving jams, suggested to add sucrose to the extract.
Eduard Buchner, Hans’s brother, put the idea of Hans into practice, and observed that the solution produced bubbles. This prompted Eduard to conclude that a fermentation was occurring, a quite surprising discovery. Indeed fermentation, according to Pasteur’s assertion in 1860, was inextricably tied to living cells, whereas it was now demonstrated that it could also occur outside them. Briefly, these two researchers refuted the vitalist dogma and had a pivotal role in starting modern biochemistry.
Eduard Buchner was awarded the Nobel Prize in Chemistry in 1907 for this research, and was the first of several researchers who won the award for their discoveries concerning the glycolytic pathway.
It was later demonstrated, working with muscle extracts, that many of the reactions of lactic fermentation  were the same of those of alcoholic fermentation , thus revealing the underlying unity in biochemistry.
As previously mentioned, glycolysis was then fully elucidated in the first half of the last century largely due to the work of researchers such as Gerty and Carl Cori, Carl Neuberg, Arthur Harden, William Young, Jacob Parnas, Otto Warburg, Hans von Euler-Chelpin, Gustav Embden and Otto Meyerhof. In particular, Warburg and von Euler-Chelpin elucidated the whole pathway in yeast, and Embden and Meyerhof in muscle in the 30’s.

Why is glycolysis so important?

Glycolysis is essential to most living cells both from the energy point of view and as a source of precursors for many other metabolic pathways. And the rate of carbon flow through glycolysis, namely, the amount of glucose converted to pyruvate per unit time, is regulated to meet these two basic needs for the cell.
From the energetic point of view, although glycolysis is a relatively inefficient pathway, it can occur in the absence of oxygen, the condition in which life evolved on Earth and that many contemporary cells, both eukaryotic and prokaryotic, experience. Here are some examples.

  • In most animals, muscles exhibit an activity-dependent anaerobiosis, namely, they can work anaerobically for short periods. For example, when animals, but also athletes, perform high intense exercises, their need for ATP exceeds body’s ability to supply oxygen to the muscle. In such situation, muscles function, albeit for a short period of time, anaerobically.
  • Another example is the cornea of the eye, a poorly vascularized tissue.
  • Many microorganisms live in environments where oxygen is low or absent, such as deep water, soil, but also skin pores. And a variety of microorganisms called obligate anaerobes cannot survive in the presence of oxygen, a highly reactive molecule. Examples are Clostridium perfringens, Clostridium tetani, and Clostridium botulinum, that cause gangrene, tetanus and botulism, respectively.

It should also be underlined that glycolysis also plays a key role in those cells and tissues in which glucose is the sole source of energy, such as:

  • red blood cells, lacking mitochondria,
  • sperm cells;
  • the brain, which can also use ketone bodies for fuel in times of low glucose;
  • the adrenal medulla.

A similar situation is also found in the plant world where many aquatic plants and some plant tissues specialized in starch accumulation, such as potato tubers, use glucose as the main source of energy.

Note: There are organisms that are facultative anaerobes, namely organisms that can survive in the presence and in the absence of oxygen, acting aerobically or anaerobically, respectively. Examples are animals belonging to the genus Mytilus, which display an habitat-dependent anaerobiosis, a condition similar to the activity-dependent anaerobiosis seen in muscle.

Finally, it should not be forgotten that under aerobic conditions, in cells with mitochondria, glycolysis constitutes the upper part of the metabolic pathway leading to the complete oxidation of glucose to carbon dioxide (CO2) and water for energy purposes.

Glycolysis as a source of building blocks for biosynthesis
Glycolysis as a Source of Building Blocks

Some glycolytic intermediates, for example glucose 6-phosphate (G-6-P), fructose 6-phosphate (F-6-P) or dihydroxyacetone phosphate (DHAP), may be used as building blocks in several metabolic pathways, such as those leading to the synthesis of glycogen, fatty acids, triglycerides, nucleotides, of some amino acids, or 2,3-bisphosphoglycerate (2,3-BPG).

The steps of glycolysis

The 10 steps that make up glycolysis can be divided into two phases.
The first, called the preparatory phase, consists of 5 steps and starts with the conversion of glucose to fructose 1,6-bisphosphate (F-1,6-BP) through three enzymatic reactions, namely, a phosphorylation at C-1, an isomerization, and a second phosphorylation, this time at C-6, with consumption of 2 ATP. Fructose 1,6-bisphosphate is then cleaved into two phosphorylated three-carbon compounds, glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. Finally, the isomerization of DHAP to a second molecule of glyceraldehyde-3- phosphate occurs. In the preparatory phase therefore a glucose is split into two molecules of glyceraldehyde 3-phosphate, and two ATP are consumed.
In the second phase, called the payoff phase, consisting of the remaining 5 steps of the pathway, the two molecules of glyceraldehyde 3-phosphate are converted into two molecules of pyruvate, with the concomitant production of 4 ATP. So, in this phase, part of the energy present in the chemical bonds of glucose is extracted and conserved in the form of ATP. Furthermore, reducing equivalents are extracted and conserved in the form of the reduced coenzyme NADH. The metabolic fate of NADH will depend on the cell type and aerobic or anaerobic conditions.

Note: Glucose metabolized in the glycolytic pathway derives both from glucose that enters the cell through specific membrane transporters, that in turn derives from the bloodstream, and glucose 6-phosphate produced by glycogen degradation.

Reaction 1: glucose phosphorylation to glucose 6-phosphate

In the first step of the glycolytic pathway glucose is phosphorylated to glucose 6-phosphate at the expense of one ATP.

Glucose + ATP → Glucose 6-phosphate + ADP + H+

In most cells this reaction is catalyzed by hexokinase (EC 2.7.1.1), enzyme present in the cells of all organisms, and in humans with four isozyme).
Hexokinase and pyruvate kinase, the other kinase of the glycolysis, like many other kinases, require the presence of magnesium ion, Mg2+, or of another bivalent metal ion such as manganese, Mn2+, for their activity. Mg2+ binds to the ATP to form the complex MgATP2-, and in fact the true substrate of the enzyme is not ATP but this complex. It should be emphasized that the nucleophilic attack by a hydroxyl group (-OH) of glucose at the terminal phosphorus atom of the ATP is facilitated by the action of Mg2+ that interacts with the negative charges of the phosphoryl groups of the nucleoside triphosphate.
The formation of the phosphoester bond between a phosphoryl group and the hydroxyl group at C-6 of glucose is thermodynamically unfavorable and requires energy to proceed, energy that is provided by the ATP. Indeed, while the phosphorylation of glucose at C-6 by inorganic phosphate has a ΔG°’ of 13.8 kJ/mol (3.3 kcal/mol), namely, it is an endergonic reaction, the hydrolysis of ATP to ADP and Pi has ΔG°’ of -30.5 kJ/mol (-7.3 kcal/mol), namely, it is an exergonic reaction. The net reaction has a ΔG°’ of (-30.5 + 13.8) = -16.7 kJ/mol (-7.3 + 3.3 = -4.0 kcal/mol). Under cellular conditions the reaction is even more favorable, with a ΔG equal to -33.5 kJ/mol (-8.0 kcal/mol).
Therefore, this is an essentially irreversible reaction.

Note: In biochemistry, phosphorylations are fundamental reactions catalyzed by enzymes called kinases, a subclass of transferases. Kinases catalyze the transfer of the terminal phosphoryl group, or γ-phosphoryl group, of a nucleoside triphosphate to an acceptor nucleophile to form a phosphoester bond. Specifically, hexokinase catalyzes the transfer of the γ-phosphoryl group of ATP to a variety of hexoses, that is, sugars with six carbons, such as fructose and mannose), in addition to glucose.

The importance of glucose phosphorylation

The phosphorylation of the glucose has some functions.

  • Glucose 6-phosphate, due to its negative charge and because there are no transporters for phosphorylated sugars in the plasma membrane, cannot diffuse out of the cell. Thus, after the initial phosphorylation, no further energy is needed to keep the phosphorylated molecule within the cell, despite the large difference between its intra- and extracellular concentrations.
    Similar considerations are valid for each of the eight phosphorylated intermediates between glucose 6-phosphate and pyruvate.
  • The rapid phosphorylation of glucose maintains a low intracellular concentration of the hexose, thus favoring its facilitated diffusion into the cell.
  • Phosphorylation causes an increase in the energy content of the molecule, that is, it starts to destabilize it, thus facilitating its further metabolism.

Other possible fates of glucose 6-phosphate

Glucose 6-phosphate is a key metabolite of glucose metabolism. In fact, in addition to be metabolized in the glycolytic pathway, in anabolic conditions it can have other fates (see Fig. 3). Here are some examples.

  • It can be used in the synthesis of:

glycogen, a polysaccharide stored mainly in the liver and muscle;
complex polysaccharides present in the extracellular matrix;
galactose;
glucosamine and other sugars used for protein glycosylation.

NADPH, needed for reductive biosynthesis, such as fatty acid, cholesterol, steroid hormone, and deoxyribonucleotide biosynthesis, and for preventing oxidative damage in cells such as erythrocytes;
ribose 5-phosphate, used in nucleotide synthesis but also in NADH, FADH2 and coenzyme A synthesis.

Reaction 2: isomerization of glucose 6-phosphate to fructose 6-phosphate

In the second step of the glycolytic pathway, the isomerization of glucose 6-phosphate, an aldose, to fructose 6-phosphate, a ketose, occurs. This reaction is catalyzed by phosphoglucose isomerase, also known as phosphohexose isomerase or glucose phosphate isomerase (EC 5.3.1.9).

Glucose 6-phosphate ⇄ Fructose 6-phosphate

Like hexokinase, phosphoglucose isomerase requires the presence of Mg2+.
The ΔG°’ of the reaction is 1.7 kJ/mol (0.4 kcal/mol), while the ΔG is -2.5 kJ/mol (-0.6 kcal/mol). These small values indicate that the reaction is close to equilibrium and is easily reversible.
The reaction essentially consists in the shift of the carbonyl group at C-1 of the open-chain form of glucose 6-phosphate to C-2 of the open-chain form of fructose 6-phosphate.

The Reaction Catalyzed by Phosphoglucose Isomerase
Phosphoglucose Isomerase Reaction

The enzymatic reaction can be divided at least into three steps. Since in aqueous solution both hexoses are primarily present in the cyclic form, the enzyme must first open the ring of G-6P, catalyze the isomerization, and, finally, the formation of the five-membered ring of F-6-P.
This isomerization is a critical step for glycolytic pathway, as it prepares the molecule for the subsequent two steps.
Why?

  • The phosphorylation that occurs in the third step requires the presence of an alcohol group at C-1, and not of a carbonyl group.
  • In the fourth step, the covalent bond between C-3 and C-4 is cleaved, and this reaction is facilitated by the presence of the carbonyl group at C-2.

Reaction 3: phosphorylation of fructose 6-phosphate to fructose 1,6-bisphosphate

In the third step of the glycolytic pathway, a second phosphorylation reaction occurs. Phosphofructokinase 1 or PFK-1 (EC 2.7.1.11) catalyzes the phosphorylation of fructose 6-phosphate at C-1 to form fructose 1,6-bisphosphate, at the expense of one ATP.

Fructose 6-phosphate + ATP → Fructose 1,6-bisphosphate + ADP + H+

PFK-1 is so named to distinguish it from phosphofructokinase 2 or PFK-2 (EC 2.7.1.105), the enzyme that catalyzes the phosphorylation of fructose 6-phosphate to fructose 2,6-bisphosphate.
Like the reaction catalyzed by hexokinase/glucokinase, this phosphorylation, too, is an essentially irreversible step, irreversibility, once again, achieved by coupling, by phosphofructokinase 1, with the hydrolysis of ATP. In fact, phosphorylation of fructose 6-phosphate by inorganic phosphate is endergonic, with a ΔG°’ of 16.3 kJ/mol (3.9 kcal/mol), whereas, when the reaction is coupled to the hydrolysis of ATP, the overall equation becomes exergonic, with a ΔG°’ of -14.2 kJ/mol (-3.4 kcal/mol) and a ΔG of -22.2 kJ/mol (-5.3 kcal/mol).
While hexokinase allows to trap glucose inside the cell, phosphofructokinase 1 prevents glucose to be used for glycogen synthesis or the production of other sugars, but is instead metabolized in the glycolytic pathway. In fact, unlike glucose 6-phosphate, fructose 1,6-bisphosphate cannot be used directly in other metabolic pathways than glycolysis/gluconeogenesis, that is, phosphofructokinase 1 catalyzes the first “committed” step of the glycolytic pathway. Such reactions are usually catalyzed by enzymes regulated allosterically, that prevent the accumulation of both intermediates and final products. PFK-1 is no exception, being subject to allosteric regulation by positive and negative effectors that signal the energy level and the hormonal status of the organism.
Some protists and bacteria, and perhaps all plants, have a phosphofructokinase that uses pyrophosphate (PPi) as a donor of the phosphoryl group in the synthesis of F-1,6-BP. This reaction has a ΔG°’ of -2.9 kJ/mol (-12.1 kcal/mol).

Fructose 6-phosphate + PPi → Fructose 1,6-bisphosphate + Pi

Note: The prefix bis– in bisphosphate, as fructose 1,6-bisphosphate, indicates that there are two phosphoryl groups are bonded to different atoms.
The prefix di– in diphosphate, as in adenosine diphosphate, indicates that there are two phosphoryl groups connected by an anhydride bond to form a pyrophosphoryl group, namely, they are directly bonded to one another.
Similar rules also apply to the nomenclature of molecules that have three phosphoryl groups standing apart, such as inositol 1,4,5-trisphosphate, or connected by anhydride bonds, such as ATP or guanosine triphosphate or GTP.

Reaction 4: cleavage of fructose 1,6-bisphosphate into two three-carbon fragments

In the fourth step of the glycolytic pathway, fructose 1,6-bisphosphate aldolase, often called simply aldolase (EC 4.1.2.13), catalyzes the reversible cleavage of fructose 1,6-bisphosphate into glyceraldehyde 3-phosphate, an aldose, and dihydroxyacetone phosphate, a ketose. The enzyme cleaves the bond between C-3 and C-4.

Fructose 1,6-bisphosphate ⇄ Dihydroxyacetone phosphate + Glyceraldehyde 3-phosphate

All glycolytic intermediates downstream to this reaction are three-carbon molecules, instead of six-carbon molecules as the previous ones.
The ΔG°’ of the reaction in the direction of glyceraldehyde 3-phosphate and dihydroxyacetone phosphate production is of 23.8 kJ/mol (5.7 kcal/mol), and the Km is approximately 10-4 M, values that would indicate that the reaction does not proceed as written from left to right. However, under normal cellular conditions, due to the lower concentrations of the reactants, the ΔG is -1.3 kJ/mol (-0.3 kcal/mol), a very small value, thus the reaction is easily reversible, that is, essentially to equilibrium.

Note: The name “aldolase” derives from the nature of the reverse reaction, from right to left as written, that is, an aldol condensation.

Reaction 5: interconversion of dihydroxyacetone phosphate and glyceraldehyde 3-phosphate

Of the two products of the previous reaction, glyceraldehyde 3-phosphate goes directly into the second phase of the glycolytic pathway. Conversely, DHAP is not on the direct pathway of glycolysis and must be converted, isomerized, to glyceraldehyde 3-phosphate to continue through the pathway. This isomerization is catalyzed by triose phosphate isomerase (EC 5.3.1.1).

Dihydroxyacetone phosphate ⇄ Glyceraldehyde 3-phosphate

Triose phosphate isomerase, in converting dihydroxyacetone phosphate into glyceraldehyde 3-phosphate, catalyzes the transfer of a hydrogen atom from C-1 to C-2, that is, catalyzes an intramolecular oxidation-reduction. And in essence, after the enzyme reaction, the carbons C-1, C-2 and C-3 of the starting glucose to become equivalent,  chemically indistinguishable, from the carbons C-6, C-5 and C-4, respectively.
Therefore, the net result of the the last two steps of glycolysis is the production of two molecules of glyceraldehyde 3-phosphate.
The ΔG°’ of the reaction is of 7.5 kJ/mol (1.8 kcal/mol), while the ΔG is 2.5 kJ/mol (0.6 kcal/mol). Although at equilibrium dihydroxyacetone phosphate represent about 96% of the trioso phosphates, the reaction proceeds readily towards the formation of glyceraldehyde 3-phosphate because of the subsequent step of the glycolytic pathway that removes the glyceraldehyde 3-phosphate produced.
One of the distinguishing features of triose phosphate isomerase is the great catalytic efficiency. The enzyme is in fact considered kinetically perfect. Why? The enzyme enhances the isomerization rate by a factor of 1010 compared with that obtained with a catalyst such as acetate ion. Indeed, the Kcat/KM ratio for the isomerization of glyceraldehyde 3-phosphate is equal to 2×108 M-1s-1, value close to the diffusion-controlled limit. Thus, the rate-limiting step in the reaction catalyzed by triose phosphate isomerase is diffusion-controlled encounter of enzyme and substrate.
From the energetic point of view, the last two steps of glycolysis are unfavorable, with ΔG°’ of 31.3 kJ/mol (7.5 kcal/mol), whereas the net ΔG°’ of the first five reactions is of 2.1 kJ/mol (0.5 kcal/mol), with a Keq of about 0.43. And it is the free energy derived from the hydrolysis two ATP that, under standard-state conditions, makes the value of the overall equilibrium constant close to one. If instead we consider ΔG, it is quite negative, -56.8 kJ/mol (-13.6 kcal/mol).

Notice that dihydroxyacetone phosphate may also be reduced to glycerol 3-phosphate (see Fig. 3) in the reaction catalyzed by cytosolic glycerol 3-phosphate dehydrogenase (EC 1.1.1.8).

Dihydroxyacetone phosphate + NADH + H+ ⇄ Glycerol 3-phosphate + NAD+

The enzyme acts as a bridge between glucose and lipid metabolism because the glycerol 3-phosphate produced is used in the synthesis of lipids such as triacylglycerols.
This reaction is an important sources of glycerol 3-phosphate in adipose tissue and small intestine.

Reaction 6: oxidation of glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate

In the sixth step of the glycolytic pathway, the first step of the second phase, the payoff phase, glyceraldehyde 3-phosphate dehydrogenase (EC 1.2.1.12) catalyses the oxidation of glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate (1,3-BPG), with the concomitant reduction of NAD+ to NADH.

Glyceraldehyde 3-phosphate + NAD+ + Pi ⇄ 1,3-Bisphosphoglycerate + NADH + H+

This is the first of the two glycolytic reactions in which the chemical energy needed for the subsequent synthesis of ATP is harvested and made available; the other reaction is catalyzed by enolase (EC 4.2.1.11). Why?
This reaction is the sum of two processes.

  • In the first reaction, the oxidation of the aldehyde group to a carboxyl group occurs, step in which NAD+ is used as oxidizing agent. The reaction is quite exergonic, with a ∆G’° of -43 kJ/mol (-10.3 kcal/mol).
  • In the second reaction, the formation of the bond between the carboxylic group at C-1 of 1,3-bisphosphoglycerate and orthophosphate occurs, to form an anhydride called acyl phosphate. The reaction is quite endergonic, with a ∆G’° of 49.3 kJ/mol (11.8 kcal/mol).

These two chemical processes must not take place in succession but must be coupled in order to allow the formation of the acyl phosphate because the oxidation of the aldehyde group is used to drive the formation of the anhydride, with an overall ΔG°’ of 6.3 kJ/mol (1.5 kcal/mol), and a ΔG of 2.5 kJ/mol (0.6 kcal/mol), both slightly endergonic.
Therefore, the free energy that might be released as heat is instead conserved by the formation of the acyl phosphate.

Note: The reversible reduction of the nicotinamide ring of NAD+ or NADP+ is due to the loss of two hydrogen atoms by another molecule, in this case the aldehyde group of glyceraldehyde 3-phosphate, that undergoes oxidation, and to the subsequent transfer of a hydride ion, the equivalent of two electrons and a proton, to the nicotinamide ring. The other proton removed from the substrate is released to the aqueous solution. Below, the half reactions for both coenzymes.

NAD+ + 2 e + 2 H+ → NADH + H+

NADP+ + 2 e + 2 H+ → NADH + H+

Reaction 7: phosphoglycerate kinase and the first ATP forming reaction

In the seventh step of the glycolytic pathway, phosphoglycerate kinase (EC 2.7.2.3) catalyzes the transfer of the high-energy phosphoryl group from the acyl phosphate of 1,3-BPG to ADP to form ATP and 3-phosphoglycerate (3-PG).

1,3-Bisphosphoglycerate + ADP + H+ ⇄ 3-Phosphoglycerate + ATP

The ΔG°’ of the reaction is of -18.5 kJ/mol (-4.4 kcal/mol), namely, it is an exergonic reaction. The ΔG is 1.3 kJ/mol (0.3 kcal/mol).
The high phosphoryl-transfer potential of the acyl phosphate is used to phosphorylate ADP. The production of ATP in this manner is called substrate-level phosphorylation. In other words, part of the energy released during the oxidation of the aldehyde group in the sixth step is now conserved by the synthesis of ATP from the ADP and Pi.
The reaction catalyzed by phosphoglycerate kinase is the first reaction of glycolysis in which part of the chemical energy present in glucose molecule is conserved as ATP. And, because the reactions catalyzed by aldolase and triose phosphate isomerase, step 4 and 5, respectively, lead to the formation of two molecules of glyceraldehyde 3-phosphate per molecule of glucose, in this step two ATP are produced and the ATP debt created by the preparatory phase, steps 1 and 3, respectively, is “paid off”.
It should be noted that the enzyme is named for the reverse reaction, from right to left as written, that is, the phosphorylation of 3-phosphoglycerate to form 1,3-bisphosphoglycerate at the expense of one ATP.
Indeed, this enzyme, like all other enzymes, is able to catalyze the reaction in both directions. And the direction leading to the synthesis of 1,3-bisphosphoglycerate occurs during the photosynthetic CO2 fixation and gluconeogenesis.

The sixth and seventh reactions of glycolysis, are, as a whole, an energy-coupling process in which the common intermediate is 1,3-bisphosphoglycerate. While the reaction leading to the synthesis of 1,3-BPG is endergonic, with a ΔG°’ of 6.3 kJ/mol (1.5 kcal/mol), the second reaction is strongly exergonic, with a ΔG°’ of -18.5 kJ/mol (-4,4 kcal/mol). The overall ΔG°’ is -12.2 kJ/mol (-2.9 kcal/mol), namely, the reaction catalyzed by phosphoglycerate kinase is sufficiently exergonic to pull even the previous one, too, making the overall reaction exergonic.

Glyceraldehyde 3-phosphate + ADP + Pi + NAD+ ⇄ 3-Phosphoglycerate + ATP + NADH + H+

In reality, phosphoglycerate kinase reaction is sufficiently exergonic to pull also the reactions catalyzed by aldolase and triose phosphate isomerase.

What is substrate-level phosphorylation?

Substrate-level phosphorylation is defined as the production of ATP by the transfer of a phosphoryl group from a substrate to ADP, a process involving chemical intermediates and soluble enzymes.
There is also a second type of phosphorylation for the synthesis of ATP called oxidative phosphorylation, a process involving not chemical intermediates and soluble enzymes but transmembrane proton gradients and membrane-bound enzymes.

Because the standard free energy of hydrolysis of the phosphoryl group of 3-phosphoglycerate is equal to 12.5 kJ/mol (-3 kcal/mol), it is not sufficient to produce ATP by phosphoryl group transfer. In the two subsequent reactions of glycolysis, 3-phosphoglycerate is converted to phosphoenolpyruvate (PEP), a molecule with a phosphoryl group transfer potential sufficiently elevated to allow the synthesis of ATP.

Reaction 8: from 3-phosphoglycerate to 2-phosphoglycerate

In the eighth step of the glycolytic pathway, 3-phosphoglycerate is converted into 2-phosphoglycerate (2-PG), in a reversible reaction catalyzed by phosphoglycerate mutase (EC 5.4.2.1). The reaction requires Mg2+, and has a very small ΔG, equal to about 0.8 kJ/mol (0.2 kcal/mol) and a ΔG°’ of 4.4 kJ/mol (1.1 kcal/mol).
Phosphoglycerate mutase is a mutase, enzymes that catalyze intramolecular group transfers, in this case the transfer of a phosphoryl group from C-3 to C-2 of the 3-phosphoglycerate. Mutases, in turn, are a subclass of isomerases.
The mechanism by which this reaction takes place depends on the type of organism studied. For example, in yeast or in rabbit muscle the reaction occurs in two steps and involves the formation of phosphoenzyme intermediates. In the first step, a phosphoryl group bound to a histidine residue in the active site of the enzyme is transferred to the hydroxyl group at C-2 of 3-PG to form 2,3-bisphosphoglycerate. In the next step, the enzyme acts as a phosphatase converting 2,3-BPG into 2-phosphoglycerate; however, the phosphoryl group at C-3 is not released but linked to the histidine residue of the active site to regenerate the intermediate enzyme-His-phosphate. Schematically:

Enzyme-His-phosphate + 3-Phosphoglycerate ⇄ Enzyme-His + 2,3-Phosphoglycerate

Enzyme-His + 2,3-Bisphosphoglycerate ⇄ Enzyme-His-phosphate + 2-Phosphoglycerate

Notice that the phosphoryl group of 2-phosphoglycerate is not the same as that of the substrate 3-phosphoglycerate.
Approximately once in every 100 catalytic cycles, 2,3-BPG dissociates from the active site of the enzyme, leaving it unphosphorylated, that is, in the inactive form. The inactive enzyme may be reactivated by binding 2,3-bisphosphoglycerate, which must, therefore, be present in the cytosol to ensure the maximal activity of the enzyme. And 2,3-BPG is present in small, but sufficient amounts in most cells, except for red blood cells, where it acts as an allosteric inhibitor, too, reducing  the affinity of hemoglobin for oxygen, and has a concentration of 4-5 mM.

Note: 3-Phosphoglycerate can also be used for the biosynthesis of serine, from which glycine and cysteine derive (see Fig. 3). The biosynthesis of serine begins with the reaction catalyzed by phosphoglycerate dehydrogenase (EC 1.1.1.95). The enzyme catalyzes the oxidation of 3-phosphoglycerate to 3-phosphohydroxypyruvate and the concomitant reduction of NAD+ to NADH. This reaction is also the rate-limiting step of this biosynthetic pathway, because serine inhibits the activity of the enzyme.

Synthesis of 2,3-bisphosphoglycerate and the Rapoport-Luebering pathway

1,3-Bisphosphoglycerate can be also converted into 2,3-bisphosphoglycerate (see Fig. 3).
In red blood cells this reaction is catalyzed by the bisphosphoglycerate mutase, one of the three isoforms of phosphoglycerate mutase found in mammals. The enzyme requires the presence of 3-phosphoglycerate as it catalyzes the intermolecular transfer of a phosphoryl group from C-1 of 1,3-bisphosphoglycerate to the C-2 of 3-phosphoglycerate. Therefore, 3-phosphoglycerate becomes 2,3-BPG, while 1,3-BPG is converted into 3-phosphoglycerate. The mutase enzyme activity has EC number 5.4.2.4.

Synthesis of 2,3-bisphosphoglycerate from 1,3-bisphosphoglycerate
Synthesis of 2,3-BPG

2,3-Bisphosphoglycerate can then be hydrolyzed to 3-phosphoglycerate in the reaction catalyzed by the phosphatase activity of bisphosphoglycerate mutase, that removes the phosphoryl group at C-2. This activity has EC number 3.1.3.13. The enzyme is also able to catalyze the interconversion of 2-phosphoglycerate and 3-phosphoglycerate, therefore, it is a trifunctional enzyme. 3-Phosphoglycerate can then re-enter the glycolytic pathway. This detour from glycolysis, also called Rapoport-Luebering pathway, that leads to the synthesis of 3-phosphoglycerate without any ATP production.
The other two isoforms of phosphoglycerate mutase, phosphoglycerate mutase 1 or type M, present in the muscle, and phosphoglycerate mutase 2 or type B, present in all other tissues, are able to catalyze, in addition to the interconversion of the 2-phosphoglycerate and 3-phosphoglycerate, the two steps of Rapoport-Luebering pathway, although with less efficacy than the glycolytic reaction. Therefore they are trifunctional enzymes.

Reaction 9: formation of phosphoenolpyruvate

In the ninth step of the glycolytic pathway, 2-phosphoglycerate is dehydrated to form phosphoenolpyruvate, an enol, in a reversible reaction catalyzed by enolase.

2-Phosphoglycerate ⇄ Phosphoenolpyruvate + H2O

The reaction requires Mg2+ that stabilizes the enolic intermediate that is formed during the process.
The ΔG°’ of the reaction is 7.5 kJ/mol (1.8 kcal/mol), while ΔG -3.3 kJ/mol (-0.8 kcal/mol).
Like 1,3-BPG, phosphoenolpyruvate has a phosphoryl group transfer potential high enough to allow ATP formation. Why does this phosphoryl group have a high free energy of hydrolysis?
Although phosphoenolpyruvate and 2-phosphoglycerate contain nearly the same amount of metabolic energy with respect to decomposition to CO2, H20 and Pi, 2-PG dehydration leads to a redistribution of energy such that the standard free energy of hydrolysis of the phosphoryl groups vary as described below:

  • -17.6 kJ/mol (-4.2 kcal/mol) for 2-phosphoglycerate, a phosphoric ester;
  • -61.9 kJ/mol (-14.8 kcal/mol) for phosphoenolpyruvate, an enol phosphate.

What happens is that the phosphoryl group traps PEP in its unstable enol form. When, in the last step of glycolysis, phosphoenolpyruvate donates the phosphoryl group to ADP, ATP and the enol form of pyruvate are formed. The enol form of pyruvate is unstable and tautomerizes rapidly and nonenzymatically to the more stable keto form, that predominates at pH 7. So, the high phosphoryl-transfer potential of PEP is due to the subsequent enol-keto tautomerization of pyruvate.

Reaction 10: the transfer of the phosphoryl group from the phosphoenolpyruvate to the ADP

In the final step of the glycolytic pathway, pyruvate kinase (EC 2.7.1.40) catalyzes the transfer of the phosphoryl group from phosphoenolpyruvate to ADP to form pyruvate and ATP. This is the second substrate-level phosphorylation of glycolysis.

Phosphoenolpyruvate + ADP + H+ → Pyruvate + ATP

The enzyme is a tetramer and, like PFK-1, is a highly regulated. Indeed, it has binding sites for numerous allosteric effectors. Moreover, in vertebrates, there are at least three isozymes of pyruvate kinase, of which the M type predominates in muscle and brain, while the L type in liver. These isozymes have many properties in common, whereas differ in the response to hormones such as glucagon, epinephrine and insulin.
The enzyme activity is stimulated by potassium ion (K+) and some other monovalent cations.
The reaction is essentially irreversible, with a ΔG°’ of -31.4 kJ/mol (-7.5 kcal/mol), and a ΔG of -16.7 kJ/mol (-4.0 kcal/mol), largely due, as anticipated in the previous paragraph, to the tautomerization of the pyruvate from the enol form to the more stable keto form.

Enol-keto tautomerization of pyruvate
Spontaneous Tautomerization of Pyruvate

And, of the -61.9 kJ/mol (14.8 kcal/mol) released from the hydrolysis of the phosphoryl group of PEP, nearly half is conserved in the formation of the phosphoanhydride bond between ADP and Pi, whose ΔG°’ is of -30.5 kJ/mol (-7.3 kcal/mol). The remaining energy, -31.4 kJ/mol (-7.5 kcal/mol), is the driving force that makes the reaction proceed towards ATP production.
While the reaction catalyzed by phosphoglycerate kinase, in the seventh step of the glycolytic pathway, pays off the ATP debt of the preparatory phase, the reaction catalyzed by pyruvate kinase allows a net gain of two ATP.

The fate of NADH and pyruvate produced in glycolysis

Glycolysis produces 2 NADH, 2 ATP, and 2 pyruvate molecules per molecule of glucose.
NADH must be reoxidized to NAD+ to allow glycolysis to proceed. NAD+, a coenzyme that is produced from the vitamin B3, also known as niacin, is present in limited amounts in the cytosol, ≤ 10-5M, a value well below than that of glucose metabolized in a few minutes, and must be continuously regenerated. Therefore, the final step of the glycolytic pathway is the regeneration of NAD+ from NADH through aerobic or anaerobic pathways, each of which involves pyruvate. Such pathways allow, therefore, maintenance of the redox balance of the cell.
Pyruvate is a versatile metabolite that can enter several metabolic pathways, both anabolic and catabolic, depending on the type of cell, the energy state of the cell and the availability of oxygen.

Three possible catabolic fates of pyruvate produced in glycolysis
Catabolic Fates of Pyruvate

With the exception of some variations encountered in bacteria, exploited, for example, in food industry for the production of various foods such as many cheeses, there are essentially three pathways in which pyruvate may enter:

  • reduction to lactate, through lactic acid fermentation;
  • reduction to ethanol or ethyl alcohol, through alcoholic fermentation;
  • aerobic oxidation.

This allows glycolysis to proceed in both anaerobic and aerobic conditions.
It is therefore possible to state that the catabolic fate of the carbon skeleton of glucose is influenced by the cell type, the energetic state of the cell, and the availability of oxygen.

Lactic acid fermentation

In animals, with few exceptions, and in many microorganisms when oxygen availability is insufficient to meet the energy requirements of the cell, or if the cell is without mitochondria, the pyruvate produced by glycolysis is reduced to lactate in the cytosol, in a reaction catalyzed by lactate dehydrogenase (EC 1.1.1.27).

Pyruvate + NADH + H+ ⇄ Lactate + NAD+

In the reaction, pyruvate, by accepting electrons from NADH, is reduced to lactate, while NAD+ is regenerated. And the overall equilibrium of the reaction strongly favors the formation of lactate, as shown by the value of ΔG°’ of -25.1 kJ/mol (-6 kcal/mol).
The conversion of glucose to lactate is called lactic acid fermentation. The overall equation of the process is:

Glucose + 2 Pi + 2 ADP + 2H+ → 2 Lactate + 2 ATP + 2 H2O

Notice that fermentation, discovered by Louis Pasteur who defined it “la vie sans l’air”, is a metabolic pathway that:

  • extracts energy from glucose and stores it as ATP;
  • does not consume oxygen;
  • does not change the concentration of NAD+ or NADH.

With regard to coenzymes, neither NAD+ nor NADH appears in the overall equation, although both are crucial in the process, that is, no net oxidation-reduction occurs. In other words, in the conversion of glucose, C6H12O6, to lactate, C3H6O3, the ratio of hydrogen to carbon atoms of the reactants and products does not change.
From an energy point of view, it should however be emphasized that fermentation extracts only a small amount of the chemical energy of glucose.

In humans, much of the lactate produced enters the Cori cycle for glucose production via gluconeogenesis. We can also state that lactate production shifts part of the metabolic load from the extrahepatic tissues, such as skeletal muscle during intense bouts of exercise, like a 200-meter, when the rate of glycolysis can almost instantly increase 2,000-fold, to the liver.
In contrast to skeletal muscle that releases lactate into the venous blood, the heart muscle is able to take up and use it for fuel, due to its completely aerobic metabolism and to the properties of the heart isozyme of lactate dehydrogenase, referred to as H4. Therefore, portion of the lactate released by skeletal muscle engaged in intense exercise is used by the heart muscle for fuel.

Note: Lactate produced by microorganisms during lactic acid fermentation is responsible for both the scent and taste of sauerkraut, namely, fermented cabbage, as well as for the taste of soured milk.

Alcoholic fermentation

In microorganisms such as brewer’s and baker’s yeast, in certain plant tissues, and in some invertebrates and protists, pyruvate, under hypoxic or anaerobic conditions, may be reduced in two steps to ethyl alcohol or ethanol, with release of CO2.
The first step involves the non-oxidative decarboxylation of pyruvate to form acetaldehyde, an essentially irreversible reaction. The reaction is catalyzed by pyruvate decarboxylase (EC 4.1.1.1), an enzyme that requires Mg2+ and thiamine pyrophosphate, a coenzyme derived from vitamin thiamine or vitamin B1. The enzyme is absent in vertebrates and in other organisms that perform lactic acid fermentation.
In the second step, acetaldehyde is reduced to ethanol in a reaction catalyzed by alcohol dehydrogenase (EC 1.1.1.1), an enzyme that contains a bound zinc atom in its active site. In the reaction, NADH supplies the reducing equivalents and is oxidized to NAD+. At neutral pH, the equilibrium of the reaction lies strongly toward ethyl alcohol formation.
The conversion of glucose to ethanol and CO2 is called alcoholic fermentation. The overall reaction is:

Glucose + 2 Pi + 2 ADP + 2 H+ → 2 Ethanol + 2 CO2 + 2 ATP + 2 H2O

And, as for lactic fermentation, even in alcoholic fermentation no net oxidation-reduction occurs.

Alcoholic fermentation is the basis of the production of beer and wine. Notice that the CO2 produced by brewer’s yeast is responsible for the characteristics “bubbles” in beer, champagne and sparkling wine, while that produced by baker’s yeast causes dough to rise.

Fate of pyruvate and NADH under aerobic conditions

In cells with mitochondria and under aerobic conditions, the most common situation in multicellular and many unicellular organisms, the oxidation of NADH and pyruvate catabolism follow distinct pathways.
In the mitochondrial matrix, pyruvate is first converted to acetyl-CoA in the reactions catalyzed by the pyruvate dehydrogenase complex, a mitochondrial multienzyme complex. In the reaction, a oxidative decarboxylation, pyruvate loses a carbon atom as CO2, and the remaining two carbon unit is bound to Coenzyme A to form acetyl-coenzyme A or acetyl-CoA.

Pyruvate + NAD+ + CoA → acetyl-CoA + CO2 + NADH + H+

The acetyl group of acetyl-CoA is then completely oxidized to CO2 in the citric acid cycle, with production of NADH and FADH2. The pyruvate dehydrogenase complex therefore represents a bridge between glycolysis, which occurs in the cytosol, and the citric acid cycle, which occurs in the mitochondrial matrix.
In turn, electrons derived from oxidations that occur during glycolysis are transported into mitochondria via the reduction of cytosolic intermediates. In this way, in the cytosol NADH is oxidized to NAD+, while the reduced intermediate, once in the mitochondrial matrix, is reoxidized through the transfer of its reducing equivalents to Complex I of the mitochondrial electron transport chain. Here the electrons flow to oxygen to form H2O, a transfer that supplies the energy needed for the synthesis of ATP through the process of oxidative phosphorylation. Of course, also the electrons carried by NADH formed by pyruvate dehydrogenase complex reactions and citric acid cycle and by FADH2 formed by citric acid cycle meet a similar fate.

Note: FADH2 transfers its reducing equivalents not to Complex I but to Complex II.

Anabolic fates of pyruvate

Under anabolic conditions, the carbon skeleton of pyruvate may have fates other than complete oxidation to CO2 or conversion to lactate. In fact, after its conversion to acetyl-CoA, it may be used, for example, for the synthesis of fatty acids, or of the amino acid alanine (see Fig. 3).

Glycolysis and ATP production

In the glycolytic pathway the glucose molecule is degraded to two molecules of pyruvate.
In the first phase, the preparatory phase, two ATP are consumed per molecule of glucose in the reactions catalyzed by hexokinase and PFK-1. In the second phase, the payoff phase, 4 ATP are produced through substrate-level phosphorylation in the reactions catalyzed by phosphoglycerate kinase and pyruvate kinase. So there is a net gain of two ATP per molecule of glucose used. In addition, in the reaction catalyzed by glyceraldehyde 3-phosphate dehydrogenase, two molecules of NADH are produced for each glucose molecule.

Standard and actual free-energy changes of glycolytic reactions
Energy Changes of Glycolytic Reactions

The overall ΔG°’ of glycolysis is -85 kJ/mol (-20.3 kcal/mol), value resulting from the difference between the ΔG°’ of the conversion of glucose into two pyruvate molecules, -146 kJ/mol (-34,9 kcal/mol), and the ΔG°’ of the formation of ATP from ADP and Pi, 2 x 30.5 kJ/mol = 61 kJ / mol (2  x 7.3 kcal/mol = 14.6 kcal/mol). Here are the two reactions.

Glucose + 2 NAD+ → 2 Pyruvate + 2 NADH + 2 H+

2 ADP + 2 Pi → 2 ATP + 2 H2O

The sum of the two reactions gives the overall equation of glycolysis.

Glucose + 2 NAD+ + 2 ADP + 2 Pi → 2 Pyruvate + 2 NADH + 2 H+ + 2 ATP + 2 H20

Thus, under standard conditions, the amount of released energy stored within ATP is (61/146) x 100 = 41.8%.
Notice that the overall equation of glycolysis can also be derived by considering all the reagents, ATP, NAD+, ADP, and Pi and all the products.

Glucose + 2 ATP + 2 NAD+ + 4 ADP + 2 Pi → 2 Pyruvate + 2 ADP + 2 NADH + 2 H+ + 4 ATP + 2 H20

Cancelling the common terms on both sides of the equation, we obtain the overall equation shown above.

Glycolysis and ATP production under anaerobic conditions

Under anaerobic conditions, regardless of what is the metabolic fate of pyruvate, conversion to lactate, ethanol or other molecules, there is no additional production of ATP downstream of glycolysis.
Therefore under these conditions, glycolysis extracts only a small fraction of the chemical energy of the glucose molecule, energy equal to 2840 kJ/mol (679 kcal/mol) released as a result of its conversion to CO2 and H2O. Indeed, only 146 kJ/mol are released in the conversion of a glucose molecule to two pyruvate molecules, equal to 5%, [(146/2,840) x 100], of the available chemical energy. Therefore,  pyruvate still contains most of the chemical energy of the hexose.
Similarly, the 4 electrons carried by NADH produced in step 6 of glycolysis cannot be used for ATP production.
In lactic acid fermentation, the ΔG°’ associated with the conversion of a glucose molecule to two molecules of lactate is -183.6 kJ/mol (-43.9 kcal/mol), and 33.2% of such free energy, [(61/183.6) x 100] is stored within ATP, whereas it is 41.8% in the conversion of a glucose molecule to two molecules of pyruvate.
It should be noted that under actual conditions the amount of free energy required for the synthesis of ATP from ADP and Pi is much higher than that required under standard conditions, namely, approximately 50%  of the energy released is stored within ATP.

Glycolysis and ATP production under aerobic conditions

Under aerobic conditions, in cells with mitochondria, the amount of chemical energy that can be extracted from glucose and stored within ATP is much greater than under anaerobic conditions.
If we consider the two NADH produced during glycolysis, the flow of their 4 reducing equivalents along the mitochondrial electron transport chain allows the production of 2-3 ATP per electron pair through oxidative phosphorylation. Therefore, 6 to 8 ATP are produced when one molecule of glucose is converted into two molecules of pyruvate, 2 from glycolysis and 4-6 from oxidative phosphorylation.

Note: The amount of ATP produced from the reducing equivalents of NADH depends upon the mechanism by which they are shuttled into mitochondria.

On the other hand, if we analyze the coordinated and consecutive action of glycolysis, the pyruvate dehydrogenase complex, citric acid cycle,  mitochondrial electron transport chain and oxidative phosphorylation, much more energy can be extracted from glucose and stored within ATP. In this case, according to what reported by Lehninger, 30 to 32 ATP are produced for each glucose molecule, although recent estimates suggest a net production equal to 29.85 ATP/glucose, or 29.38 ATP/glucose if also ATP formed from GTP, in turn produced by the citric acid cycle, is exported. Considering both estimates, the production of ATP is about 15 times greater than under anaerobic condition.

Feeder pathways for glycolysis

Other carbohydrates besides glucose, both simple and complex, can be catabolized via glycolysis, after enzymatic conversion to one of the glycolytic intermediates. Among the most important are:

  • glycogen and starch, two storage polysaccharides;
  • some disaccharides such as sucrose, maltose, lactose and trehalose;
  • the monosaccharides galactose, fructose, and the less common mannose.
Metabolic pathways to catabolizy carbohydrates other than glucose in glycolysis
Feeder Pathways for Glycolysis

Dietary starch and disaccharides must be hydrolyzed in the intestine to the respective monosaccharides before being absorbed. Once in the venous circulation, monosaccharides reach the liver through the portal vein; this organ is the main site where they are metabolized.

Glycogen and starch

Regarding the phosphorolytic breakdown of starch and endogenous glycogen refer to the corresponding articles.

Fructose

Under physiological conditions, the liver removes much of the ingested fructose from the bloodstream before it can reach extrahepatic tissues.
The hepatic pathway for the conversion of the monosaccharide to intermediates of glycolysis consists of several steps.
In the first step fructose is phosphorylated to fructose 1-phosphate at the expense of one ATP. This reaction is catalyzed by fructokinase (EC 2.7.1.4), and requires the presence of Mg2+.

Fructose + ATP → Fructose 1-phosphate + ADP + H+

As for glucose, fructose phosphorylation traps the molecule inside the cell.
In the second step fructose 1-phosphate aldolase catalyzes the hydrolysis, an aldol cleavage, of fructose 1-phosphate to dihydroxyacetone phosphate and glyceraldehyde.

Fructose 1-phosphate → Dihydroxyacetone Phosphate + Glyceraldehyde

Dihydroxyacetone phosphate is an intermediate of the glycolytic pathway and, after conversion to glyceraldehyde 3-phosphate, may flow through the pathway. Conversely, glyceraldehyde is not an intermediate of the glycolysis, and is phosphorylated to glyceraldehyde 3-phosphate at the expense of one ATP. The reaction is catalyzed by triose kinase (EC 2.7.1.28), and requires the presence of Mg2+.

Glyceraldehyde + ATP → Glyceraldehyde 3-phosphate + ADP + H+

In hepatocytes, therefore, a molecule of fructose is converted to two molecules of glyceraldehyde 3-phosphate, at the expense of two ATP, as for glucose.

Fructose + 2 ATP → 2 Glyceraldehyde 3-phosphate +2 ADP +2  H+

Fructose and hexokinase

In extrahepatic sites, such as skeletal muscle, kidney or adipose tissue, fructokinase is not present, and fructose enters the glycolytic pathway as fructose 6-phosphate. In fact, as previously seen, hexokinase can catalyzes the phosphorylation of fructose at C-6.

Fructose + ATP → Fructose 6-phosphate + ADP + H+

However, the affinity of the enzyme for fructose is about 20 times lower than for glucose, so in the hepatocyte, where glucose is much more abundant than fructose, or in the skeletal muscle under anaerobic conditions, that is, when glucose is the preferred fuel, little amounts of fructose 6-phosphate are formed.
Conversely, in adipose tissue, fructose is more abundant than glucose, so that its phosphorylation by hexokinase is not competitively inhibited to a significant extent by glucose. In this tissue, therefore, fructose 6-phosphate synthesis is the entry point into glycolysis for the monosaccharide.
With regard to the metabolic effects of fructose, it is important to underline that in the liver the monosaccharide, being phosphorylated at C-1, enters glycolysis at triose phosphate level, thus downstream to the reaction catalyzed by PFK-1, an enzyme that plays a key role in the regulation of the flow of carbon through this metabolic pathway. Conversely, when fructose is phosphorylated at C-6, it enters the glycolytic pathway upstream of PFK-1.

Sorbitol

Fructose is the entry point into glycolysis for sorbitol, a sugar present in many fruits and vegetables, and used as a sweetener and stabilizer, too. In the liver, sorbitol dehydrogenase (EC 1.1.99.21) catalyzes the oxidation of sorbitol to fructose.

Sorbitol + NAD+ → Fructose + NADH + H+

The reaction requires the presence of zinc ion, and occurs in the cytosol.

Galactose

Galactose, for the most part derived from intestinal digestion of the lactose, once in the liver is converted, via the Leloir pathway, to glucose 1-phosphate.
For a more in-depth discussion of the Leloir pathway, see the article on galactose.
The metabolic fate of glucose 1-phosphate depends on the energy status of the cell.
Under conditions promoting glucose storage, glucose 1-phosphate can be channeled to glycogen synthesis. Conversely, under conditions that favor the use of glucose as fuel, glucose 1-phosphate is isomerized to glucose 6-phosphate in the reversible reaction catalyzed by phosphoglucomutase (EC 5.4.2.2).

Glucose 1-phosphate ⇄ Glucose 6-phosphate

In turn, glucose 6-phosphate can be channeled to glycolysis and be used for energy production, or dephosphorylated to glucose in the reaction catalyzed by glucose 6-phosphatase, and then released into the bloodstream.

Mannose

Mannose is present in various dietary polysaccharides, glycolipids and glycoproteins. In the intestine, it is released from these molecules, absorbed, and, once reached the liver, is phosphorylated at C-6 to form mannose 6-phosphate, in the reaction catalyzed by hexokinase.

Mannose + ATP → Mannose 6-phosphate + ADP + H+

Mannose 6-phosphate is then isomerized to fructose 6-phosphate in the reaction catalyzed by mannose 6-phosphate isomerase (EC 5.3.1.8.).

Mannose 6-phosphate ⇄ Fructose 6-phosphate

Regulation of glycolysis

The flow of carbon through the glycolytic pathway is regulated in response to metabolic conditions, both inside and outside the cell, essentially to meet two needs: the production of ATP and the supply of precursors for biosynthetic reactions.
And in the liver, to avoid wasting energy, glycolysis and gluconeogenesis are reciprocally regulated so that when one pathway is active, the other slows down. As explained in the article on gluconeogenesis, during evolution this was achieved by selecting different enzymes to catalyze the essentially irreversible reactions of the two pathways, whose activity are regulated separately. Indeed, if these reactions proceeded simultaneously at high speed, they would create a futile cycle or substrate cycle. A such fine regulation could not be achieved if a single enzyme operates in both directions.
The control of the glycolytic pathway involves essentially the reactions catalyzed by hexokinase, PFK-1, and pyruvate kinase, whose activity is regulated through:

  • allosteric modifications, that occur on a time scale of  milliseconds and are instantly reversible;
  • covalent modifications, that is, phosphorylations and dephosphorylation, that occur on a time scale of seconds;
  • changes in enzyme concentrations, resulting from changes in the rate of their synthesis and/or degradation, that occur on a time scale of hours.

Note: The main regulatory enzymes of gluconeogenesis are pyruvate carboxylase (EC 6.4.1.1) and fructose 1,6-bisphosphatase (EC 3.1.3.11).

Hexokinase

In humans, hexokinase has four tissue specific isozymes, designated as hexokinase I, II, III, and IV, encoded by as many genes.
Hexokinase I is the predominant isozyme in the brain, whereas in skeletal muscle hexokinase I and II are present, accounting for 70-75% and 25-30% of the isozymes, respectively.
Hexokinase IV, also known as glucokinase (EC 2.7.1.2), is mainly present in hepatocytes and β cells of the pancreas, where it is the predominant isozyme. In the liver it catalyzes, with glucose 6-phosphatase, the substrate cycle between glucose and glucose 6-phosphate. Glucokinase differs from the other hexokinase isozymes in kinetic and regulatory properties.

Note: Isoenzymes or isozymes are different proteins that catalyze the same reaction, and that generally differ in kinetic and regulatory properties, subcellular distribution, or in the cofactors used. They may be present in the same species, in the same tissue or even in the same cell.

Comparison of the kinetic properties of hexokinase isozymes

The kinetic properties of hexokinase I, II, and III are similar.
Hexokinase I and II have a Km for glucose of 0.03 mM and 0.1 mM, respectively. Therefore these isoenzymes work very efficiently at normal blood glucose levels, 4-5 mM.
Conversely, glucokinase has a high Km for glucose, approximately 10 mM; this means that the enzyme works efficiently only when blood glucose concentration is high, for example after a meal rich in carbohydrates with a high glycemic index.

Regulation of the activity of hexokinases I-III

Hexokinases I-III are allosterically inhibited by glucose 6-phosphate, the product of their reaction. This ensures that glucose 6-phosphate does not accumulate in the cytosol when glucose is not needed for energy, for glycogen synthesis, for the pentose phosphate pathway, or as a source of precursors for biosynthetic pathways, leaving, at the same time, the monosaccharide in the blood, available for other organs and tissues. For example, when PFK-1 is inhibited, fructose 6-phosphate accumulates and then, due to phosphoglucose isomerase reaction, glucose 6-phosphate accumulates. Therefore, inhibition of PFK-1 leads to inhibition of hexokinases I-III.

In skeletal muscle, the activity of hexokinase I and II is coordinated with that of GLUT4, a low Km glucose transporter (5mM), whose translocation to the plasma membrane is induced by both insulin and physical activity. The combined action of GLUT4 on plasma membrane and hexokinase in the cytosol maintains a balance between glucose uptake and its phosphorylation. Because blood glucose concentration is between 4 and 5 mmol/L, its entry into the myocyte through GLUT4 may cause an increase in its concentration sufficient to saturate, or near saturate the enzyme, which therefore operates at or near its Vmax.

Regulation of the activity of hepatic glucokinase

Glucokinase differs in three respects from hexokinases I-III, and is particularly suitable for the role that the liver plays in glycemic control. Why?

  • As previously said, glucokinase has a Km for glucose of about 10 mM, much higher than the Km for glucose of hexokinases I-III, and higher than the value of fasting blood glucose levels (4-5 mM) as well. In the liver, where it is the predominant hexokinase isoenzyme, its role is to provide glucose 6-phosphate for the synthesis of glycogen and fatty acids. The activity of glucokinase is linked to that of GLUT2, the major glucose transporter in hepatocytes, with a high Km for glucose, approximately 10 mM. Hence, GLUT2 is very active when blood glucose concentration is high, rapidly equilibrating sugar concentrations in cytosol of hepatocytes and blood. Under such conditions glucokinase is active and converts glucose to glucose 6-phosphate, and, due to high Km for glucose, its activity continues to increase even when the intracellular concentration of the monosaccharide reaches or exceeds 10 mM.  Therefore, the rate at which glucose uptake and phosphorylation occurs are determined by the value of blood glucose level itself. On the other hand, when glucose availability is low, its concentration in the cytosol of hepatocytes is just as low, much lower than the Km for glucose of glucokinase, so that glucose produced through gluconeogenesis and/or glycogenolysis is not phosphorylated and can leave the cell.
    A similar situation also occurs in pancreatic β cells, where the GLUT2/glucokinase system causes the intracellular G-6-P concentration to equalize with glucose concentration in the blood, allowing the cells to detect and respond to hyperglycemia.
  • Unlike hexokinases I-III, glucokinase is not inhibited by glucose 6-phosphate, that is, is not product inhibited, and catalyzes its synthesis even when it accumulates.
  • Glucokinase is inhibited by the reversible binding of glucokinase regulatory protein or GKRP, a liver-specific regulatory protein. The mechanism of inhibition by GKRP occurs via the anchorage of glucokinase inside the nucleus, where it is separated from the other glycolytic enzymes.
    Regulation of the activity of hepatic isoform of hexokinase or glucokinase
    Regulation Glucokinase Activity

    The binding between glucokinase and GKRP is much tighter in the presence of fructose 6-phosphate, whereas it is weakened by glucose and fructose 1-phosphate.
    In the absence of glucose, glucokinase is in its super-opened conformation that has low activity. The rise in cytosolic glucose concentration causes a concentration dependent transition of glucokinase to its close conformation, namely, its active conformation that is not accessible for glucokinase regulatory protein. Hence, glucokinase is active and no longer inhibited.
    Notice that fructose 1-phosphate is present in the hepatocyte only when fructose is metabolized. Hence, fructose relieves the inhibition of glucokinase by glucokinase regulatory protein.
    Example
    After a meal rich in carbohydrates, blood glucose levels rise, glucose enters the hepatocyte through GLUT2, and then moves inside the nucleus through the nuclear pores. In the nucleus glucose determines the transition of glucokinase to its close conformation, active and not accessible to GKRP, allowing glucokinase to diffuse in the cytosol where it phosphorylates glucose.
    Conversely, when glucose concentration declines, such as during fasting when blood glucose levels may drop below 4 mM, glucose concentration in hepatocytes is low, and fructose 6-phosphate binds to GKRP allowing it to bind tighter to glucokinase. This results in a strong inhibition of the enzyme. This mechanism ensures that the liver, at low blood glucose levels, does not compete with other organs, primarily the brain, for glucose.
    In the cell, fructose 6-phosphate is in equilibrium with glucose 6-phosphate, due to phosphoglucose isomerase reaction. Through its association with GKRP, fructose 6-phosphate allows the cell to decrease glucokinase activity, so preventing the accumulation of intermediates.

To sum up, when blood glucose levels are normal, glucose is phosphorylated mainly by hexokinases I-III, whereas when blood glucose levels are high glucose can be phosphorylated by glucokinase as well.

Regulation of phosphofructokinase 1 activity

Phosphofructokinase 1 is the key control point of carbon flow through the glycolytic pathway.
The enzyme, in addition to substrate binding sites, has several binding sites for allosteric effectors.
ATP, citrate, and hydrogen ions are allosteric inhibitors of the enzyme, whereas AMP, Pi and fructose 2,6-bisphosphate are allosteric activators.

Regulation of phosphofructokinase 1 and fructose 1,6-bisphosphatase activities
Regulation of PFK 1 and Fructose 1,6-bisphosphatase

It should be noted that ATP, an end product of glycolysis, is also a substrate of phosphofructokinase 1. Indeed, the enzyme has two binding sites for the nucleotide: a low-affinity regulatory site, and a high affinity substrate site.
What do allosteric effectors signal?

  • ATP, AMP and Pi signal the energy status of the cell.
    The activity of PFK-1 increases when the energy charge of the cell is low, namely, when there is a need for ATP, whereas it decreases when the energy charge of the cell is high, namely when ATP concentration in the cell is high. How?
    When the nucleotide is produced faster than it is consumed, its cellular concentration is high. Under such condition ATP, binding to its allosteric site, inhibits PFK-1 by reducing the affinity of the enzyme for fructose 6-phosphate. From the kinetic point of view, the increase in ATP concentration modifies the relationship between enzyme activity and substrate concentration, chancing the hyperbolic fructose 6-phosphate velocity curve into a sigmoidal one, and then, increasing Km for the substrate. However, under most cellular conditions, ATP concentration does not vary much. For example, during a vigorous exercise ATP concentration in muscle may lower of about 10% compared to the resting state, whereas glycolysis rate varies much more than would be expected by such reduction.
    When ATP consumption exceeds its production, ADP and AMP concentrations rise, in particular that of AMP, due to the reaction catalyzed by adenylate kinase (EC 2.7.4.3), that form ATP from ADP.

ADP + ADP ⇄ ATP + AMP

The equilibrium constant, Keq, of the reaction is:

Keq = [ATP][AMP]/[ADP]2= 0.44

Under normal conditions, ADP and AMP concentrations are about 10% and often less than 1% of ATP concentration, respectively. Therefore, considering that the total adenylate pool is constant over the short term, even a small reduction in ATP concentration leads, due to adenylate kinase activity, to a much larger relative increase in AMP concentration. In turn, AMP acts by reversing the inhibition due to ATP.
Therefore, the activity of phosphofructokinase 1 depends on the cellular energy status:

when ATP is plentiful, enzyme activity decreases;

when AMP levels increase and ATP levels fall, enzyme activity increases.

Why is not ADP a positive effector of PFK-1? There are two reasons.
When the energy charge of the cell falls, ADP is used to regenerate ATP, in the reaction catalyzed by adenylate kinase Moreover, as previously said, a small reduction in ATP levels leads to larger-percentage changes in ADP levels and, above all, in AMP levels.

  • Hydrogen ions inhibit PFK-1. Such inhibition prevents, by controlling the rate of glycolysis, excessive lactate buildup and the consequent fall of blood pH.
  • Citrate is an allosteric inhibitor of PFK-1 that acts by enhancing the inhibitory effect of ATP.
    It is the product of the first step of the citric acid cycle, a metabolic pathway that provides building blocks for biosynthetic pathways and directs electrons into mitochondrial electron transport chain for ATP synthesis via oxidative phosphorylation. High citrate levels in the cytosol mean that, in the mitochondria, an overproduction of building blocks is occurring and the current energy are met, namely, the citric acid cycle has reached saturation. Under such conditions glycolysis, that feeds the cycle under aerobic condition, can slow down, sparing glucose.
    So, it should be noted that PFK-1 couples glycolysis and the citric acid cycle.
  • In the liver, the central control point of glycolysis and gluconeogenesis is the substrate cycle between F-6-P and F-1,6-BP, catalyzed by PFK-1 and fructose 1,6-bisphosphatase.
    The liver plays a pivotal role in maintaining blood glucose levels within the normal range.
    When blood glucose levels drop, glucagon stimulates hepatic glucose synthesis, via both glycogenolysis and gluconeogenesis, and at the same time signals the liver to stop consuming glucose to meet its needs.
    Conversely, when blood glucose levels are high, insulin causes the liver to use glucose for energy, and to synthesize glycogen, and triglycerides.
    In this context, the regulation of glycolysis and gluconeogenesis is mediated by fructose 2,6-bisphosphate, a molecule that allows the liver to play a major role in regulating blood glucose levels, and whose levels are controlled by insulin and glucagon.
    As a result of binding to its allosteric site on PFK-1, fructose 2,6-bisphosphate increases the affinity of the enzyme for fructose 6-phosphate, its substrate, while decreases its affinity for the allosteric inhibitors citrate and ATP. It is remarkable to note that under physiological concentrations of the substrates and positive and negative allosteric effectors, phosphofructokinase 1 would be virtually inactive in the absence of fructose 2,6-bisphosphate.
    On the other hand, the binding of fructose 2,6-bisphosphate to fructose 1,6-bisphosphatase inhibits the enzyme, even in the absence of AMP, another allosteric inhibitor of the enzyme.
    Due to these effects, fructose 2,6-bisphosphate increases the net flow of glucose through glycolysis.
    For an more in-depth analysis of fructose 2,6-bisphosphate metabolism, refer to the article on gluconeogenesis.
  • Another metabolite involved in the control of the flow of carbon through glycolysis and gluconeogenesis is xylulose 5-phosphate, a product of the pentose phosphate pathway, whose concentration in hepatocytes rises after ingestion of a carbohydrate-rich meal. The molecule, by activating protein phosphatase 2A, finally leads to an increase in the concentration of fructose 2,6-bisphosphate, and then to an increase in the flow of carbon through glycolysis and to a reduction in the flow of carbon through gluconeogenesis.

Regulation of pyruvate kinase activity

A further control point of carbon flow through glycolysis and gluconeogenesis is the substrate cycle between phosphoenolpyruvate and pyruvate, catalyzed by pyruvate kinase for glycolysis, and by the combined action of pyruvate carboxylase and phosphoenolpyruvate carboxykinase (EC 4.1.1.32) for gluconeogenesis.
All isozymes of pyruvate kinase are allosterically inhibited by high concentrations of ATP, long-chain fatty acids, and acetyl-CoA, all signs that the cell is in an optimal energy status. Alanine, too, that can be synthesized from pyruvate through a transamination reaction, is an allosteric inhibitor of pyruvate kinase; its accumulation signals that building blocks for biosynthetic pathways are abundant.

Regulation of hepatic pyruvate kinase activity
Regulation of Pyruvate Kinase Activity

Conversely, pyruvate kinase is allosterically activated by fructose 1,6-bisphosphate, the product of the first committed step of glycolysis. Therefore, F-1,6-BP allows pyruvate kinase to keep pace with the flow of intermediates. It should be underlined that, at physiological concentration of PEP, ATP and alanine, the enzyme would be completely inhibited without the stimulating effect of F-1,6-BP.
The hepatic isoenzyme, but not the muscle isoenzyme, is also subject to regulation through phosphorylation by:

  • protein kinase A or PKA, activated by the binding of glucagon to the specific receptor or epinephrine to β-adrenergic receptors;
  • calcium/calmodulin dependent protein kinase or CAMK, activated by the binding of epinephrine to α1-adrenergic receptors.

Phosphorylation of the enzyme decreases its activity, by increasing the Km for phosphoenolpyruvate, and slows down glycolysis.
For example, when the blood glucose levels are low, glucagon-induced phosphorylation decreases pyruvate kinase activity. The phosphorylated enzyme is also less readily stimulated by fructose 1,6-bisphosphate but more readily inhibited by alanine and ATP. Conversely, the dephosphorylated form of pyruvate kinase is more sensitive to fructose 1,6-bisphosphate, and less sensitive to ATP and alanine. In this way, when blood glucose levels are low, the use of glucose for energy in the liver slows down, and the sugar is available for other tissues and organs, such as the brain. However, it should be noted that pyruvate kinase does not undergo glucagon-induced phosphorylation in the presence of fructose 1,6-bisphosphate.
An increase in the insulin/glucagon ratio, on the other hand, leads to dephosphorylation of the enzyme and then to its activation. The dephosphorylated enzyme is more readily stimulated by its allosteric activators F-1,6-BP, and less readily inhibited by allosteric inhibitors alanine and ATP.

References

de la Iglesia N., Mukhtar M., Seoane J., Guinovart J.J., & Agius L. The role of the regulatory protein of glucokinase in the glucose sensory mechanism of the hepatocyte. J Biol Chem 2000;275(14):10597-10603. doi: 10.1074/jbc.275.14.10597

Garrett R.H., Grisham C.M. Biochemistry. 4th Edition. Brooks/Cole, Cengage Learning, 2010

Kabashima T., Kawaguchi T., Wadzinski B.E., Uyeda K. Xylulose 5-phosphate mediates glucose-induced lipogenesis by xylulose 5-phosphate-activated protein phosphatase in rat liver. Proc Natl Acad Sci USA 2003;100:5107-5112. doi:10.1073/pnas.0730817100

Kaminski M.T., Schultz J., Waterstradt R., Tiedge M., Lenzen S., Baltrusch S. Glucose-induced dissociation of glucokinase from its regulatory protein in the nucleus of hepatocytes prior to nuclear export. BBA – Molecular Cell Research 2014;1843(3):554-564. doi:10.1016/j.bbamcr.2013.12.002

Nelson D.L., M. M. Cox M.M. Lehninger. Principles of biochemistry. 6th Edition. W.H. Freeman and Company, 2012

Oslund R.C., Su X., Haugbro M., Kee J-M., Esposito M., David Y., Wang B., Ge E., Perlman D.H., Kang Y., Muir T.W., & Rabinowitz J.D. Bisphosphoglycerate mutase controls serine pathway flux via 3-phosphoglycerate. Nat Chem Biol 2017;13:1081-1087. doi:10.1038/nchembio.2453

Rich P.R. The molecular machinery of Keilin’s respiratory chain. Biochem Soc Trans 2003;31(6):1095-1105. doi:10.1042/bst0311095

Stipanuk M.H., Caudill M.A. Biochemical, physiological, and molecular aspects of human nutrition. 3rd Edition. Elsevier health sciences, 2012

Van Schaftingen E., and Hers H-G. Inhibition of fructose-1,6-bisphosphatase by fructose-2,6-bisphosphate. Proc Natl Acad Sci USA 1981;78(5):2861-2863. doi:10.1073/pnas.78.5.2861

Van Schaftingen E., Jett M-F., Hue L., and Hers, H-G. Control of liver 6-phosphofructokinase by fructose 2,6-bisphosphate and other effectors. Proc Natl Acad Sci USA 1981;78(6):3483-3486. doi:10.1073/pnas.78.6.3483

The gluconeogenesis pathway: steps and regulation

Gluconeogenesis is a metabolic pathway that leads to the synthesis of glucose from pyruvate and other non-carbohydrate precursors, even in non-photosynthetic organisms.
It occurs in all microorganisms, fungi, plants and animals, and the reactions are essentially the same, leading to the synthesis of one glucose molecule from two pyruvate molecules. Therefore, it is in essence glycolysis in reverse, which instead goes from glucose to pyruvate, and shares seven enzymes with it.

Glycolysis vs Gluconeogenesis
Gluconeogenesis and Glycolysis Pathways

Glycogenolysis is quite distinct from gluconeogenesis: it does not lead to de novo production of glucose from non-carbohydrate precursors, as shown by its overall reaction:

Glycogen or (glucose)n → n glucose molecules

The following discussion will focus on gluconeogenesis that occurs in higher animals, and in particular in the liver of mammals.

CONTENTS

Why is gluconeogenesis important?

Gluconeogenesis is an essential metabolic pathway for at least two reasons.

  • It ensures the maintenance of appropriate blood glucose levels when the liver glycogen is almost depleted and no carbohydrates are ingested.
  • Maintaining blood glucose within the normal range, 3.3 to 5.5 mmol/L (60 and 99 mg/dL), is essential because many cells and tissues depend, largely or entirely, on glucose to meet their ATP demands; examples are red blood cells, neurons, skeletal muscle working under low oxygen conditions, the medulla of the kidney, the testes, the lens and the cornea of the eye, and embryonic tissues. For example, glucose requirement of the brain is about 120 g/die that is equal to:

over 50% of the total body stores of the monosaccharide, about 210 g, of which 190 g are stored as muscle and liver glycogen, and 20 g are found in free form in body fluids;
about 75% of the daily glucose requirement, about 160 g.

During fasting, as in between meals or overnight, the blood glucose levels are maintained within the normal range due to hepatic glycogenolysis, and to the release of fatty acids from adipose tissue and ketone bodies by the liver. Fatty acids and ketone bodies are preferably used by skeletal muscle, thus sparing glucose for cells and tissues that depend on it, primarily red blood cells and neurons. However, after about 18 hours of fasting or during intense and prolonged exercise, glycogen stores are depleted and may become insufficient. At that point, if no carbohydrates are ingested, gluconeogenesis becomes important.
And, the importance of gluconeogenesis is further emphasized by the fact that if the blood glucose levels fall below 2 mmol/L, unconsciousness occurs.

  • The excretion of pyruvate would lead to the loss of the ability to produce ATP through aerobic respiration, i.e. more than 10 molecules of ATP for each molecule of pyruvate oxidized.

Where does gluconeogenesis occur?

In higher animals, gluconeogenesis occurs in the liver, kidney cortex and epithelial cells of the small intestine, that is, the enterocytes.
Quantitatively, the liver is the major site of gluconeogenesis, accounting for about 90% of the synthesized glucose, followed by kidney cortex, with about 10%. The key role of the liver is due to its size; in fact, on a wet weight basis, the kidney cortex produces more glucose than the liver.
In the kidney cortex, gluconeogenesis occurs in the cells of the proximal tubule, the part of the nephron immediately following the glomerulus. Much of the glucose produced in the kidney is used by the renal medulla, while the role of the kidney in maintaining blood glucose levels becomes more important during prolonged fasting and liver failure. It should, however, be emphasized that the kidney has no significant glycogen stores, unlike the liver, and contributes to maintaining blood glucose homeostasis only through gluconeogenesis and not through glycogenolysis.
Part of the gluconeogenesis pathway also occurs in the skeletal muscle, cardiac muscle, and brain, although at very low rate. In adults, muscle is about 18 the weight of the liver; therefore, its de novo synthesis of glucose might have quantitative importance. However, the release of glucose into the circulation does not occur because these tissues, unlike liver, kidney cortex, and enterocytes, lack glucose 6-phosphatase (EC 3.1.3.9), the enzyme that catalyzes the last step of gluconeogenesis (see below).
Therefore, the production of glucose 6-phosphate, including that from glycogenolysis, does not contribute to the maintenance of blood glucose levels, and only helps to restore glycogen stores, in the brain small and limited mostly to astrocytes. For these tissues, in particular for skeletal muscle due to its large mass, the contribution to blood glucose homeostasis results only from the small amount of glucose released in the reaction catalyzed by enzyme debranching (EC 3.2.1.33) of glycogenolysis.
With regard to the cellular localization, most of the reactions occur in the cytosol, some in the mitochondria, and the final step) within the endoplasmic reticulum cisternae.

Irreversible steps of gluconeogenesis

As previously said, gluconeogenesis is in essence glycolysis in reverse. And, of the ten reactions that constitute gluconeogenesis, seven are shared with glycolysis; these reactions have a ΔG close to zero, therefore easily reversible. However, under intracellular conditions, the overall ΔG of glycolysis is about -63 kJ/mol (-15 kcal/mol) and of gluconeogenesis about -16 kJ/mol (-3.83 kcal/mol), namely, both the pathways are irreversible.
The irreversibility of the glycolytic pathway is due to three strongly exergonic reactions, that cannot be used in gluconeogenesis, and listed below.

  • The phosphorylation of glucose to glucose 6-phosphate, catalyzed by hexokinase (EC 2.7.1.1) or glucokinase (EC 2.7.1.2).
    ΔG = -33.4 kJ/mol (-8 kcal/mol)
    ΔG°’ = -16.7 kJ/mol (-4 kcal/mol)
  • The phosphorylation of fructose 6-phosphate to fructose 1,6-bisphosphate, catalyzed by phosphofructokinase-1 or PFK-1 (EC 2.7.1.11)
    ΔG = -22.2 kJ/mol (-5.3 kcal/mol)
    ΔG°’ = -14.2 kJ/mol (-3.4 kcal/mol)
  •  The conversion of phosphoenolpyruvate or PEP to pyruvate, catalyzed by pyruvate kinase (EC 2.7.1.40)
    ΔG = -16.7 kJ/mol (-4.0 kcal/mol)
    ΔG°’ = -31.4 kJ/mole (-7.5 kcal/mol)

In gluconeogenesis, these three steps are bypassed by enzymes that catalyze irreversible steps in the direction of glucose synthesis: this ensures the irreversibility of the metabolic pathway.
Below, such reactions are analyzed.

From pyruvate to phosphoenolpyruvate

The first step of gluconeogenesis that bypasses an irreversible step of glycolysis, namely the reaction catalyzed by pyruvate kinase, is the conversion of pyruvate to phosphoenolpyruvate.
Phosphoenolpyruvate is synthesized through two reactions catalyzed, in order, by the enzymes:

  • pyruvate carboxylase (EC 6.4.1.1);
  • phosphoenolpyruvate carboxykinase or PEP carboxykinase (EC 4.1.1.32).

Pyruvate → Oxaloacetate → Phosphoenolpyruvate

Pyruvate carboxylase catalyzes the carboxylation of pyruvate to oxaloacetate, with the consumption of one ATP. The enzyme requires the presence of magnesium or manganese ions

Pyruvate + HCO3+ ATP → Oxaloacetate + ADP + Pi

The enzyme, discovered in 1960 by Merton Utter, is a mitochondrial protein composed of four identical subunits, each with catalytic activity. The subunits contain a biotin prosthetic group, covalently linked by amide bond to the ε-amino group of a lysine residue, that acts as a carrier of activated CO2 during the reaction. An allosteric binding site for acetyl-CoA is also present in each subunit.
It should be noted that the reaction catalyzed by pyruvate carboxylase, leading to the production of oxaloacetate, also provides intermediates for the citric acid cycle or Krebs cycle.
Phosphoenolpyruvate carboxykinase is present, approximately in the same amount, in mitochondria and cytosol of hepatocytes. The isoenzymes are encoded by separate nuclear genes.
The enzyme catalyzes the decarboxylation and phosphorylation of oxaloacetate to phosphoenolpyruvate, in a reaction in which GTP acts as a donor of high-energy phosphate. PEP carboxykinase requires the presence of both magnesium and manganese ions. The reaction is reversible under normal cellular conditions.

Oxaloacetate + GTP ⇄ PEP + CO2 + GDP

During this reaction, a CO2 molecule, the same molecule that is added to pyruvate in the reaction catalyzed by pyruvate carboxylase, is removed. Carboxylation-decarboxylation sequence is used to activate pyruvate, since decarboxylation of oxaloacetate facilitates, makes thermodynamically feasible, the formation of phosphoenolpyruvate.
More generally, carboxylation-decarboxylation sequence promotes reactions that would otherwise be strongly endergonic, and also occurs in the citric acid cycle, in the pentose phosphate pathway, also called the hexose monophosphate pathway, and in the synthesis of fatty acids.
The levels of PEP carboxykinase before birth are very low, while its activity increases several fold a few hours after delivery. This is the reason why gluconeogenesis is activated after birth.
The sum of the reactions catalyzed by pyruvate carboxylase and phosphoenolpyruvate carboxykinase is:

Pyruvate + ATP + GTP + HCO3 → PEP + ADP + GDP + Pi + CO2

ΔG°’ of the reaction is equal to 0.9 kJ/mol (0.2 kcal/mol), while standard free energy change associated with the formation of pyruvate from phosphoenolpyruvate by reversal of the pyruvate kinase reaction is + 31.4 kJ/mol (7.5 kcal/mol).
Although the ΔG°’ of the two steps leading to the formation of PEP from pyruvate is slightly positive, the actual free-energy change (ΔG), calculated from intracellular concentrations of the intermediates, is very negative, -25 kJ/mol (-6 kcal/mol). This is due to the fast consumption of phosphoenolpyruvate in other reactions, that maintains its concentration at very low levels. Therefore, under cellular conditions, the synthesis of PEP from pyruvate is irreversible.
It is noteworthy that the metabolic pathway for the formation of phosphoenolpyruvate from pyruvate depends on the precursor: pyruvate or alanine, or lactate.

Phosphoenolpyruvate precursors: pyruvate or alanine

The bypass reactions described below predominate when alanine or pyruvate is the glucogenic precursor.
Pyruvate carboxylase is a mitochondrial enzyme, therefore pyruvate must be transported from the cytosol into the mitochondrial matrix. This is mediated by transporters located in the inner mitochondrial membrane, referred to as MPC1 and MPC2. These proteins, associating, form a hetero-oligomer that facilitates pyruvate transport.
Pyruvate can also be produced from alanine in the mitochondrial matrix by transamination, in the reaction catalyzed by alanine aminotransferase (EC 2.6.1.2).

Conversion of pyruvate and alanine to phosphoenolpyruvate in gluconeogenesis
Conversion of Pyruvate and Alanine to Phosphoenolpyruvate

Since the enzymes involved in the later steps of gluconeogenesis, except glucose-6-phosphatase, are cytosolic, the oxaloacetate produced in the mitochondrial matrix is transported into the cytosol. However, there are no oxaloacetate transporters in the inner mitochondrial membrane. The transfer to the cytosol occurs as a result of its reduction to malate, that, on the contrary, can cross the inner mitochondrial membrane. The reaction is catalyzed by mitochondrial malate dehydrogenase (EC 1.1.1.37), an enzyme also involved in the citric acid cycle, where the reaction proceeds in the reverse direction. In the reaction NADH is oxidized to NAD+.

Oxaloacetate + NADH + H+ ⇄ Malate + NAD+

Although ΔG°’ of the reaction is highly positive, under physiological conditions, ΔG is close to zero, and the reaction is easily reversible.
Malate crosses the inner mitochondrial membrane through a component of the malate-aspartate shuttle, the malate-α-ketoglutarate transporter. Once in the cytosol, the malate is re-oxidized to oxaloacetate in the reaction catalyzed by cytosolic malate dehydrogenase. In this reaction NAD+ is reduced to NADH.

Malate + NAD+ → Oxaloacetate + NADH + H+

Note: Malate-aspartate shuttle is the most active shuttle for the transport of NADH-reducing equivalents from the cytosol into the mitochondria. It is found in mitochondria of liver, kidney, and heart.
The reaction enables the transport into the cytosol of mitochondrial reducing equivalents in the form of NADH. This transfer is needed for gluconeogenesis to proceed, as in the cytosolic the NADH, oxidized in the  reaction catalyzed by glyceraldehydes 3-phosphate dehydrogenase (EC 1.2.1.12), is present in very low concentration, with a [NADH]/[NAD+] ratio equal to 8×10-4, about 100,000 times lower than that observed in the mitochondria.
Finally, the oxaloacetate is converted to phosphoenolpyruvate in the reaction catalyzed by PEP carboxykinase.

Phosphoenolpyruvate precursor: lactate

Lactate is one of the major gluconeogenic precursors. It is produced for example by:

  • red blood cells, that are completely dependent on anaerobic glycolysis for ATP production;
  • skeletal muscle during intense exercise, that is, under low oxygen condition, when the rate of glycolysis exceeds the rate of the citric acid cycle and oxidative phosphorylation.

When lactate is the gluconeogenic precursor, PEP synthesis occurs through a different pathway than that previously seen. In the hepatocyte cytosol NAD+ concentration is high and the lactate is oxidized to pyruvate in the reaction catalyzed by the liver isoenzyme of lactate dehydrogenase (EC 1.1.1.27). In the reaction NAD+ is reduced to NADH.

Lactate + NAD+ → Pyruvate + NADH + H+

The production of cytosolic NADH makes unnecessary the export of reducing equivalents from the mitochondria.
Pyruvate enters the mitochondrial matrix to be converted to oxaloacetate in the reaction catalyzed by pyruvate carboxylase. In the mitochondria, oxaloacetate is converted to phosphoenolpyruvate in the reaction catalyzed by mitochondrial pyruvate carboxylase. Phosphoenolpyruvate exits the mitochondria through an anion transporter located in the inner mitochondrial membrane, and, once in the cytosol, continues in the gluconeogenesis pathway.
Note: The synthesis of glucose from lactate may be considered as the part of  the Cori cycle that takes place in the liver.

From fructose 1,6-bisphosphate to fructose 6-phosphate

The second step of gluconeogenesis that bypasses an irreversible step of the glycolytic pathway, namely the reaction catalyzed by PFK-1, is the dephosphorylation of fructose 1,6-bisphosphate to fructose 6-phosphate.
This reaction, catalyzed by fructose 1,6-bisphosphatase or FBPasi-1 (EC 3.1.3.11), a Mg2+ dependent enzyme located in the cytosol, leads to the hydrolysis of the C-1 phosphate of fructose 1,6-bisphosphate, without production of ATP.

Fructose 1,6-bisphosphate + H2O → Fructose 6-phosphate + Pi

The ΔG°’ of the reaction is -16.3 kJ/mol (-3.9 kcal/mol), therefore an irreversible reaction.

From glucose 6-phosphate to glucose

The third step of gluconeogenesis that bypasses an irreversible step of the glycolytic pathway, namely the reaction catalyzed by hexokinase or glucokinase, is the dephosphorylation of glucose 6-phosphate to glucose.
This reaction is catalyzed by the catalytic subunit of glucose 6-phosphatase, a protein complex located in the membrane of the endoplasmic reticulum of hepatocytes, enterocytes and cells of the proximal tubule of the kidney. Glucose 6-phosphatase complex is composed of a glucose 6-phosphatase catalytic subunit and a glucose 6-phosphate transporter called glucose 6-phosphate translocase or T1.
Glucose 6-phosphatase catalytic subunit has the active site on the luminal side of the organelle. This means that the enzyme catalyzes the release of glucose not in the cytosol but in the lumen of the endoplasmic reticulum.
Glucose 6-phosphate, both resulting from gluconeogenesis, produced in the reaction catalyzed by glucose 6-phosphate isomerase or phosphoglucose isomerase (EC 5.3.1.9), and glycogenolysis, produced in the reaction catalyzed by phosphoglucomutase (EC 5.4.2.2), is located in the cytosol, and must enter the lumen of the endoplasmic reticulum to be dephosphorylated. Its transport is mediated by glucose-6-phosphate translocase.

The catalytic subunit of glucose 6-phosphatase, a Mg2+-dependent enzyme, catalyzes the last step of both gluconeogenesis and glycogenolysis. And, like the reaction catalyzed by fructose 1,6-bisphosphatase, this reaction leads to the hydrolysis of a phosphate ester.

Glucose 6-phosphate + H2O → Glucose + Pi

It should also be underlined that, due to orientation of the active site, the cell separates this enzymatic activity from the cytosol, thus avoiding that glycolysis, that occurs in the cytosol, is aborted by enzyme action on glucose 6-phosphate.
The ΔG°’ of the reaction is -13.8 kJ/mol (-3.3 kcal/mol), therefore it is an irreversible reaction. If instead the reaction were that catalyzed by hexokinase/glucokinase in reverse, it would require the transfer of a phosphate group from glucose 6-phosphate to ADP. Such a reaction would have a ΔG equal to +33.4 kJ/mol (+8 kcal/mol), and then strongly endergonic. Similar considerations can be made for the reaction catalyzed by FBPase-1.
Glucose and Pi group seem to be transported into the cytosol via different transporters, referred to as T2 and T3, the last one an anion transporter.
Finally, glucose leaves the hepatocyte via the membrane transporter GLUT2, enters the bloodstream and is transported to tissues that require it. Conversely, under physiological conditions, as previously said, glucose produced by the kidney is mainly used by the medulla of the kidney itself.

Gluconeogenesis: energetically expensive

Like glycolysis, much of the energy consumed is used in the irreversible steps of the process.
Six high-energy phosphate bonds are consumed: two from GTP and four from ATP. Furthermore, two molecules of NADH are required for the reduction of two molecules of 1,3-bisphosphoglycerate in the reaction catalyzed by glyceraldehyde 3-phosphate dehydrogenase. The oxidation of NADH causes  the lack of production of 5 molecules of ATP that are synthesized when the electrons of the reduced coenzyme are used in oxidative phosphorylation.
Also these energetic considerations show that gluconeogenesis is not simply glycolysis in reverse, in which case it would require the consumption of two molecules of ATP, as shown by the overall glycolytic equation.

Glucose + 2 ADP + 2 Pi + 2 NAD+ → 2 Pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O

Below, the overall equation for gluconeogenesis:

2 Pyruvate + 4 ATP + 2 GTP + 2 NADH+ + 2 H+ + 4 H2O → Glucose + 4 ADP + 2 GDP + 6 Pi + 2 NAD+

At least in the liver, ATP needed for gluconeogenesis derives mostly from the oxidation of fatty acids or of the carbon skeletons of the amino acids, depending on the available “fuel”.

Coordinated regulation of gluconeogenesis and glycolysis

If glycolysis and gluconeogenesis were active simultaneously at a high rate in the same cell, the only products would be ATP consumption and heat production, in particular at the irreversible steps of the two pathways, and nothing more.
For example, considering PFK-1 and FBPasi-1:

ATP + Fructose 6-phosphate → ADP + Fructose 1,6-bisphosphate

Fructose 1,6-bisphosphate + H2O → Fructose 6-phosphate + Pi

The sum of the two reactions is:

ATP + H2O → ADP + Pi + Heat

Two reactions that run simultaneously in opposite directions result in a futile cycle or substrate cycle. These apparently uneconomical cycles allow to regulate opposite metabolic pathways. In fact, a substrate cycle involves different enzymes, at least two, whose activity can be regulated separately. A such regulation would not be possible if a single enzyme would operate in both directions. The modulation of the activity of involved enzymes occurs through:

  • allosterical mechanisms;
  • covalent modifications, such as phosphorylation and dephosphorylation;
  • changes in the concentration of the involved enzymes, due to changes in their synthesis/degradation ratio.

Allosteric mechanisms are very rapid and instantly reversible, taking place in milliseconds. The others, triggered by signals from outside the cell, such as hormones, like insulin, glucagon, or epinephrine, take place on a time scale of seconds or minutes, and, for changes in enzyme concentration, hours.
This allows a coordinated regulation of the two pathways, ensuring that when pyruvate enters gluconeogenesis, the flux of glucose through the glycolytic pathway slows down, and vice versa.

Regulation of gluconeogenesis

The regulation of gluconeogenesis and glycolysis involves the enzymes unique to each pathway, and not the common ones.
While the major control points of glycolysis are the reactions catalyzed by PFK-1 and pyruvate kinase, the major control points of gluconeogenesis are the reactions catalyzed by fructose 1,6-bisphosphatase and pyruvate carboxylase.
The other two enzymes unique to gluconeogenesis, glucose-6-phosphatase and PEP carboxykinase, are regulated at transcriptional level.

Pyruvate carboxylase

In the mitochondrion, pyruvate can be converted to:

  • acetyl-CoA, in the reactions catalyzed by pyruvate dehydrogenase complex, reaction that connects glycolysis to the Krebs cycle;
  • oxaloacetate, in the reaction catalyzed by pyruvate carboxylase, to continue in the gluconeogenesis pathway.

The metabolic fate of pyruvate depends on the availability of acetyl-CoA, that is, by the availability of fatty acids in the mitochondrion.
When fatty acids are available, their β-oxidation leads to the production of acetyl-CoA, that enters the Krebs cycle and leads to the production of GTP and NADH. When the energy needs of the cell are met, oxidative phosphorylation slows down, the [NADH]/[NAD+] ratio increases, NADH inhibits the citric acid cycle, and acetyl-CoA accumulates in the mitochondrial matrix. Acetyl-CoA is a positive allosteric effector of pyruvate carboxylase, and a negative allosteric effector of pyruvate kinase. Moreover, it inhibits pyruvate dehydrogenase complex both through feedback inhibition and phosphorylation through the activation of a specific kinase.

Two fates for pyruvate: synthesis of glucose or energy production, and role of acetil-CoA
Fates for Pyruvate

This means that when the energy charge of the cell is high, the formation of acetyl-CoA from pyruvate slows down, while the conversion of pyruvate to glucose is stimulated. Therefore acetyl-CoA is a molecule that signals that additional glucose oxidation for energy is not required and that glucogenic precursors can be used for the synthesis and storage of glucose.
Conversely, when acetyl-CoA levels decrease, the activity of pyruvate kinase and of the pyruvate dehydrogenase complex increases, and therefore also the flow of metabolites through the citric acid cycle. This supplies energy to the cell.
Summarizing, when the energy charge of the cell is high pyruvate carboxylase is active, and that the first control point of gluconeogenesis determines what will be the fate of pyruvate in the mitochondria.

Fructose 1,6-bisphosphatase

The second major control point in gluconeogenesis is the reaction catalyzed by fructose 1,6-bisphosphatase. The enzyme is allosterically inhibited by AMP. Therefore, when AMP levels are high, and consequently ATP levels are low, gluconeogenesis slows down. This means that, as previously seen, FBPase-1 is active when the energy charge of the cell is sufficiently high to support de novo synthesis of glucose.
Conversely, PFK-1, the corresponding glycolytic enzyme, is allosterically activated by AMP and ADP and allosterically inhibited by ATP and citrate, the latter resulting from the condensation of acetyl-CoA and oxaloacetate. Therefore:

  • when AMP levels are high, gluconeogenesis slows down, and glycolysis accelerates;
  • when ATP levels are high or when acetyl-CoA or citrate are present in adequate concentrations, gluconeogenesis is promoted, while glycolysis slows down.
    The increase in citrate levels indicates that the activity of the citric acid cycle can slow down; in this way,  pyruvate can be used in glucose synthesis.

PFK-1, FBPase-1 and fructose 2,6-bisphosphate

The liver plays a key role in maintaining blood glucose homeostasis: this requires regulatory mechanisms that coordinate glucose consumption and production. Two hormones are mainly involved: glucagon and insulin. They act intracellularly through fructose 2,6-bisphosphate or F2,6BP, an allosteric effector of PFK-1 and FBPase-1. This molecule is structurally related to fructose 1,6-bisphosphate, but is not an intermediate in glycolysis or gluconeogenesis.
It was discovered in 1980 by Emile Van Schaftingen and Henri-Gery Hers, as a potent activator of PFK-1. In the subsequent year, the same researchers showed that it is also a potent inhibitor of FBPase-1.
Fructose 2,6-bisphosphate, by binding to the allosteric site on PFK-1, reduces the affinity of the enzyme for ATP and citrate, allosteric inhibitors, and at the same time increases the affinity of the enzyme for fructose 6-phosphate, its substrate. PFK-1, in the absence of fructose 2,6-bisphosphate, and in the presence of physiological concentrations of ATP, fructose 6-phosphate, and of allosteric effectors AMP, ATP and citrate, is practically inactive. Conversely, the presence of fructose 2,6-bisphosphate activates PFK-1, thus stimulating glycolysis in the hepatocytes. At the same time fructose 2,6-bisphosphate slows down gluconeogenesis by inhibiting fructose 1,6-bisphosphatase, even in the absence of AMP. However, the effects of fructose-2,6-bisphosphate and AMP on FBPase-1 activity  are synergistic.

Role of fructose 2,6-bisphosphate in the regulation of gluconeogenesis and glycolysis
F2,6BP: Regulation of Glycolysis and Gluconeogenesis

Fructose-2,6-bisphosphate concentration is regulated by the relative rates of synthesis and degradation. It is synthesized from fructose 6-phosphate in the reaction catalyzed by phosphofructokinase-2 or PFK-2 (EC 2.7.1.105), and is hydrolyzed to fructose 6-phosphate in the reaction catalyzed by fructose 2,6-bisphosphatase or FBPasi-2 (EC 3.1.3.46). These two enzymatic activities are located on a single bifunctional enzyme or tandem enzyme. In the liver, the balance of these two enzymatic activities is regulated by insulin and glucagon, as described below.

  • Glucagon
    It is released into the circulation when blood glucose levels drop, signaling the liver to reduce glucose consumption for its own needs and to increase de novo synthesis of glucose and its release from glycogen stores.
    After binding to specific membrane receptors, glucagon stimulates hepatic adenylate cyclase (EC 4.6.1.1) to synthesize 3′,5′-cyclic AMP or cAMP, that activates cAMP-dependent protein kinase or protein kinase A or PKA (EC 2.7.11.11). The kinase catalyzes the phosphorylation, at the expense of one molecule of ATP, of a specific serine residue (Ser32) of PFK-2/FBPase-2. As a result of the phosphorylation, phosphatase activity increases while kinase activity decreases. Such reduction, due to the increase in the Km for fructose 6-phosphate, causes a decrease in the levels of fructose 2,6-bisphosphate, that, in turn, inhibits glycolysis and stimulates gluconeogenesis. Therefore, in response to glucagon, hepatic production of glucose increases, enabling the organ to counteract the fall in blood glucose levels.
    Note: glucagon, like adrenaline, stimulates gluconeogenesis also by increasing the availability of substrates such as glycerol and amino acids.
  • Insulin
    After binding to specific membrane receptors, insulin activates a protein phosphatase, the phosphoprotein phosphatase 2A or PP2A, that catalyzes the removal of the phosphate group from PFK-2/FBPase-2, thus increasing PFK-2 activity and decreasing FBPase-2 activity. (At the same time, insulin also stimulates a cAMP phosphodiesterase that hydrolyzes cAMP to AMP). This increases the level of fructose 2,6-bisphosphate, that, in turn, inhibits gluconeogenesis and stimulates glycolysis.
    In addition, fructose 6-phosphate allosterically inhibits FBPase-2, and activates PFK-2. It should be noted that the activities of PFK-2 and FBPase-2 are inhibited by their reaction products. However, the main effectors are the level of fructose 6-phosphate and the phosphorylation state of the enzyme.

Glucose 6-phosphatase

Unlike pyruvate carboxylase and fructose-1,6-bisphosphatase, the catalytic subunit of glucose-6-phosphatase is not subject to allosteric or covalent regulation. The modulation of its activity occurs at the transcriptional level. Low blood glucose levels and glucagon, namely, factors that lead to increased glucose production, and glucocorticoids stimulate its synthesis, that, conversely, is inhibited by insulin.
Also, the Km for glucose 6-phosphate is significantly higher than the range of physiological concentrations of glucose 6-phosphate itself. This is why it is said that the activity of the enzyme is almost linearly dependent on the concentration of the substrate, that is, enzyme is controlled by the level of substrate.

PEP carboxykinase

The enzyme is regulated mainly at the level of synthesis and degradation. For example, high levels of glucagon or fasting increase protein production through the stabilization of its mRNA and the increase in its transcription rate. High blood glucose levels or insulin have opposite effects.

Xylulose 5-phosphate

Xylulose 5-phosphate, a product of the pentose phosphate pathway, is a recently discovered regulatory molecule. It stimulates glycolysis and inhibits gluconeogenesis by controlling the levels of fructose 2,6-bisphosphate in the liver.
When blood glucose levels increase, e.g. after a meal high in carbohydrates, the activation of glycolysis and hexose monophosphate pathway occurs in the liver. Xylulose 5-phosphate produced activates protein phosphatase 2A, that, as previously said, dephosphorylates PFK-2/FBPase-2, thus inhibiting FBPase-2 and stimulating PFK-2. This leads to an increase in the concentration of fructose 2,6-bisphosphate, and then to the inhibition of gluconeogenesis and stimulation of glycolysis, resulting in increased production of acetyl-CoA, the main substrate for lipid synthesis. At the same time, an increase in flow through the hexose monophosphate shunt occurs, leading to the production of NADPH, a source of electrons for lipid synthesis. Finally, PP2A also dephosphorylates carbohydrate-responsive element-binding protein or ChREBP, a transcription factor that activates the expression of hepatic genes for lipid synthesis. Therefore, in response to an increase in blood glucose levels, lipid synthesis is stimulated.

It is therefore evident that xylulose 5-phosphate is a key regulator of carbohydrate and fat metabolism.

Precursors of gluconeogenesis

Besides the aforementioned pyruvate, the major gluconeogenic precursors are lactate, glycerol, the majority of the amino acids, and, more generally, any compound that can be converted to pyruvate or oxaloacetate.

Glycerol

Glycerol is released by lipolysis in adipose tissue. With the exception of propionyl-CoA, it is the only part of the lipid molecule that can be used for de novo synthesis of glucose in animals.
Glycerol enters gluconeogenesis, or glycolysis, depending on the cellular energy charge, as dihydroxyacetone phosphate or DHAP, whose synthesis occurs in two steps.
In the first step, glycerol is phosphorylated to glycerol 3-phosphate, in the reaction catalyzed by glycerol kinase (EC 2.7.1.30), with the consumption of one ATP.

Glycerol + ATP → Glycerol 3-phosphate + ADP + Pi

The enzyme is absent in adipocytes but present in the liver; this means that glycerol needs to reach the liver to be further metabolized.
Glycerol 3-phosphate is then oxidized to dihydroxyacetone phosphate, in the reaction catalyzed by glycerol 3-phosphate dehydrogenase (EC 1.1.1.8). In this reaction NAD+ is reduced to NADH.

Glycerol 3-phosphate + NAD+ ⇄ Dihydroxyacetone phosphate + NADH + H+

During prolonged fasting, glycerol is the major gluconeogenic precursor, accounting for about 20% of glucose production.

Glucogenic amino acids

Pyruvate and oxaloacetate are the entry points for the glucogenic amino acids, i.e. those whose carbon skeleton or part of it can be used for de novo synthesis of glucose.
Amino acids result from the catabolism of proteins, both food and endogenous proteins, like those of skeletal muscle during the fasting state or during intense and prolonged exercise.
The catabolic processes of each of the twenty amino acids that made up the proteins converge to form seven major products, acetyl-CoA, acetoacetyl-CoA, α-ketoglutarate, succinyl-CoA, fumarate, oxaloacetate, and pyruvate.
Except acetyl-CoA, acetoacetyl-CoA , the other five molecules can be used for gluconeogenesis. This means that gluconeogenic amino acids may also be defined as those whose carbon skeleton or part of it can be converted to one or more of the above molecules.
Below, the entry points of the gluconeogenic amino acids are shown.

  • Pyruvate: alanine, cysteine, glycine, serine, threonine and tryptophan.
  • Oxaloacetate: aspartate and asparagine.
  • α-Ketoglutarate: glutamate, arginine, glutamine, histidine and proline.
  • Succinyl-CoA: isoleucine, methionine, threonine and valine.
  • Fumarate: phenylalanine and tyrosine.
Glucogenic and ketogenic amino acids and their entry to the citric acid cycle
Glucogenic and Ketogenic Amino Acids

α-Ketoglutarate, succinyl-CoA and fumarate, intermediates of the citric acid cycle, enter the gluconeogenic pathway after conversion to oxaloacetate.
The utilization of the carbon skeletons of the amino acids requires the removal of the amino group. Alanine and glutamate, the key molecules in the transport of amino groups from extrahepatic tissues to the liver, are major glucogenic amino acids in mammals. Alanine is the main gluconeogenic substrate for the liver; this amino acid is shuttled to the liver from muscle and other peripheral tissues through the glucose-alanine cycle.

Ketogenic amino acids

Acetyl-CoA and acetoacetyl-CoA cannot be used for gluconeogenesis and are precursors of fatty acids and ketone bodies. The stoichiometry of the citric acid cycle elucidates why they cannot be used for de novo synthesis of glucose.
Acetyl-CoA, in the reaction catalyzed by citrate synthase, condenses with oxaloacetate to form citrate, a molecule with 6 carbon atoms instead of 4 as oxaloacetate. However, although the two carbon atoms from acetyl-CoA become part of the oxaloacetate molecule, two carbon atoms are oxidized and removed  as CO2, in the reactions catalyzed by isocitrate dehydrogenase (EC 1.1.1.42) and α-ketoglutarate dehydrogenase complex. Therefore, acetyl-CoA does not yield any net carbon gain for the citric acid cycle.
Furthermore, the reaction leading to the formation of acetyl-CoA from pyruvate, catalyzed by the pyruvate dehydrogenase complex, that is the bridge between glycolysis and the Krebs cycle, is irreversible, and there is no other pathway to convert acetyl-CoA to pyruvate.

Pyruvate + NAD+ + CoASH → Acetyl-CoA + NADH + H+ + C02

For this reason, amino acids whose catabolism produces acetyl-CoA and/or acetoacetyl-CoA, are termed ketogenic.
Only leucine and lysine are exclusively ketogenic.

Note: Plants, yeasts, and many bacteria can use acetyl-CoA for de novo synthesis of glucose as they do have the glyoxylate cycle. This cycle has four reactions in common with the citric acid cycle, two unique enzymes, isocitrate lyase (EC 4.1.3.1) and malate synthase (EC 2.3.3.9), but lacks the decarboxylation reactions. Therefore, organisms that have such pathway are able to use fatty acids for gluconeogenesis.

Five amino acids, isoleucine, phenylalanine, tyrosine, threonine and tryptophan, are both glucogenic and ketogenic, because part of their carbon backbone can be used for gluconeogenesis, while the other gives rise to ketone bodies.

Propionate

Propionate, a three carbon fatty acid, is a gluconeogenic precursor because, as propionyl-CoA, the active molecule, can be converted to succinyl-CoA.
Below, the different sources of propionate are analyzed.

  • It may arise from β-oxidation of odd-chain fatty acids such as margaric acid, a saturated fatty acid with 17 carbon atoms. Such fatty acids are rare compared to even-chain fatty acids, but present in significant amounts in the lipids of some marine organisms, ruminants, and plants. In the last pass through the β-oxidation sequence, the substrate is a five carbon fatty acid. This means that, once oxidized and cleaved to two fragments, it produces an acetyl-CoA and propionyl-CoA.
  • Another source is the oxidation of branched-chain fatty acids, with alkyl branches with an odd number of carbon atoms. An example is phytanic acid, produced in ruminants by oxidation of phytol, a breakdown product of chlorophyll.
  • In ruminants, propionate is also produced from glucose. Glucose is released from breakdown of cellulose by bacterial cellulase (EC 3.2.1.4) in the rumen, one of the four chambers that make up the stomach of these animals. These microorganisms then convert, through fermentation, glucose to propionate, which, once absorbed, may be used for gluconeogenesis, synthesis of fatty acids, or be oxidized for energy.
    In ruminants, in which gluconeogenesis tends to be a continuous process, propionate is the major gluconeogenic precursor.
  • Propionate may also result from the catabolism of valine, leucine, and isoleucine (see above).

The oxidation of propionyl-CoA to succinyl-CoA involves three reactions that occur in the liver and other tissues.
In the first reaction, propionyl-CoA is carboxylated to D-methylmalonyl-CoA in the reaction catalyzed by propionyl-CoA carboxylase (EC 6.4.1.3), a biotin-requiring enzyme. This reaction consumes one ATP.

Propionyl-CoA + HCO3 + ATP → D-methylmalonyl-CoA+ ADP + Pi

In the subsequent reaction, catalyzed by methylmalonyl-CoA epimerase (EC 5.1.99.1), D-methylmalonyl-CoA is epimerized to its L-stereoisomer.

D-Methylmalonyl-CoA ⇄ L-Methylmalonyl-CoA

Finally, L-methylmalonyl-CoA undergoes an intramolecular rearrangement to succinyl-CoA, in the reaction catalyzed by methylmalonyl-CoA mutase (EC 5.4.99.2). This enzyme requires 5-deoxyadenosylcobalamin or coenzyme B12, a derivative of cobalamin or vitamin B12, as a coenzyme.

L-Methylmalonyl-CoA ⇄ Succinyl-CoA

References

Bender D.A. Introduction to nutrition and metabolism. 3rd Edition. Taylor & Francis, 2004

Garrett R.H., Grisham C.M. Biochemistry. 4th Edition. Brooks/Cole, Cengage Learning, 2010

Kabashima T., Kawaguchi T., Wadzinski B.E., Uyeda K. Xylulose 5-phosphate mediates glucose-induced lipogenesis by xylulose 5-phosphate-activated protein phosphatase in rat liver. Proc Natl Acad Sci USA 2003;100:5107-5112. doi:10.1073/pnas.0730817100

Kuriyama H. et all. Coordinated regulation of fat-specific and liver-specific glycerol channels, aquaporin adipose and aquaporin 9. Diabetes 2002;51(10):2915-2921. doi:10.2337/diabetes.51.10.2915

McCommis K.S. and Finck B.N. Mitochondrial pyruvate transport: a historical perspective and future research directions. Biochem J 2015;466(3):443-454. doi:10.1042/BJ20141171

Nelson D.L., M. M. Cox M.M. Lehninger. Principles of biochemistry. 6th Edition. W.H. Freeman and Company, 2012

Rosenthal M.D., Glew R.H. Medical biochemistry – Human metabolism in health and disease. John Wiley J. & Sons, Inc., Publication, 2009

Soty M., Chilloux J., Delalande F., Zitoun C., Bertile F., Mithieux G., and Gautier-Stein A. Post-Translational regulation of the glucose-6-phosphatase complex by cyclic adenosine monophosphate is a crucial determinant of endogenous glucose production and is controlled by the glucose-6-phosphate transporter. J Proteome Res  2016;15(4):1342-1349. doi:10.1021/acs.jproteome.6b00110

Stipanuk M.H., Caudill M.A. Biochemical, physiological, and molecular aspects of human nutrition. 3rd Edition. Elsevier health sciences, 2012

Van Schaftingen E., and Hers H-G. Inhibition of fructose-1,6-bisphosphatase by fructose-2,6-bisphosphate. Proc Natl Acad Sci USA 1981;78(5):2861-2863. doi:10.1073/pnas.78.5.2861

Van Schaftingen E., Jett M-F., Hue L., and Hers, H-G. Control of liver 6-phosphofructokinase by fructose 2,6-bisphosphate and other effectors. Proc Natl Acad Sci USA 1981;78(6):3483-3486. doi:10.1073/pnas.78.6.3483

Glucose-alanine cycle: steps and importance

The glucose-alanine cycle, or Cahill cycle, proposed for the first time by Mallette, Exton and Park, and Felig et al. between 1969 and 1970, consists of a series of steps through which extrahepatic tissues, for example the skeletal muscle, export pyruvate and amino groups as alanine to the liver, and receive glucose from the liver via the bloodstream.
The main steps of the glucose-alanine cycle are summarized below.

  • When in extrahepatic tissues amino acids are used for energy, pyruvate, derived from glycolysis, is used as amino group acceptor, forming alanine, a nonessential amino acid.
  • Alanine diffuses into the bloodstream and reaches the liver.
  • In the liver, the amino group of alanine is transferred to α-ketoglutarate to form pyruvate and glutamate, respectively.
  • The amino group of glutamate mostly enters the urea cycle, and in part acts as a nitrogen donor in many biosynthetic pathways.
    Pyruvate enters gluconeogenesis and is used for glucose synthesis.
  • The newly formed glucose diffuses into the bloodstream and reaches the peripheral tissues where, due to glycolysis, is converted into pyruvate that can accept amino groups from the free amino acids, thus closing the cycle.

Therefore, the glucose-alanine cycle provides a link between carbohydrate and amino acid metabolism, as schematically described below.

Glucose → Pyruvate → Alanine → Pyruvate → Glucose

The steps of glucose-alanine cycle in liver and muscle
Glucose-Alanine Cycle

The glucose-alanine cycle occurs not only between the skeletal muscle, the first tissue in which it was observed, and the liver, but involves other cells and extrahepatic tissues including cells of the immune system, such as lymphoid organs.

CONTENTS

The steps of the glucose-alanine cycle

The analysis of the steps of the glucose-alanine cycle is made considering the cycle between skeletal muscle and the liver.
Both intracellular and extracellular proteins are continuously hydrolyzed to the constituent amino acids and resynthesized, and the rate at which these processes occur is balanced precisely, thereby preventing loss of fat free mass.
However, under catabolic conditions, such as intense and prolonged exercise or fasting, the rate of muscle protein breakdown exceeds synthesis. This leads to the liberation of amino acids, some of which are used for energy and others for gluconeogenesis. And the oxidation of the carbon skeletons of amino acids, in particular branched chain amino acids or BCAA, leucine, isoleucine and valine, may be a significant source of energy for the muscle. For example, after about 90 minutes of strenuous exercise, amino acid oxidation in muscle provides 10-15% of the energy needed for contraction.
The utilization of the carbon skeletons of amino acids for energy involves the removal of the amino group, and then the excretion of amino nitrogen in a non-toxic form.
The removal of the α-amino group occurs by transamination, that can be summarized as follows:

α-Keto acid + Amino acid ⇄ New amino acid + New α-keto acid

Such reactions, catalyzed by enzymes called aminotransferases or transaminases (EC 2.6.1) are freely reversible.
Branched chain amino acids, for example, transfer the amino group to α-ketoglutarate or 2-oxoglutaric acid, to form glutamate and the α-keto acid derived from the original amino acid, in a reaction catalyzed by branched chain aminotransferase or BCAT (EC 2.6 .1.42).

The Cahill cycle in skeletal muscle

In skeletal muscle, the newly formed glutamate may react with ammonia to form glutamine, for many tissues and organs, such as the brain, the major vehicle for interorgan transport of nitrogen. The reaction is catalyzed by the cytosolic enzyme glutamine synthetase (EC 6.3.1.2), and consumes an ATP.

Glutamate + NH4+ + ATP → Glutamine + ADP + Pi

In this case, glutamate leaves the Cahill cycle.
Alternatively, and in contrast to what happens in most of the other tissues, the newly formed glutamate may transfer the amino group to pyruvate, derived from glycolysis, to form alanine and α-ketoglutarate. This transamination is catalyzed by alanine aminotransferase or ALT (EC 2.6.1.2), an enzyme found in most animal and plant tissues.

Glutamate + Pyruvate ⇄ Alanine + α-Ketoglutarate

The alanine produced and that derived directly from protein breakdown, and muscle proteins are rich in alanine, can leave the cell and be carried by the bloodstream to the liver; in this way the amino group reaches the liver. And the rate at which alanine formed by transamination of pyruvate is transferred into the circulation is proportional to the intracellular pyruvate production.
Note: Alanine and glutamine are the major sources of nitrogen and carbon in interorgan amino acid metabolism.

The Cahill cycle in the liver

Once in the liver, a hepatic alanine aminotransferase catalyzes a transamination in which alanine, the major gluconeogenic amino acid, acts as an amino group donor and α-ketoglutarate as an α-keto acid acceptor. The products of the reaction are pyruvate, i.e. the carbon skeleton of alanine, and glutamate.

Alanine + α-Ketoglutarate ⇄ Glutamate + Pyruvate

Glutamate, in the reaction catalyzed by glutamate dehydrogenase (EC 1.4.1.2), an enzyme present in the mitochondrial matrix, forms ammonium ion, which enters the urea cycle, and α-ketoglutarate, which can enter the Krebs cycle. This reaction is an anaplerotic reaction that links amino acid metabolism with the Krebs cycle.

Glutamate + H2O + NAD+ ⇄ α-Ketoglutarate + NH4+ + NADH + H+

However, glutamate can also react with oxaloacetate to form aspartate and α-ketoglutarate, in a reaction catalyzed by aspartate aminotransferase (EC 2.6.1.1). Aspartate is involved in the formation of urea as well as in the synthesis of purines and pyrimidines.

Glutamate + Oxaloacetate ⇄ Aspartate + α-Ketoglutarate

Also the pyruvate produced may have different metabolic fates: it can be oxidized for ATP production, and then leave the glucose-alanine cycle, or enter the gluconeogenesis pathway, and thus continue in the cycle.
The glucose produced is released from the liver into the bloodstream and delivered to various tissues that require it, as the skeletal muscle, in which it is used for pyruvate synthesis. In turn, the newly formed pyruvate may react with glutamate, thus closing the cycle.

Transaminases

As previously mentioned, the removal of the amino group from amino acids occurs through transamination (see above for the general reaction). These reactions are catalyzed by enzymes called aminotransferases or transaminases.
They are cytosolic enzymes, present in all cells and particularly abundant in the liver, kidney, intestine and muscle; they require pyridoxal phosphate or PLP, the active form of vitamin B6 or pyridoxine, as a coenzyme, which is tightly bound to the active site.
In transamination reactions, the amino group of free amino acids, except of threonine and lysine, is channeled towards a small number of α-keto acids, notably pyruvate, oxaloacetate and α-ketoglutarate.
Cells contain different types of aminotransferases: many are specific for α-ketoglutarate as α-keto acid acceptor, but differ in specificity for the amino acid, from which they are named. Examples are the aforementioned alanine aminotransferase, also called alanine transaminase and glutamic pyruvic transferase or GPT, and aspartate aminotransferase or AST, also called glutamic-oxaloacetic transaminase or GOT.
It should be underlined that there is no net deamination in these reactions, no loss of amino groups, as the α-keto acid acceptor is aminated and the amino acid deaminated.

Functions of the glucose-alanine cycle

This cycle has various functions.

  • It transports nitrogen in a non-toxic form from peripheral tissues to the liver.
  • It transports pyruvate, a gluconeogenic substrate, to the liver.
  • It removes pyruvate from peripheral tissues.  This leads to a higher production of ATP from glucose in these tissues. In fact, the NADH produced during glycolysis can enter the mitochondria and be oxidized through oxidative phosphorylation.
  • It allows to maintain a relatively high concentration of alanine in hepatocytes, sufficient to inhibit protein degradation.
  • It may play a role in host defense against infectious diseases.

Finally, it is important to underline that there is no net synthesis of glucose in the glucose-alanine cycle.

Energy cost of the glucose-alanine cycle

Like the Cori cycle, also the glucose-alanine cycle has an energy cost, equal to 3-5 ATP.
The part of the cycle that takes place in peripheral tissues involves the production of 5-7 ATP per molecule of glucose:

  • 2 ATP are produced by glycolysis;
  • 3-5 ATP derive from NADH/FADH2 (see below).

Instead in the liver, gluconeogenesis and the urea cycle cost 10 ATP:

  • 6 ATP are consumed in the during gluconeogenesis per molecule of glucose synthesized;
  • 4 ATP are consumed in the urea cycle per molecule of urea synthesized.

The glucose-alanine cycle, like the Cori cycle, shifts part of the metabolic burden from extrahepatic tissues to the liver. However, the energy cost paid by the liver is justified by the advantages that the cycle brings to the whole body, as it allows, in particular conditions, an efficient breakdown of proteins in extrahepatic tissues (especially skeletal muscle), which in turn allows to obtain gluconeogenic substrates as well as the use of amino acids for energy in extrahepatic tissues.

Similarities and differences between Cahill cycle and Cori cycle

There are some analogies between the two cycles, which are listed below.

  • The Cahill cycle partially overlaps the Cori cycle when pyruvate is converted to glucose and the monosaccharide is transported to extrahepatic tissues, in which it is converted again to pyruvate via the glycolytic pathway.
  • The entry into gluconeogenesis pathway is similar for the two cycles: both alanine and lactate are converted to pyruvate.
  • Like the Cori cycle, the glucose-alanine cycle occurs between different cell types, unlike metabolic pathways such as glycolysis, Krebs cycle or gluconeogenesis that occur within individual cells
Similarities and differences between glucose-alanine cycle and Cori cycle
Cori cycle vs Glucose-Alanine Cycle

Below, some differences between the two cycles.

  • The main difference concerns the three carbon intermediate that from peripheral tissues reach the liver: lactate in the Cori cycle, and alanine in the glucose-alanine cycle.
  • Another difference concerns the fate of the NADH produced by glycolysis in peripheral tissues.
    In the Cori cycle, the coenzyme acts as reducing agent to reduce pyruvate to lactate, in the reaction catalyzed by lactate dehydrogenase (EC 1.1.1.27).
    In the glucose-alanine cycle, this reduction does not occur and the electrons of NADH can be transported into the mitochondria via the malate-aspartate and glycerol 3-phosphate shuttles, generating NADH, the first shuttle, and FADH2, the other shuttle. And the yield of ATP from NADH and FADH2 is 2.5 and 1.5, respectively.
  • Finally, from the previous point, it is clear that, unlike the Cori cycle, the Cahill cycle requires the presence of oxygen and mitochondria in the peripheral tissues.

References

Felig P., Pozefsk T., Marlis E., Cahill G.F. Alanine: key role in gluconeogenesis. Science 1970;167(3920):1003-1004. doi:10.1126/science.167.3920.1003

Gropper S.S., Smith J.L., Groff J.L. Advanced nutrition and human metabolism. Cengage Learning, 2009

Lecker S.H., Goldberg A.L. and Mitch W.E. Protein degradation by the ubiquitin–proteasome pathway in normal and disease states. J Am Soc Nephrol 2006;17(7):1807-1819. doi:10.1681/ASN.2006010083

Mallette L. E., Exton J.H., and Park C.R. Control of gluconeogenesis from amino acids in the perfused rat liver. J Biol Chem 1969;244(20):5713-5723.

Nelson D.L., Cox M.M. Lehninger. Principles of biochemistry. 6th Edition. W.H. Freeman and Company, 2012

Raju S.M., Madala B. Illustrated medical biochemistry. Jaypee Brothers Publishers, 2005

Wu G. Amino acids: biochemistry and nutrition. CRC Press, 2010

Cori cycle: where it occurs, enzymes, and significance

The Cori cycle, or glucose-lactate cycle, was discovered by Carl Ferdinand Cori and Gerty Theresa Radnitz, a husband-and-wife team, in the ‘30s and ‘40s of the last century . They demonstrated the existence of a metabolic cooperation between the skeletal muscle working under low oxygen conditions and the liver. This cycle can be summarized as follows:

  • the conversion of glucose to lactic acid, or lactate, by anaerobic glycolysis in skeletal muscle cells;
  • the diffusion of lactate from muscle cells into the bloodstream, by which it is transported to the liver;
  • the conversion of lactate to glucose by hepatic gluconeogenesis;
  • the diffusion of glucose from the hepatocytes into the bloodstream, by which it is transported back to the skeletal muscle cells, thereby closing the cycle.

Summarizing, we have: part of the lactate produced in skeletal muscle is converted to glucose in the liver, and transported back to skeletal muscle, thus closing the cycle.

Glucose → Lactate → Glucose

The importance of this cycle is demonstrated by the fact that it may account for about 40% of plasma glucose turnover.

CONTENTS

Where does the Cori cycle occur?

In addition to skeletal muscle, this metabolic cooperation was also demonstrated between other extrahepatic tissues and liver.  Indeed, like the glucose-alanine cycle, the glucose-lactate cycle is active between the liver and all those tissues that do not completely oxidize glucose to CO2 and H2O, in which case pyruvate for conversion to lactate or, by transamination, to alanine would lack (see below).
In addition to skeletal muscle cells, examples of cells that continually produce lactic acid are red blood cells, immune cells in the lymph nodules,  proliferating cells in the bone marrow, and epithelial cells in the skin.
Notice that skeletal muscle produces lactate even at rest, although at low rate.

The steps of the Cori cycle or glucose-lactate cycle
The Cori Cycle

From a biochemical point of view, the Cori cycle links gluconeogenesis with anaerobic glycolysis, using different tissues to compartmentalize opposing metabolic pathways. In fact, in the same cell, regardless of the cell type, these metabolic pathways are not very active simultaneously. Glycolysis is more active when the cell requires ATP; by contrast, when the demand for ATP is low, gluconeogenesis, in those cells where it occurs, is more active.
And it is noteworthy that, although traditionally the metabolic pathways, such as glycolysis, citric acid cycle, or gluconeogenesis, are considered to be confined within individual cells, the Cori cycle, as well as the glucose-alanine cycle, occurs between different cell types.
Finally, it should be underscored that the Cori cycle also involves the renal cortex, particularly the proximal tubules, another site where gluconeogenesis occurs.

Steps of the Cori cycle

The analysis of the steps of the Cori cycle is made considering the lactate produced by red blood cells and skeletal muscle cells.
Mature red blood cells are devoid of mitochondria, nucleus and ribosomes, and obtain the necessary energy only by glycolysis. The availability of NAD+ is essential for glycolysis to proceed as well as for its rate: the oxidized form of the coenzyme is required for the oxidation of glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate in the reaction catalyzed by glyceraldehyde 3-phosphate dehydrogenase (EC 1.2.1.12).

Glyceraldehyde 3-phosphate + NAD+ → 1,3-Bisphosphoglycerate + NADH + H+

The accumulation of NADH is avoided by the reduction of pyruvate to lactate, in the reaction catalyzed by lactate dehydrogenase (EC 1.1.1.27), where NADH acts as reducing agent.

Pyruvate + NADH + H+ → Lactate + NAD+

The skeletal muscle, particularly fast-twitch fibers which contain a reduced number of mitochondria, under low oxygen condition, such as during intense exercise, produces significant amounts of lactate. In fact, in such conditions:

  • the rate of pyruvate production by glycolysis  exceeds the rate of its oxidation by the citric acid cycle, so that less than 10% of the pyruvate enters the citric acid cycle;
  • the rate at which oxygen is taken up by the cells is not sufficient to allow aerobic oxidation of all the NADH  produced.

And, like in red blood cells, the reaction catalyzed by lactate dehydrogenase, regenerating NAD+, allows glycolysis to proceed.
However, lactate is an end product of metabolism that must be converted back into pyruvate to be used.
The plasma membrane of most cells is freely permeable to both pyruvate and lactate that can thus reach the bloodstream. And, regarding for example the skeletal muscle, the amount of lactate that leaves the cell is greater than that of pyruvate due to the high NADH/NAD+ ratio in the cytosol and to the catalytic properties of the skeletal muscle isoenzyme of LDH.
Once into the bloodstream, lactate reaches the liver, which is its major user, where it is oxidized to pyruvate in the reaction catalyzed by the liver isoenzyme of lactate dehydrogenase.

Lactate + NAD+ → Pyruvate + NADH + H+

In the hepatocyte, this oxidation is favored by the low NADH/NAD+ ratio in the cytosol.
Then, pyruvate enters the gluconeogenesis pathway to be converted into glucose.
Glucose leaves the liver, enters into the bloodstream and is delivered to the muscle, as well as to other tissues and cells that require it, such as red blood cells and neurons, thus closing the cycle.

Lactate dehydrogenase

The enzyme is a tetramer composed of two different types of subunits, designed as:

  • H subunit (heart) or B chain;
  • M subunit (muscle) or A chain.

The H subunit predominates in the heart, whereas the M subunit predominates in the  skeletal muscle and liver. Typically, tissues in which a predominantly or exclusively aerobic metabolism occurs, such as the heart, synthesize H subunits to a greater extent than M subunits, whereas tissues in which anaerobic metabolism is important, such as skeletal muscle, synthesize M subunits to a greater extent than H subunits.
The two subunits associate in 5 different ways to form homopolymers, that is, macromolecules formed by repeated, identical subunits, or heteropolymers, that is, macromolecules formed by different subunits. Different LDH  isoenzymes have different catalytic properties, as well as different distribution in various tissues, as indicated below:

  • H4, also called type 1, LDH1, or A4, a homopolymer of H subunits, is found in cardiac muscle, kidney, and red blood cells;
  • H3M1, also called type 2, LDH2, or A3B, has a tissue distribution similar to that of LDH1;
  • H2M2, also called type 3, LDH3, or A2B2, is found in the spleen, brain, white cells, kidney, and lung;
  • H1M3, also called type 4, LDH4, or AB3, is found in the spleen, lung, skeletal muscle, lung, red blood cells, and kidney;
  • M4, also called type 5, LDH5, or B4, a homopolymer of M subunits, is found in the liver, skeletal muscle, and spleen.

The H4 isoenzyme has a higher substrate affinity than the M4 isoenzyme.
The H4 isoenzyme is allosterically inhibited by high levels of pyruvate (its product), whereas the M4 isoenzyme is not.
The other LDH isoenzymes have intermediate properties, depending on the ratio between the two types of subunits.
It is thought that the H4 isoenzyme is the most suitable for catalyzing the oxidation of lactate to pyruvate that, in the heart, due to its exclusively aerobic metabolism, is then completely oxidized to CO2 and H2O. Instead, the M4 isoenzyme is the main isoenzyme found in skeletal muscle, most suitable for catalyzing the reduction of pyruvate to lactate, thus allowing glycolysis to proceed in anaerobic conditions.

Other metabolic fates of lactate

From the above, it is clear that lactate is not a metabolic dead end, a waste product of glucose metabolism.
And it may have a different fate from that entering the Cori cycle.
For example, in skeletal muscle during recovery following an exhaustive exercise, that is, when oxygen is again available, or if the exercise is of low intensity, lactate is re-oxidized to pyruvate, due to NAD+ availability, and then completely oxidized to CO2 and H20, with a greater production of ATP than in anaerobic condition. In such conditions, the energy stored in NADH will be released, yielding on average 2.5 ATP per molecule of NADH.
In addition, lactate can be taken up by exclusively aerobic tissues, such as heart, to be oxidized to CO2 and H20.

Energy cost of the glucose-lactate cycle

The Cori cycle results in a net consumption of 4 ATP.
The gluconeogenic leg of the cycle consumes 2 GTP and 4 ATP per molecule of glucose synthesized, that is, 6 ATP.
The ATP-consuming reactions are catalyzed by:

  • pyruvate carboxylase (EC 6.4.1.1): one ATP;
  • phosphoenolpyruvate carboxykinase (EC 4.1.1.32): one GTP;
  • glyceraldehyde 3-phosphate dehydrogenase (EC 1.2.1.12): one ATP.

Since two molecules of lactate are required for the synthesis of one molecule of glucose, the net cost is 2 x 3 = 6 high energy bonds per molecule of glucose.
Conversely, the glycolytic leg of the cycle produces only 2 ATP per molecule of glucose.
Therefore, more energy is required to produce glucose from lactate than that obtained by anaerobic glycolysis in extrahepatic tissues. This explains why the Cori cycle cannot be sustained indefinitely.

Is the Cori cycle a futile cycle?

The continuous breakdown and resynthesis of glucose, feature of the Cori cycle, might seem like a waste of energy. Indeed, this cycle allows the effective functioning of many extrahepatic cells at the expense of the liver and partly of the renal cortex. Below, two examples.

  • Red blood cells
    These cells, lacking a nucleus, ribosomes, and mitochondria, are smaller than most other cells. Their small size allows them to pass through tiny capillaries. However, the lack of mitochondria makes them completely dependent on anaerobic glycolysis for ATP production. Then, the lactate is partly disposed of by the liver and renal cortex.
  • Skeletal muscle
    Its cells, and particularly fast-twitch fibers contracting under low oxygen conditions, such as during intense exercise, produce much lactate.
    In such conditions, anaerobic glycolysis leads to the production of 2 ATP per molecule of glucose, 3 if the glucose comes from muscle glycogen, therefore, much lower than the 29-30 ATP produced by the complete oxidation of the monosaccharide. However, the rate of ATP production by anaerobic glycolysis is greater than that produced by the complete oxidation of glucose. Therefore, to meet the energy requirements of contracting muscle, anaerobic glycolysis is an effective means of ATP production. But this could lead to an intracellular accumulation of lactate, and a consequent reduction in intracellular pH. Obviously, such accumulation does not occur, due also to the Cori cycle, in which the liver pays the cost of the disposal of a large part of the muscle lactate, thereby allowing the muscle to use ATP for the contraction.
    And the oxygen debt, which always occurs after a strenuous exercise, is largely due to the increased oxygen demand of the hepatocytes, in which the oxidation of fatty acids, their main fuel, provides the ATP required for gluconeogenesis from lactate.
  • During trauma, sepsis, burns, or after major surgery, an intense cell proliferation occurs in the wound, that is a hypoxic tissue, and in bone marrow. This in turn results in greater production of lactate, an increase in the flux through the Cori cycle and an increase in ATP consumption in the liver, which, as previously said, is supported by an increase in fatty acid oxidation. Hence, the nutrition plan provided to these patients must be taken into account this increase in energy consumption.
  • A similar condition seems to occur also in cancer patients with progressive weight loss.
  • The Cori cycle is also important during overnight fasting and starvation.

Cori cycle and glucose-alanine cycle

These cycles are metabolic pathways that contribute to ensure a continuous delivery of glucose to tissues for which the monosaccharide is  the primary source of energy.
The main difference between the two cycles consists in the three carbon intermediate which is recycled: in the Cori cycle, carbon returns to the liver in the form of pyruvate, whereas in the glucose-alanine cycle in the form of alanine.
For more information, see: glucose-alanine cycle.

References

American Chemical Society National Historic Chemical Landmarks. Carl and Gerty Cori and Carbohydrate Metabolism. http://www.acs.org/content/acs/en/education/whatischemistry/landmarks/carbohydratemetabolism.html

Bender D.A. Introduction to nutrition and metabolism. 3rd Edition. Taylor & Francis, 2004

Iqbal S.A., Mido Y. Biochemistry. Discovery Publishing House, 2005

Nelson D.L., M. M. Cox M.M. Lehninger. Principles of biochemistry. 6th Edition. W.H. Freeman and Company, 2012

Newsholme E.A., Leech T.R. Functional biochemistry in health and disease. John Wiley J. & Sons, Inc., Publication, 2010

Rosenthal M.D., Glew R.H. Medical biochemistry – Human metabolism in health and disease. John Wiley J. & Sons, Inc., Publication, 2009

Stipanuk M.H., Caudill M.A. Biochemical, physiological, and molecular aspects of human nutrition. 3rd Edition. Elsevier health sciences, 2012

Bile salts: structure, function, synthesis, and enterohepatic circulation

Bile salts and bile acids are polar cholesterol derivatives, and represent the major route for the elimination of the steroid from the body.
They are molecules with similar but not identical structures, and diverse physical and biological characteristics.
They are synthesized in the liver, stored in the gallbladder, secreted into the duodenum, and finally, for the most part, reabsorbed in the ileum.
Because at physiological pH these molecules are present as anions, the terms bile acid and bile salts are used herein as synonyms.

CONTENTS

Chemical structure of bile salts

Bile salts have similarities and differences with cholesterol molecule.
Like the steroid, they have a nucleus composed of four fused rings: three cyclohexane rings, labeled A, B and C, and a cyclopentane ring, labeled D. This structure is the perhydrocyclopentanophenanthrene, more commonly known as steroid nucleus.

Structures and names of the most abundant bile acids and their conjugates
Bile Acids and Their Conjugates

In higher vertebrates, they have 24 carbon atoms, as the side chain is three carbons shorter than the original. In lower vertebrates, bile acids have 25, 26, or 27 carbon atoms. The side chain ends with a carboxyl group, ionized at pH 7, that can be linked to the amino acid glycine or taurine (see below).
In addition to the hydroxyl group at position 3, they have hydroxyl groups at positions 7 and/or 12.
All this makes them much more polar than cholesterol.
Since A and B rings are fused in cis configuration, the planar structure of the steroid nucleus is curved, and it is possible to identify:

  • a concave side, which is hydrophilic because the hydroxyl groups and the carboxyl group of the side chain, with or without the linked amino acid, are oriented towards it;
  • a convex side, which is hydrophobic because the methyl groups present at position 18 and 19 are orientated towards it.
Cholic Acid Structure
Cholic Acid Structure

Therefore, having both polar and nonpolar groups, they are amphiphilic molecules and excellent surfactants. However, their chemical structure makes them different from many other surfactants, often composed of a polar head region and a nonpolar tail.

Primary, conjugated and secondary bile salts

Primary bile acids are those synthesized directly from cholesterol in the hepatocytes. In humans, the most important are cholic acid and chenodeoxycholic acid, which make up 80% of all bile acids. Before being secreted into the biliary tree, they are almost completely conjugated, up to 98%, with the glycine or taurine, to form glycoconjugates and tauroconjugates, respectively. In particular, approximately 75% of cholic acid and chenodeoxycholic acid are conjugated with glycine, to form glycocholic acid  and glycochenodeoxycholic acid, the remaining 25% with taurine, to form taurocholic acid and taurochenodeoxycholic.

Synthesis of taurine- and glycine-conjugated bile acids
Synthesis of Conjugated Bile Acids

Conjugated bile acids are molecules with more hydrophilic groups than unconjugated bile acids, therefore with a increased emulsifying capacity. In fact, conjugation decreases the pKa of bile acids, from about 6, a value typical of non-conjugated molecules, to about 4 for glycocholic acid, and about 2 for taurocholic acid. This makes that conjugated bile acids are ionized in a broader range of pH to form the corresponding salts.
The hydrophilicity of the common acid and bile salts decreases in the following order: glycine-conjugated < taurine-conjugated < lithocholic acid  < deoxycholic acid  < chenodeoxycholic acid < cholic acid <ursodeoxycholic acid.
Finally, conjugation also decreases the cytotoxicity of primary bile acids.

Secondary bile acids  are formed from primary bile acids which have not been reabsorbed from the small intestine. Once they reach the colon, they can undergo several modifications by gut  microbiota to form secondary bile acids (see below). They make up the remaining 20% of the body’s bile acid pool.

Another way of categorizing bile salts is based on their conjugation with glycine and taurine and their degree of hydroxylation. On this basis, three categories are identified.

  • Trihydroxy conjugates, such as taurocholic acid and glycocholic acid.
  • Dihydroxy conjugates, such as glycodeoxycholic acid, glycochenodeoxycholic acid, taurochenodeoxycholic acid, and taurodeoxycholic acid. They account for about 60% of bile salts present in the bile.
  • Unconjugated forms, such as cholic acid, deoxycholic acid, chenodeoxycholic acid, and lithocholic acid.

Function of bile acids

All their physiological functions are performed in the conjugated form.

  • They are the major route for the elimination of cholesterol from human body.
    Indeed, humans do not have the enzymes to break open the cyclohexane rings or  the cyclopentane ring of the steroid nucleus, nor to oxidize cholesterol to CO2 and water.
    The other mechanism to eliminate the steroid from the body is as cholesterol per se in the bile.
  • Bile salts are strong surfactants. And in particular, di- and trihydroxy conjugates are the best surfactants among bile acids, much more effective than unconjugated counterparts, since they have more polar groups.
    Once in contact with apolar lipids in the lumen of the small intestine, the convex apolar surface interacts with the apolar lipids, such as triglycerides, cholesterol esters, and ester of fat-soluble vitamins, whereas the concave polar surface interacts with the surrounding aqueous medium. This increases the dispersion of apolar lipids in the aqueous medium, as it allows the formation of tiny lipid droplets, increasing the surface area for:

lipase activity, mainly pancreatic lipase, (bile salts also play a direct role in the activation of this enzyme);

intestinal esterase activity.

Subsequently, they facilitate the absorption of lipid digestion products, as well as of fat soluble vitamins by the intestinal mucosa thanks to the formation of mixed micelles.
Bile acids perform a similar function in the gallbladder where, forming mixed micelles with phospholipids, they prevent the precipitation of cholesterol.
Note: As a consequence of the arrangement of polar and nonpolar groups, bile acids form micelles in aqueous solution, usually made up of less than 10 monomers, as long as their concentration is above the so-called critical micellar concentration or CMC.

  • At the intestinal level, they modulate the secretion of pancreatic enzymes and cholecystokinin.
  • In the small and large intestine, they have a potent antimicrobial activity, mainly deoxycholic acid, in particular against Gram-positive bacteria. This activity may be due to oxidative DNA damage, and/or to the damage of the cell membrane. Therefore, they play an important role in the prevention of bacterial overgrowth, but also in the regulation of gut microbiota composition.
  • In the last few years, it becomes apparent their regulatory role in the control of energy metabolism, and in particular for the hepatic glucose handling.

Enterohepatic circulation of bile salts

After fat intake, enteroendocrine cells of the duodenum secrete cholecystokinin into the blood stream. Hormone binding to receptors on smooth muscle cells of the gallbladder promotes their contraction; the hormone also causes the relaxation of the sphincter of Oddi. All this results in the secretion of the bile, and therefore of bile acids into the duodenum.
Under physiological conditions, human bile salt pool is constant, and equal to about 3-5 g. This is made possible by two processes:

  • their intestinal reabsorption;
  • their de novo synthesis (see below).

Up to 95% of the secreted bile salts is reabsorbed from the gut, not together with the products of lipid digestion, but through a process called enterohepatic circulation.
It is an extremely efficient recycling system, which seems to occur at least two times for each meal, and includes the liver, the biliary tree, the small intestine, the colon, and the portal circulation through which reabsorbed molecules return to the liver. Such recirculation is necessary since liver’s capacity to synthesize bile acids is limited and insufficient to satisfy intestinal needs if the bile salts were excreted in the feces in high amounts.
Most of the bile salts are reabsorbed into the distal ileum, the lower part of the small intestine, by a sodium-dependent transporter within the brush border of the enterocytes, called sodium-dependent bile acid transporter or ASBT, which carries out the cotransport of a molecule of bile acid and two sodium ions.
Within the enterocyte, it is thought that bile acids are transported across the cytosol to the basolateral membrane by the ileal bile acid-binding protein or IBABP. They cross the basolateral membrane by the organic solute transporter alpha-beta or OSTα/OSTβ, pass into the portal circulation, and, bound to albumin, reach the liver.
It should be noted that a small percentage of bile acids reach the liver through the hepatic artery.
A hepatic level, their extraction is very efficient, with a first-pass extraction fraction ranging from 50 to 90%, a percentage that depends on bile acid structure. The uptake of conjugated bile acids is mainly mediated by a Na+-dependent active transport system, that is, the sodium-dependent taurocholate cotransporting polypeptide or NTCP. However, a sodium-independent uptake can also occur, carried out by proteins of the family of organic anion transporting polypeptides or OATP, mainly OATP1B1 and OATP1B3.
The rate limiting step in the enterohepatic circulation is their canalicular secretion, largely mediated by the bile salt export pump or BSEP, in an ATP-dependent process. This pump carries monoanionic bile salts, which are the most abundant. Bile acids conjugated with glucuronic acid or sulfate, which are dianionic, are transported by different carriers, such as MRP2 and BCRP.

Note: Serum levels of bile acids vary on the basis of the rate of their reabsorption, and therefore they are higher during meals, when the enterohepatic circulation is more active.

Intestinal metabolism of bile acids

Bile acids which escape ileal absorption pass into the colon where they partly undergo modifications by intestinal microbiota and are converted to secondary bile acids.
The main reactions are listed below.

  • Deconjugation
    On the side chain, hydrolysis of the C24 N-acyl amide bond can occur, with release of unconjugated bile acids and glycine or taurine. This reaction is catalyzed by bacterial hydrolases present both in the small intestine and in the colon.
  • 7α-Dehydroxylation
    Quantitatively, it is the most important reaction, carried out by colonic bacterial dehydratases that remove the hydroxyl group at position 7 to form 7-deoxy bile acids. In particular, deoxycholic acid is formed from cholic acid, and lithocholic acid, a toxic secondary bile acid, from chenodeoxycholic acid.
    It should be noted that 7α-dehydroxylation, unlike oxidation and epimerization (see below), can only occur on unconjugated bile acids, and therefore, deconjugation is an essential prerequisite.
  • Oxidation and epimerization
    They are reactions involving the hydroxyl groups at positions 3, 7 and 12, catalyzed by bacterial hydroxysteroid dehydrogenases. For example, ursodeoxycholic acid derives from the epimerization of chenodeoxycholic acid.
Conversion of bile acids to secondary bile acids by intestinal microbiota
Intestinal Metabolism of Bile Acids

Some of the secondary bile acids are then reabsorbed from the colon and return to the liver. In the hepatocytes, they are reconjugated, if necessary, and resecreted. Those that are not reabsorbed, are excreted in the feces.
Whereas oxidations and deconjugations are carried out by a broad spectrum of anaerobic bacteria, 7α-dehydroxylations is carried out by a limited number of colonic anaerobes.
7α-Dehydroxylations and deconjugations increase the pKa of the bile acids, and therefore their hydrophobicity, allowing a certain degree of passive absorption across the colonic wall.
The increase of hydrophobicity is also associated with an increased toxicity of these molecules. And a high concentration of secondary bile acids in the bile, blood, and feces has been associated to the pathogenesis of colon cancer.

Soluble fibers and reabsorption of bile salts

The reabsorption of bile salts can be reduced by chelating action of soluble fibers, such as those found in fresh fruits, legumes, oats and oat bran, which bind them, decreasing their uptake. In turn, this increases bile acid de novo synthesis, up-regulating the expression of the 7α-hydroxylase and sterol 12α-hydroxylase (see below), and thereby reduces hepatocyte cholesterol concentration.
The depletion of hepatic cholesterol increases the expression of the LDL receptor, and thus reduces plasma concentration of LDL cholesterol. On the other hand, it also stimulates the synthesis of HMG-CoA reductase, the key enzyme in cholesterol biosynthesis.
Note: Some anti-cholesterol drugs act by binding bile acids in the intestine, thereby preventing their reabsorption.

Synthesis of primary bile acids

Quantitatively, bile acids are the major product of cholesterol metabolism.
As previously said, enterohepatic circulation and their de novo synthesis maintain a constant bile acid pool size. In particular, de novo synthesis allows the replacement of bile salts excreted in the faces, about 5-10% of the body pool, namely ~ 0.5 g/day.
Below, the synthesis of cholic acid and chenodeoxycholic acid, and their conjugation with the amino acids taurine and glycine, is described.
There are two main pathways for bile acid synthesis: the classical pathway and the alternative pathway. In addition, some other minor pathways will also be described.

De novo synthesis of primary bile acids and their conjugates: classical and alternative pathways
De Novo Synthesis of Primary Bile Acids and Their Conjugates

The classical or neutral pathway

In humans, up to 90% of bile salts are produced via the classical pathway (see fig. 5), also referred to as “neutral” pathway since intermediates are neutral molecules.
It is a metabolic pathway present only in the liver, that consists of reactions catalyzed by enzymes localized in the cytosol, endoplasmic reticulum, peroxisomes, and mitochondria, and whose end products are the conjugates of cholic acid and chenodeoxycholic acid.

  • The first reaction is the hydroxylation at position 7 of cholesterol, to form 7α-hydroxycholesterol. The reaction is catalyzed by cholesterol 7α-hydroxylase or CYP7A1 (E.C. 1.14.14.23). It is an enzyme localized in the endoplasmic reticulum, and catalyzes the rate-limiting step of the pathway.

Cholesterol + NADPH + H+ + O2 → 7α-Hydroxycholesterol + NADP+ + H2O

  • 7α-Hydroxycholesterol undergoes oxidation of the 3β-hydroxyl group and the shift of the double bond from the 5,6 position to the 4,5 position, to form 7α-hydroxy-4-cholesten-3-one. The reaction is catalyzed by 3β-hydroxy-Δ5-C27-steroid oxidoreductase or HSD3B7 (E.C. 1.1.1.181), an enzyme localized in the endoplasmic reticulum.
  • 7α-Hydroxy-4-cholesten-3-one can follow two routes:

to enter the pathway that leads to the synthesis of cholic acid, through the reaction catalyzed by 7α-hydroxy-4-cholesten-3-one 12α-monooxygenase or sterol 12α-hydroxylase or CYP8B1 (E.C. 1.14.18.8), an enzyme localized in the endoplasmic reticulum;

to enter the pathway that leads the synthesis of chenodeoxycholic acid, through the reaction catalyzed by 3-oxo-Δ4-steroid 5β-reductase or AKR1D1 (E.C. 1.3.1.3), a cytosolic enzyme.

It should be underlined that the activity of sterol 12α-hydroxylase determines the ratio of cholic acid to chenodeoxycholic acid, and, ultimately, the detergent capacity of bile acid pool. And in fact, the regulation of sterol 12α-hydroxylase gene transcription is one of the main regulatory step of the classical pathway.

Therefore, if 7α-hydroxy-4-cholesten-3-one proceeds via the reaction catalyzed by sterol 12α-hydroxylase, the following reactions will occur.

  • 7α-Hydroxy-4-cholesten-3-one is hydroxylated at position 12 by sterol 12α-hydroxylase, to form 7α,12α-dihydroxy-4-cholesten-3-one.
  • 7α,12α-Dihydroxy-4-cholesten-3-one undergoes reduction of the double bond at 4,5 position, in the reaction catalyzed by 3-oxo-Δ4-steroid 5β-reductase, to form 5β-cholestan-7α,12α-diol-3-one.
  • 5β-Cholestan-7α,12α-diol-3-one undergoes reduction of the hydroxyl group at position 4, in the reaction catalyzed by 3α-hydroxysteroid dehydrogenase or AKR1C4 (EC 1.1.1.213), a cytosolic enzyme, to form 5β-cholestan-3α,7α,12α-triol.
  • 5β-Cholestan-3α,7α,12α-triol undergoes oxidation of the side chain via three reactions catalyzed by sterol 27-hydroxylase or CYP27A1 (EC 1.14.15.15). It is a mitochondrial enzyme also present in extrahepatic tissues and macrophages, which introduces a hydroxyl group at position 27. The hydroxyl group is oxidized to aldehyde, and then to carboxylic acid, to form 3α,7α,12α-trihydroxy-5β-cholestanoic acid.
  • 3α,7α,12α-Trihydroxy-5β-cholestanoic  acid is activated to its coenzyme A ester, 3α,7α,12α-trihydroxy-5β-cholestanoyl-CoA, in the reaction catalyzed by either very long chain acyl-CoA synthetase or VLCS (EC 6.2.1.-), or bile acid CoA synthetase or BACS (EC 6.2.1.7), both localized in the endoplasmic reticulum.
  • 3α,7α,12α-Trihydroxy-5β-cholestanoyl-CoA is transported to peroxisomes where it undergoes five successive reactions, each catalyzed by a different enzyme. In the last two reactions, the side chain is shortened to four carbon atoms, and finally cholylCoA is formed.
  • In the last step, the conjugation, via amide bond, of the carboxylic acid group of the side chain with the amino acid glycine or taurine occurs. The reaction is catalyzed by bile acid-CoA:amino acid N-acyltransferase or the BAAT (EC 2.3.1.65), which is predominantly localized in peroxisomes.
    The reaction products are thus the conjugated bile acids: glycocholic acid and taurocholic acid.

If 7α-hydroxy-4-cholesten-3-one does not proceed via the reaction catalyzed by sterol 12α-hydroxylase, it enters the pathway that leads to the synthesis of chenodeoxycholic acid conjugates, through the reactions described below.

  • 7α-Hydroxy-4-cholesten-3-one is converted to 7α-hydroxy-5β-cholestan-3-one in the reaction catalyzed by 3-oxo-Δ4-steroid 5β-reductase.
  • 7α-Hydroxy-5β-cholestan-3-one is converted to 5β-cholestan-3α,7α-diol in the reaction catalyzed by 3α-hydroxysteroid dehydrogenase.

Then, the conjugated bile acids glycochenodeoxycholic acid and taurochenodeoxycholic acid are formed by modifications similar to those seen for the conjugation of cholic acid, and catalyzed mostly by the same enzymes.

Note: Unconjugated bile acids formed in the intestine must reach the liver to be reconjugated.

The alternative or acidic pathway

It is prevalent in the fetus and neonate, whereas in adults it leads to the synthesis of less than 10% of the bile salts.
This pathway  (see fig. 5) differs from the classical pathway in that:

  • the intermediate products are acidic molecules, from which the alternative name “acidic pathway”;
  • the oxidation of the side chain is followed by modifications of the steroid nucleus, and not vice versa;
  • the final products are conjugates of chenodeoxycholic acid.

The first step involves the conversion of cholesterol into 27-hydroxycholesterol in the reaction catalyzed by sterol 27-hydroxylase.
27-Hydroxycholesterol can follow two routes.

Route A

  • 27-hydroxycholesterol is converted to 3β-hydroxy-5-cholestenoic acid in a reaction catalyzed by sterol 27-hydroxylase.
  • 3β-Hydroxy-5-cholestenoic acid is hydroxylated at position 7 in the reaction catalyzed by oxysterol 7α-hydroxylase or CYP7B1 (EC 1.14.13.100), an enzyme localized in the endoplasmic reticulum, to form 3β-7α-dihydroxy-5-colestenoic acid.
  • 3β-7α-Dihydroxy-5-cholestenoic acid is converted to 3-oxo-7α-hydroxy-4-cholestenoic acid, in the reaction catalyzed by 3β-hydroxy-Δ5-C27-steroid oxidoreductase.
  • 3-Oxo-7α-hydroxy-4-cholestenoic acid, as a result of side chain modifications, forms chenodeoxycholic acid, and then its conjugates.

Route B

  • 27-Hydroxycholesterol is converted to 7α,27-dihydroxycholesterol in the reaction catalyzed by oxysterol 7α-hydroxylase and cholesterol 7α-hydroxylase.
  • 7α,27-Dihydroxycholesterol is converted to 7α,26-dihydroxy-4-cholesten-3-one in the reaction catalyzed by 3β-hydroxy-Δ5-C27-steroid oxidoreductase;

7α, 26-Dihydroxy-4-cholesten-3-one can be transformed directly to conjugates of chenodeoxycholic acid, or can be converted to 3-oxo-7α-hydroxy-4-colestenoic acid,  and then undergo side chain modifications and other reactions that lead to the synthesis of the conjugates of chenodeoxycholic acid.

Minor pathways

There are also minor pathways (see fig. 5) that contribute to bile salt synthesis, although to a lesser extent than classical and alternative pathways.

For example:

  • A cholesterol 25-hydroxylase (EC 1.14.99.38) is expressed in the liver.
  • A cholesterol 24-hydroxylase or CYP46A1 (EC 1.14.14.25) is expressed in the brain, and therefore, although the organ cannot export cholesterol, it exports oxysterols.
  • A nonspecific 7α-hydroxylase has also been discovered. It is  expressed in all tissues and appears to be involved in the generation of oxysterols, which may be transported to hepatocytes to be converted to chenodeoxycholic acid.

Additionally, sterol 27-hydroxylase is expressed in various tissues, and therefore its reaction products must be transported to the liver to be converted to bile salts.

Bile salts: regulation of synthesis

Regulation of bile acid synthesis occurs via a negative feedback mechanism, particularly on the expression of cholesterol 7α-hydroxylase and sterol 12α-hydroxylase.
When an excess of bile acids, both free and conjugated, occurs, these molecules bind to the nuclear receptor farnesoid X receptor or FRX, activating it: the most efficacious bile acid is chenodeoxycholic acid, while others, such as ursodeoxycholic acid, do not activate it.
FRX induces the expression of the transcriptional repressor small heterodimer partner or SHP, which in turn interacts with other transcription factors, such as liver receptor homolog-1 or LRH-1, and hepatocyte nuclear factor-4α or HNF-4α. These transcription factors bind to a sequence in the promoter region of 7α-hydroxylase and 12α-hydroxylase genes, region called bile acid response elements or BAREs, inhibiting their transcription.
One of the reasons why bile salt synthesis is tightly regulated is because many of their metabolites are toxic.

References

Chiang J.Y.L. Bile acids: regulation of synthesis. J Lipid Res 2009;50(10):1955-1966. doi:10.1194/jlr.R900010-JLR200

Gropper S.S., Smith J.L. Advanced nutrition and human metabolism. 6th Edition. Cengage Learning, 2012

Moghimipour E., Ameri A., and Handali S. Absorption-enhancing effects of bile salts. Molecules 2015;20(8); 14451-14473. doi:10.3390/molecules200814451

Monte M.J., Marin J.J.G., Antelo A., Vazquez-Tato J. Bile acids: Chemistry, physiology, and pathophysiology. World J Gastroenterol 2009;15(7):804-816. doi:10.3748/wjg.15.804

Rosenthal M.D., Glew R.H. Medical biochemistry – Human metabolism in health and disease. John Wiley J. & Sons, Inc., Publication, 2009

Sundaram S.S., Bove K.E., Lovell M.A. and Sokol R.J. Mechanisms of Disease: inborn errors of bile acid synthesis. Nat Clin Pract Gastroenterol Hepatol 2008;5(8):456-468. doi:10.1038/ncpgasthep1179

Human gut microbiota: definition, composition, and diet

The human gastrointestinal tract is one of the most fierce and competitive ecological niches. It harbors viruses, eukaryotes, bacteria, and one member of Archaebacteria, Methanobrevibacter smithii.
Bacteria vary in proportion and amount all along the gastrointestinal tract; the greatest amount is found in the colon, which contains over 400 different species belonging to 9 phyla or divisions (of the 30 recognized phyla), and hereafter you refer to them as gut microbiota.
These are the phyla and some of their most represented genera.

  • Actinobacteria (Gram-positive bacteria); Bifidobacterium, Collinsella, Eggerthella, and Propionibacterium.
  • Bacteroidetes (Gram-negative bacteria); more than 20 genera including Bacteroides, Prevotella and Corynebacterium.
  • Cyanobacteria (Gram-negative bacteria).
  • Firmicutes (Gram-positive bacteria); at least 250 genera, including Mycoplasma, Bacillus, Clostridium, Dorea, Faecalibacterium, Ruminococcus, Eubacterium, Staphylococcus, Streptococcus, Lactobacillus, Lactococcus, Enterococcus, Sporobacter, and Roseburia.
  • Fusobacteria (Gram-negative bacteria);
  • Lentisphaerae (Gram-negative bacteria).
  • Proteobacteria (Gram-negative bacteria); Escherichia, Klebsiella, Shigella, Salmonella, Citrobacter, Helicobacter, and Serratia.
  • Spirochaeates (Gram-negative bacteria).
  • Verrucomicrobia (Gram-negative bacteria).

The presence of a small subset of the bacterial world in the colon is the result of a strong selective pressure which acted, during evolution, on both the microbial colonizers, selecting organisms very well adapted to this environment, and the intestinal niche. And nevertheless, each individual harbors an unique bacterial community in his gut.
Despite the high variability existing both with regard to taxa and between individuals, it has been proposed, but not accepted by all researchers, that in most adults the bacterial gut microbiota can be classified into variants or “enterotypes”, on the basis of the ratio of the abundance of the genera Bacteroides and Prevotella. This seems to indicate that there is a limited number of well balanced symbiotic states, which could respond differently to factors such as diet, age, genetics, and drug intake (see below).

Adult’s gut harbors a large and diverse community of DNA and RNA viruses made up of about 2,000 different genotypes, none of which is dominant. Indeed, the most abundant virus accounts for only about 6% of the community, whereas in infants the most abundant virus accounts over 40% of the community. The majority of DNA viruses are bacteriophages or phages, that is, viruses that infect bacteria (they are the most abundant biological entity on earth, with an estimated population of about 1031 units), whereas the majority of RNA viruses are plant viruses.

CONTENTS

Factors affecting gut microbiota composition and development

The intestinal bacterial community is regulated by several factors, most of which are listed below.

  • The diet of the host.
    It seems to be the most important factor.
    Traditionally considered sterile, mother’s milk harbors a rich microbiota consisting of more than 700 species, dominated by staphylococci, streptococci, bifidobacteria and lactic acid bacteria. Therefore, it is a major source for the colonization of the breastfed infant gut, and it was suggested that this mode of colonization is closely correlated with infant’s health status, because, among other functions, it could protect against infections and contribute to the maturation of the immune system. Breast milk affects intestinal microbiota also indirectly, through the presence of oligosaccharides with prebiotic activity that stimulate the growth of specific bacterial groups including staphylococci and bifidobacteria.
    A recent study has compared the intestinal microbiota of European and African children (respectively from Florence and a rural village in Burkina Faso) between the ages of 1 and 6 years old. It has highlighted the dominant role of diet over variables such as climate, geography, hygiene and health services (it was also observed the absence of significant differences in the expression of key genes regulating the immune function, which suggests a functional similarity between the two groups). Indeed infants, as long as they are breastfed, have a very similar gut microbiota, rich in Actinobacteria, mainly Bifidobacterium (see below). The subsequent introduction of solid foods in the two groups, a Western diet rich in animal fats and proteins in European children, and low in animal proteins but rich in complex carbohydrates in African children, leads to a differentiation in the Firmicutes/Bacteroidetes ratio between the two groups. Gram-positive bacteria, mainly Firmicutes, were more abundant than Gram-negative bacteria in European children, whereas Gram-negative bacteria, mainly Bacteroidetes, prevailed over Gram-positive bacteria in African children.
    And the long-term diets are strongly associated to the enterotype partitioning. Indeed, it has been observed that:

a diet high in animal fats and proteins, i.e. a Western-type diet, leads to a gut microbiota dominated by the Bacteroides enterotype;
a diet high in complex carbohydrates, typical of agrarian societies, leads to the prevalence of the Prevotella enterotype.

Similar results emerged from the aforementioned study on children. In the Europeans, gut microbiota was dominated by taxa typical of Bacteroides enterotype, whereas in the Burkina Faso children, Prevotella enterotype dominates.
With short-term changes in the diet (10 days), such as the switch from a low-fat and high-fiber diet to a high-fat and low-fiber diet and vice versa, changes were observed in the composition of the microbiome (within 24 hours), but no stable change in the enterotype partitioning. And this underlines as a long-term diet is needed for a change in the enterotypes of the gut microbiota.
Dietary interventions can also result in changes in the gut virome, which moves to a new state, that is, changes occur in the proportions of the pre-existing viral populations, towards which subjects on the same diet converge.

  • pH, bile salts and digestive enzymes.
    The stomach, due to its low pH, is a hostile environment for bacteria, which are not present in high numbers, about 102-103 bacterial cells/gram of tissue. In addition to Helicobacter pylori, able to cause gastritis and gastric ulcers, microorganisms of the genus Lactobacillus are also present.
    Reached the duodenum, an increase in bacterial cell number occurs, 104-105 bacterial cells/gram of tissue; and similar bacterial concentrations are present in the jejunum and proximal ileum. The low number of microorganisms present in the small intestine is due to the inhospitable environment, consequent to the fact that there is the opening of the ampulla of Vater in the descending part of the duodenum, which pours pancreatic juice and bile into the duodenum, that is, pancreatic enzymes and bile salts, which damage microorganisms.
    In the terminal portion of the ileum, where the activities of pancreatic enzymes and bile salts are lower, there are about 107 bacterial cells/gram of tissue, and up to 1012-1014 bacterial cells/gram of tissue in the colon, so that bacteria represent a large proportion, about 40%, of the fecal mass.
    The distribution of bacteria along the intestine is strategic. In the duodenum and jejunum, the amount of available nutrients is much higher than that found in the terminal portion of the ileum, where just water, fiber, and electrolytes remain. Therefore, the presence of large number of bacteria in the terminal portion of the ileum, and even more in the colon, is not a problem. The problem would be to find a high bacterial concentration in the duodenum, jejunum, and proximal parts of the ileum; and there is a disease condition, called small intestinal bacterial overgrowth or SIBO, in which the number of bacteria in the small intestine increases by about 10-15 times. This puts them in a position to compete with the host for nutrients and give rise to gastrointestinal disturbances such as diarrhea.
  • The geographical position and the resulting differences in lifestyle, diet, religion etc.
    For example, a kind of geographical gradient occurs in the microbiota of European infants, with a higher number of Bifidobacterium species and some of Clostridium in Northern infants, whereas Southern infants have higher levels of Bacteroides, Lactobacillus and Eubacterium.
  • The mode of delivery (see below).
  • The genetics of the host.
  • The health status of the infant and mother.
    For example, in mothers with inflammatory bowel disease or IBD, Faecalibacterium prausnitzii, a bacterium that produces butyrate (an important source of energy for intestinal cells), and with anti-inflammatory activity is depleted, whereas there is an increase in the number of adherent Escherichia coli.
  • The treatment with antibiotics.
  • Bacterial infections and predators.
    Bacteriocins, i.e. proteins with antibacterial activity, and bacteriophages.
    Phages play an important role in controlling the abundance and composition of the gut microbiota. In particular, they could play a major role in the colonization of the newborn, infecting the dominant bacteria thus allowing to another bacterial strain to become abundant.
    This model of predator-prey dynamics, called “kill the winner”, suggests that the blooms of a specific bacterial species would lead to blooms of their corresponding bacteriophages, followed by a decline in their abundance. Therefore, the most abundant bacteriophage genotype will not be the same at different times. And although some the gene sequences present in the infant gut virome are stable over the first three months of life, dramatic changes occur in the overall composition of the viral community between the first and second week of life. During this time period also the bacterial community is extremely dynamic (see below).
  • The competition for space and nutrients.

Composition throughout life

The development of the intestinal microbial ecosystem is a complex and crucial event in human life, highly variable from individual to individual, and influenced by the factors outlined above.

Development and modifications of gut microbiota throughout life

In utero, the gut is considered sterile, but is rapidly colonized by microbes at birth, as the infant is born with an immunological tolerance instructed by the mother.
However, recent studies show the presence of bacteria in the placental tissue, umbilical cord blood, fetal membranes and amniotic fluid from healthy newborns without signs of infection or inflammation. And for example, the meconium of premature infants, born to healthy mothers, contains a specific microbiota, with Firmicutes as the main phylum, and predominance of staphylococci, whereas Proteobacteria, in particular species such as Escherichia coli, Klebsiella pneumoniae, Serratia marcescens, but also enterococci are more abundant in the faeces.
Note: The meconium is free of detectable viruses.
It seems that both vaginal and gut bacteria may gain access to the fetus, although via different route of entry: by ascending entry the vaginal ones, by dendritic cells of the immune system the gut ones. Therefore, there could exist a fetal microbiota.

Colonization occurs during delivery by a maternal inoculum, generally composed of aerobic and facultative bacteria (the newborn’s gut initially contains oxygen), then replaced by obligate anaerobes,  bacteria typically present in adulthood, to which they have created a hospitable environment.
Furthermore, there is a small number of different taxa, with a relative dominance of the phyla Actinobacteria and Proteobacteria, that remains unchanged during the first month of life, but not in the subsequent ones as there is a large increase in variability and new genetic variants. Many studies underline that the initial exposure is important in defining the “trajectories” which will lead to the adult ecosystems. Additionally, these initial communities may act as a source of protective or pathogenic microorganisms.

Mother’s vaginal and fecal microbiotas are the main sources of inoculum in vaginally delivered infants. Indeed, infants harbor microbial communities dominated by species of the genera Lactobacillus (the most abundant genus in the vaginal microbiota and early gut microbiota) Bifidobacterium, Prevotella, or Sneathia. And it seems likely that anaerobes, such as members of the phyla Firmicutes and Bacteroidetes, not growing outside of their host, rely on the close contact between mother and offspring for transmission. Finally, due to the presence of oxygen in infant gut, the transmission of strict anaerobes could occur not directly at birth but at a later stage by means of spores.
The first bacteria encountered by infants born by caesarean section are those of the skin and hospital environment, and gut microbiota is dominated by species of the genera Corynebacterium, Staphylococcus and Propionibacterium, with a lower bacterial count and diversity in first weeks of life than infants born vaginally.
Further evidence supporting the hypothesis of vertical transmission is the similarity between the microbiota of meconium and samples obtained from possible sites of contamination.
These “maternal bacteria” do not persist indefinitely, and are replaced by other populations within the first year of life.
Objects, animals, mouths and skin of relatives, and breast milk are secondary sources of inoculum; and breast milk (see below) seems to have a primary role in determining the microbial succession in the gut.
The variation and diversity among children reflect instead the individuality of these microbial exposures.
Note: The delivery mode seems also to influence the immune system during the first year of life, perhaps via the influence on the development of gut microbiota. Infants born by cesarean section have:

  • a lower bacterial count in stool samples at one month of age, mainly due to the higher number of bifidobacteria in infants born vaginally;
  • a higher number of antibody secreting cells, which could reflect an excessive antigen exposure (the intestinal barrier would be more vulnerable to the passage of antigens).

Within a days after birth, a thriving community is established. This community is less stable over time and more variable in composition than that of adults. Very soon, it will be more numerous than that of the child’s cells, evolving according to a temporal pattern highly variable from individual to individual.
Viruses, absent at birth, reach about 108 units/gram wet weight of faeces by the end of the first week of life, therefore representing a dynamic and abundant component of the developing gut microbiota. However, viral community has an extremely low diversity, like bacteria, and is dominated by phages, which probably influence the abundance and diversity of co-occurring bacteria, as seen above. The initial source of the viruses is unknown; of course, maternal and/or environmental inocula are among the possibilities. Notably, the earliest viruses could be the result of induction of prophages from the “newborn” gut bacterial flora, hypothesis supported by the observation that more than 25% of the phage sequences seem to be very similar to those of phages infecting bacteria such as Lactococcus, Lactobacillus, Enterococcus, and Streptococcus, which are abundant in breast milk.

By the end of the first month of life it is thought that the initial phase of rapid acquisition of microorganism is over.
In 1-month-old-infants, the most abundant bacteria belong to the genera Bacteroides and Escherichia, whereas Bifidobacterium, along with Ruminococcus, appear and grow to become dominant in the gastrointestinal tract of the breastfed infants between 1 and 11 months. Bifidobacteria such as Bifidobacterium longum subspecies infantis:

  • are known to be closely related to breastfeeding;
  • are among the best characterized commensal bacteria;
  • are considered probiotics, that is, microorganisms which can confer health benefits to the host.

Their abundance confers also benefits through competitive exclusion, that is, they are an obstacle to colonization by pathogens. And indeed, Escherichia and Bacteroides can become preponderant if Bifidobacterium is not adequately present in the gut.
In contrast, bacteria of the genera Escherichia (e.g. E. coli), Clostridium (e.g. C. difficile), Bacteroides (e.g. B. fragilis) and Lactobacillus are present in higher levels in formula-fed infants than in breastfed infants.
Although breast-fed infants receive only breast milk until weaning, their microbiota can show a large variability in the abundances of bacterial taxa, with differences between individuals also with regard to the temporal patterns of variation. These variations may be due to diseases, treatments with antibiotics, changes in host lifestyle, random colonization events, as well as differences in immune responses to the gut colonizing microbes. However, it is not yet clear how these factors contribute to shape infant gut microbiota.
It seems that also the virome changes rapidly after birth, as the majority of the viral sequences present in the first week of life are not found after the second week. Moreover, the repertoire expands rapidly in number and diversity during the first three months. This is in contrast with the stability observed in the adult virome, where 95% of the sequences are conserved over time.

In normal condition, towards the end of the first year of life, babies have consumed an adult-like diet for a significant time period and should have developed a microbial community with characteristics similar to those found in the adult gut, such as:

  • a more stable composition, phylogenetically more complex, and progressively more similar among different subjects;
  • a preponderance of Firmicutes and Bacteroidetes, followed by Verrucomicrobia and a very low abundance of Proteobacteria;
  • an increase in short-chain fatty acid (SCFA) levels and bacterial load in the feces;
  • an increase of genes associated with xenobiotic degradation, vitamin biosynthesis, and carbohydrate utilization.

Interestingly, the significant turnover of taxa occurring from birth to the end of the first year is accompanied by a remarkable constancy in the overall functional capabilities.
Towards the end of the first year of life also the early viral colonizers were replaced by a community specific to the child.

The gut microbiota reaches maturity at about 2.5 years of age, fully resembling the adult gut microbiota.
The selection of the most adapted bacteria is the result of various factors.

  • The transition to an adult diet.
  • An increased fitness to the intestinal environment of the taxa that typically dominate the adult gut microbiota than the early colonizers.
  • The significant changes in the intestinal environment, result of the developmental changes in the intestinal mucosa.
  • The effects of the microbiota itself.

Therefore, the first 2-3 years of life are the most critical period in which you can intervene to shape the microbiota as best as possible, and so optimize child growth and development.

From a chaotic beginning, all this leads to the establishment of the gut ecosystem typical of the young adult, which is relatively stable over time until old age (viral, archaeal and eukaryotic components included), and dominated, at least in the western population, by members of the phyla Firmicutes, about 60% of the bacterial communities, Bacteroidetes and Actinobacteria (mainly belonging to the Bifidobacterium genus), each comprising about 10% of the bacterial community, followed by Proteobacteria and Verrucomicrobia. The genera Bacteroides, Clostridium, Faecalibacterium, Ruminococcus and Eubacterium make up, together with Methanobrevibacter smithii, the large majority of the adult gut microbial community.
It should be noted that different data were obtained from analysis of populations of African rural areas, as seen above.
And the gut microbiota is sufficiently similar among subjects to allow the identification of a shared core microbiome.
Stability and resilience, however, are subject to numerous variables among which, as previously said, diet seems to be one of the most important. Therefore, in order to maintain the stability of the gut microbiota, the variables have to be kept constant, or in the case of diseases prevented (also through vaccinations). However, the stability and resilience could be harmful if the dominant community is pathogenic.

The gut microbiota undergoes substantial changes in the elderly. In a study conducted in Ireland on 161 healthy people aged 65 years and over, the gut microbiota is distinct from that of younger adults in the majority of subjects, with a composition that seems to be dominated by the phyla Bacteroidetes, the main ones, and Firmicutes, with almost inverted percentages than those found in younger adults (although large variations across subjects were observed). And there are Faecalibacterium, about 6% of the main genera, followed by species of the genera Ruminococcus, Roseburia and Bifidobacterium (the latter about 0.4%) among the most abundant genera.
Also the variability in the composition of the community is greater than in younger adults; this could be due to the increase in morbidities associated with aging and the subsequent increased intake of medications, as well as to changes in the diet.

References

Breitbart M., Haynes M., Kelley S., Angly F., Edwards R.A., Felts B., Mahaffy J.M., Mueller J., Nulton J., Rayhawk S., Rodriguez-Brito B., Salamon P., Rohwer F. Viral diversity and dynamics in an infant gut. Res Microbiol 2008;159:367-373. doi:10.1016/j.resmic.2008.04.006

Claesson M.J., Cusack S., O’Sullivan O., Greene-Diniz R., de Weerd H., Flannery E., Marchesi J.R., Falush D., Dinan T., Fitzgerald G., et al. Composition, variability, and temporal stability of the intestinal microbiota of the elderly. Proc Natl Acad Sci USA 2011;108(Suppl 1);4586-4591. doi:10.1073/pnas.1000097107

Clemente J.C., Ursell L.K., Wegener Parfrey L., and Knight R. The impact of the gut microbiota on human health: an integrative view. Cell 2012;148:1258-1270. doi:10.1016/j.cell.2012.01.035

De Filippo c., Cavalieri D., Di Paola M., Ramazzotti M., Poullet J.B., Massart S., Collini S., Pieraccini G., and Lionetti P. Impact of diet in shaping gut microbiota revealed by a comparative study in children from Europe and rural Africa. Proc Natl Acad Sci 2010;107(33):14691-14696. doi:10.1073/pnas.1005963107

Dominguez-Bello M.G., Costello E.K., Contreras M., Magris M., Hidalgo G., Fierer N., and Knight R. Delivery mode shapes the acquisition and structure of the initial microbiota across multiple body habitats in newborns. Proc Natl Acad Sci 2010;107:11971-11975. doi:10.1073/pnas.1002601107

Fernández L., Langa S., Martín V., Maldonado A., Jiménez E., Martín R., Rodríguez J.M. The human milk microbiota: origin and potential roles in health and disease. Pharmacol Res 2013;69(1):1-10. doi:10.1073/pnas.1002601107

Huurre A., Kalliomäki M., Rautava S., Rinne M., Salminen S., and Isolauri E. Mode of delivery-effects on gut microbiota and humoral immunity. Neonatology 2008;93:236-240. doi:10.1159/000111102

Koenig J.E., Spor A., Scalfone N., Fricker A.D., Stombaugh J., Knight R., Angenent L.T., and Ley R.E. Succession of microbial consortia in the developing infant gut microbiome. Proc Natl Acad Sci 2011;108(1):4578-4585. doi:10.1073/pnas.1000081107

Ley R.E., Peterson D.A., and Gordon J.I. Ecological and evolutionary forces shaping microbial diversity in the human intestine. Cell 2006;124(4):837-848. doi:10.1016/j.cell.2006.02.017

Minot S., Sinha R., Chen J., Li H., Keilbaugh S.A., Wu G.D., Lewis J.D., and Bushman F.D. The human gut virome: inter-individual variation and dynamic response to diet. Genome Res 2011;21:1616-1625. doi:10.1101/gr.122705.111

Moreno-Indias I.M., Cardona F., Tinahones F.J. and Queipo-Ortuño M.I. Impact of the gut microbiota on the development of obesity and type 2 diabetes mellitus. Front Microbiol 2014;5(190):1-10. doi:10.3389/fmicb.2014.00190

Newburg D.S. & Morelli L. Human milk and infant intestinal mucosal glycans guide succession of the neonatal intestinal microbiota. Pediatr Res 2015;77:115-120. doi:10.1038/pr.2014.178

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Rodrıguez J.M., Murphy K., Stanton C., Ross R.P., I. Kober O.I., Juge N., Avershina E., Rudi K., Narbad A., Jenmalm M.C., Marchesi J.R. and Collado M.C. The composition of the gut microbiota throughout life, with an emphasis on early life. Microb Ecol Health Dis 2015;26:26050. doi:10.3402/mehd.v26.26050

Wu G.D., Chen J., Hoffmann C., Bittinger K., Chen Y.Y., Keilbaugh S.A., Bewtra M., Knights D., Walters W.A., Knight R., et al. Linking long-term dietary patterns with gut microbial enterotypes. Science 2011;334:105-108. doi:10.1126/science.1208344

Human microbiota: definition, composition, function, and effect of antibiotics

It has been known for almost a century that humans harbor a microbial ecosystem, known as human microbiota, remarkably dense and diverse, made up of a  number of viruses and cells much higher than those of the human body, and that accounts for one to three percent of body weight. All the genes encoded by the human body’s microbial ecosystem, which are about 1,000 times more numerous than those of our genome, make up the human microbiome. Microorganisms colonize all the surfaces of the body that are exposed to the environment. Indeed, distinct microbial communities are found on the skin, in the vagina, in the respiratory tract, and along the whole intestinal tract, from the mouth up to rectum, the last part of the intestine.

CONTENTS

Composition of the human microbiota

It is composed of organisms from all taxa.

  • Bacteria, at least 100 trillion (1014) cells, a number ten times greater than that of the human body. They are found in very high concentration in the intestinal tract, up to 1012-1014/gram of tissue, where they form one of the most densely populated microbial habitats on Earth. In the gut, bacteria mainly belong to the Firmicutes, Bacteroidetes and Actinobacteria phyla. Fusobacteria (oropharynx), Tenericutes, Proteobacteria, and Verrucomicrobia are other phyla present in our body.
    Note: Bacterial communities in a given body region resemble themselves much more across individuals than those from different body regions of the same individual; for example, bacterial communities of the upper respiratory tract are much more similar across individuals than those of the skin or intestine of the same individual.
  • Viruses, by far the most numerous organisms, about quadrillion units. The genomes of all the viruses harbored in the human body make up the human virome. In the past, viruses and eukaryotes (see below) have been studied focusing on pathogenic microorganisms, but in recent years the attention has also shifted on many non-pathogenic members of these groups. And many of the viral gene sequences found are new, which suggests that there is still much to learn about the human virome. Finally, just like for bacteria, there is considerable interpersonal variability.
  • Archaebacteria, primarily those belonging to the order Methanobacteriales, with Methanobrevibacter smithii predominant in the human gut (up to 10% of all anaerobes).
  • Eukaryotes, and the parasites of the genera Giardia and Entamoeba have probably been among the first to be identified. But there is also a great abundance and diversity of fungal species, belonging to genera such as Candida, Penicillium, Aspergillus, Hemispora, Fusarium, Geotrichum, Hormodendrum, Cryptococcus, Saccharomyces, and Blastocystis.

Candida albicans, a component of Human Microbiota

Based on the relationships with the human host, microorganisms may be classified as commensals or pathogens.

  • Commensals cause no harm to the host, with which they establish a symbiotic relationship that generally brings benefits to both.
  • On the contrary, pathogens are able to cause diseases, but fortunately represent a small percentage of the human microbiota. These microorganisms establish a symbiosis with the human host and benefit from it at the expense of the host. They can cause disease:

if they move from their niche, such as the intestine, into another one where they do not usually reside, such as the vagina or bladder (as in the case of Candida albicans, normally present in the intestine, but in very small quantities);
in patients with impaired immunological defenses, such as after an immunosuppressive therapy.

Functions of the human microbiota

Sometimes referred to as “the forgotten organ“, human microbiota, mainly with its intestinal bacterial members, plays many important functions that can lead to nutritional, immunological, and developmental benefits, but can also cause diseases. Here are some examples.

  • It is involved in the development of the gastrointestinal system of the newborn, as shown by experiments carried out on germ-free animals in which, for example, the thickness of the intestinal mucosa is thinner than that of colonized animals, therefore more easily subject to rupture.
  • It contributes to energy harvest from nutrients, due to its ability to ferment indigestible carbohydrates, promote the absorption of monosaccharides and the storage of the derived energy. This has probably been a very strong evolutionary force that has played a major role in favor of the fact that these bacteria became our symbionts.
  • It contributes to the maintenance of the acidic pH of the skin and in the colon.
  • It is involved in the metabolism of xenobiotics and several polyphenols.
  • It improves water and mineral absorption in the colon.
  • It increases the speed of intestinal transit, slower in germ-free animals.
  • It has an important role in resistance to colonization by pathogens, primarily in the vagina and gut.
  • It is involved in the biosynthesis of isoprenoids and vitamins through the methylerythritol phosphate pathway.
  • It stimulates angiogenesis.
  • In the intestinal tract, it interacts with the immune system, providing signals for promoting the maturation of immune cells and the normal development of immune functions. And this is perhaps the most important effect of the symbiosis between the human host and microorganisms. Experiments carried out on germ-free animals have shown, for example, that:

macrophages, the cells that engulf pathogens and then present their antigens to the immune system, are found in much smaller amounts than those present in the colonized intestine, and if placed in the presence of bacteria they fail to find and therefore engulf them, unlike macrophages extracted from a colonized intestine;
there is not the chronic non-specific inflammation, present in the normal intestine as a result of the presence of bacteria (and of what we eat).

  • Changes in its composition can contribute to the development of obesity and metabolic syndrome.
  • It protects against the development of type I diabetes.
  • Many diseases, both in children and adults, such as stomach cancer, lymphoma of mucosa-associated lymphoid tissue, necrotizing enterocolitis (an important cause of morbidity and mortality in premature babies) or chronic intestinal diseases, are, and others seem to be, related to the gut microbiota.

In conclusion, it seems very likely that the human body represents a superorganism, result of years of evolution and made up of human cells, and the resulting metabolic and physiological capacities, as well as an additional organ, the microbiota.

Human Microbiome Project

The bacterial component of the human microbiota is the subject of most studies including a large-scale project started in 2008 called “Human Microbiome Project“, whose aim is to characterize the microbiome associated with multiple body sites, such as the skin, mouth, nose, vagina and intestine, in 242 healthy adults. These studies have shown a great variability in the composition of the human microbiota; for example, twins share less than 50% of their bacterial taxa at the species level, and an even smaller percentage of viruses. The factors that shape the composition of bacterial communities begin to be understood: for example, the genetic characteristics of the host play an important, although this is not true for the viral community. And metagenomic studies have shown that, despite the great interpersonal variability in microbial community composition, there is a core of shared genes encoding signaling and metabolic pathways. It appears namely that the assembly and the structure of the microbial community does not occur according to the species but the more functional set of genes. Therefore, disease states of these communities might be better identified by atypical distribution of functional classes of genes.

Effect of antibiotics

The microbiota in healthy adult humans is generally stable over time. However, its composition can be altered by factors such as dietary changes, urbanization, travel, and especially the use of broad-spectrum antibiotics. Here are some examples of the effect of antibiotic treatments.

  • There is a long-term reduction in microbial diversity.
  • The taxa affected vary from individual to individual (even up to a third of the taxa).
  • Several taxa do not recover even after 6 months from treatment.
  • Once the bacterial communities have reshaped, a reduced resistance to colonization occurs. This allows foreign and/or pathogen bacteria, able to grow more than the commensals, to cause permanent changes in human microbiota structure, as well as acute diseases, such as the dangerous pseudomembranous colitis, and chronic diseases, as it is suspected for asthma following the use and abuse of antibiotics in childhood. Moreover, their repeated use has been suggested to increase the pool of antibiotic-resistance genes in our microbiome. In support of this hypothesis, a decrease in the number of antibiotic-resistant pathogens has been observed in some European countries following the reduction in the number of antibiotics prescribed.

Finally, you must not underestimate the fact that the intestinal microflora is involved in many chemical transformations, and its alteration could be implicated in the development of cancer and obesity. However, regarding use of antibiotics, you should be underlined that if western population has a life expectancy higher than in the past is also because you do not die of infectious diseases!

References

Burke C., Steinberg P., Rusch D., Kjelleberg S., and Thomas T. Bacterial community assembly based on functional genes rather than species. Proc Natl Acad Sci USA 2011;108:14288-14293. doi:10.1073/pnas.1101591108

Clemente J.C., Ursell L.K., Wegener Parfrey L., and Knight R. The impact of the gut microbiota on human health: an integrative view. Cell 2012;148:1258-1270. doi:10.1016/j.cell.2012.01.035

Gill S.R., Pop M., Deboy R.T., Eckburg P.B., Turnbaugh P.J., Samuel B.S., Gordon J.I., Relman D.A., Fraser-Liggett C.M., and Nelson K.E. Metagenomic analysis of the human distal gut microbiome. Science 2006;312:1355-1359. doi:10.1126/science.1124234

Palmer C., Bik E.M., DiGiulio D.B., Relman D.A., and Brown P.O. Development of the human infant intestinal microbiota. PLoS Biol 2007;5(7):e177. doi:10.1371/journal.pbio.0050177

Turnbaugh P.J., Gordon J.I. The core gut microbiome, energy balance and obesity. J Physiol 2009;587:4153-4158. doi:10.1113/jphysiol.2009.174136

Zhang, T., Breitbart, M., Lee, W., Run, J.-Q., Wei, C., Soh, S., Hibberd, M., Liu, E., Rohwer, F., Ruan, Y. Prevalence of plant viruses in the RNA viral community of human feces. PLoS Biol 2006;4(1):e3. doi:10.1371/journal.pbio.0040003

Flavonoid biosynthesis pathway: genes and enzymes

The biosynthesis of flavonoids, probably the best characterized pathway of plant secondary metabolism, is part of the phenylpropanoid pathway that, in addition to flavonoids, leads to the formation of a wide range of phenolic compounds, such as hydroxycinnamic acids, stilbenes, lignans and lignins.
Flavonoid biosynthesis is linked to primary metabolism through both mitochondria- and plastid-derived molecules. Since it seems that most of the involved enzymes characterized to date operate in protein complexes located in the cell cytosol, these molecules must be exported to the cytoplasm to be used.
The end products are transported to different intracellular or extracellular locations, with flavonoids involved in pigmentation usually transported into the vacuoles.
The biosynthesis of this group of polyphenols requires one p-coumaroyl-CoA and three malonyl-CoA molecules as initial substrates.

Flavonoid biosynthesis pathway
Flavonoid Biosynthesis

CONTENTS

Biosynthesis of p-coumaroyl-CoA

p-Coumaroyl-CoA is the pivotal branch-point metabolite in the phenylpropanoid pathway, being the precursor of a wide variety of phenolic compounds, both flavonoid and non-flavonoid polyphenols.
It is produced from phenylalanine via three reactions catalyzed by cytosolic enzymes collectively called group I or early-acting enzymes, in order of action:

  • phenylalanine ammonia lyase (EC 4.3.1.24);
  • trans-cinnamate 4-monooxygenase (EC:1.14.14.91);
  • 4-coumarate-CoA ligase (EC 6.2.1.12).
Biosynthesis of p-coumaroyl-CoA from phenylalanine
Biosynthesis of p-coumaroyl-CoA

They seems to be associated in a multienzyme complex anchored to the endoplasmic reticulum membrane. The anchoring is probably ensured by cinnamate 4-hydroxylase that inserts its N-terminal domain into the membrane of the endoplasmic reticulum itself. These complexes, referred to as “metabolons”, allow the product of a reaction to be channeled directly as substrate to the active site of the enzyme that catalyzes the consecutive reaction in the metabolic pathway.
With the exception of cinnamate 4-hydroxylase, the enzymes which act downstream of phenylalanine ammonia lyase are encoded by small gene families in all species analyzed so far.
The different isoenzymes show distinct temporal, tissue, and elicitor-induced patterns of expression. It seems, in fact, that each member of each family can be used mainly for the synthesis of a specific compound, thus acting as a control point for carbon flux among the metabolic pathways leading to lignan, lignin, and flavonoid biosynthesis.

Note: Phenylalanine is a product of the shikimic acid pathway, which converts simple precursors derived from carbohydrate metabolism, phosphoenolpyruvate and erythrose-4-phosphate, into the aromatic amino acids phenylalanine, tyrosine and tryptophan. Unlike plants and microorganisms, animals do not possess the shikimic acid pathway, and are not able to synthesize the three above-mentioned amino acids, which are therefore essential nutrients.

Phenylalanine ammonia lyase (PAL)

It is one of the most studied and best characterized enzymes of plant secondary metabolism. It requires no cofactors and catalyzes the reaction that links primary and secondary metabolism: the reversible deamination of phenylalanine to trans-cinnamic acid, with the release of nitrogen as ammonia and introduction of a trans double bond between carbon atoms 7 and 8 of the side chain.

Phenylalanine ⇄ trans-Cinnamic Acid + NH3

Therefore, it directs the flow of carbon from the shikimic acid pathway to the different branches of the phenylpropanoid pathway. The released ammonia is probably fixed in the reaction catalyzed by glutamine synthetase.
The enzyme from monocots is also able to act as tyrosine ammonia lyase (EC 4.3.1.25), converting tyrosine to p-coumaric acid directly, (therefore without the 4-hydroxylation step), but with a lower efficiency.
In all plant species investigated,  several copies of phenylalanine ammonia lyase gene are found, copies that probably respond differentially to internal and external stimuli. Indeed, gene transcription, and then enzyme activity, are under the control of both internal developmental and external environmental stimuli. Here are some examples that require increased enzyme activity.

  • The flowering.
  • The  production of lignin to strengthen the secondary wall of xylem cells.
  • The production of flower pigments that attract pollinators.
  • Pathogen infections, that require the production of phenylpropanoid phytoalexins, or exposure to UV rays.

trans-Cinnamate 4-monooxygenase

It belongs to the cytochrome P450 superfamily (EC 1.14.-.-), is a microsomal monooxygenase containing a heme cofactor, and dependent on both O2  and NADPH. It catalyzes the formation of p-coumaric acid  through the introduction of a hydroxyl group in 4-position of trans-cinnamic acid (this hydroxyl group is present in most flavonoids).

trans-Cinnamic Acid + NADPH + H+ + O2 ⇄ p-Coumaric Acid + NADP+ + H2O

This reaction is also part of the biosynthesis of hydroxycinnamic acids.
Increases in transcription rates and enzyme activity are observed in correlation with the synthesis of phytoalexins (in response to fungal infections), lignification as well as wounding.

4-Coumarate:CoA ligase (4CL)

With Mg2+ as a cofactor, it catalyzes the ATP-dependent activation of the carboxyl group of p-coumaric acid and other hydroxycinnamic acids, metabolically rather inert molecules, through the formation of the corresponding CoA-thioester.

p-Coumaric Acid + ATP + CoA ⇄ p-Coumaroyl-CoA + AMP + PPi

Generally, p-coumaric acid and caffeic acid are the preferred substrates, followed by ferulic acid and 5-hydroxyferulic acid, low activity against trans-cinnamic acid and none against sinapic acid. These CoA-thioesters are able to enter various reactions such as:

  • reduction to alcohol (monolignols) or aldehydes;
  • stilbene and flavonoid biosynthesis;
  • transfer to acceptor molecules.

It should finally be pointed out that the activation of the carboxyl group can also be obtained through an UDP-glucose-dependent transfer to glucose.

Biosynthesis of malonyl-CoA

Malonyl-CoA does not derived from the phenylpropanoid pathway, but from the reaction catalyzed by acetyl-CoA carboxylase (EC 6.4.1.2, the cytosolic form, see below). The enzyme, with biotin and Mg2+ as cofactors, catalyzes the ATP-dependent carboxylation of acetyl-CoA, using bicarbonate as a source of carbon dioxide (CO2).

Acetyl-CoA + HCO3 + ATP → Malonyl-CoA + ADP + Pi

It is found both in the plastids, where it participates in the synthesis of fatty acids, and the cytoplasm, and is the latter that catalyzes the formation of malonyl-CoA that is used in the biosynthesis of flavonoids and other compounds. Increases in the transcription rate of the gene and enzyme activity are induced in response to stimuli that increase the biosynthesis of these polyphenols, such as exposure to pathogenic fungi or UV-rays.
In turn, acetyl-CoA is produced in plastids, mitochondria, peroxisomes and cytosol through different metabolic pathways. The molecules used in the biosynthesis of malonyl-CoA, and therefore of the flavonoids, are  the cytosolic ones, produced in the reaction catalyzed by ATP-citrate lyase (EC 2.3.3.8) that cleaves citrate, in the presence of CoA and ATP, to form oxaloacetate and acetyl-CoA, plus ADP and inorganic phosphate.

First steps in flavonoid biosynthesis

The first step in flavonoid biosynthesis is catalyzed by chalcone synthase (EC 2.3.1.74), an enzyme anchored to the endoplasmic reticulum and with no known cofactors.
From one p-coumaroyl-CoA and three malonyl-CoA, it catalyzes sequential condensation and decarboxylation reactions in the course of which a polyketide intermediate is formed. The polyketide undergoes cyclizations and aromatizations leading to the formation of the A ring. The product of the reactions is naringenin chalcone (2′,4,4′,6′-tetrahydroxychalcone), a 6′-hydroxychalcone and the first flavonoid to be synthesized by plants.

p-Coumaroyl-CoA + 3 Malonyl-CoA → Naringenin Chalcone + 4 CoA + 3 CO2

The reaction, cytosolic, is irreversible due to the release of three CO2 and 4 CoA.
The B ring and the three-carbon bridge of the molecule originate from p-coumaroyl-CoA (and therefore from phenylalanine), the A ring from the three malonyl-CoA units.

Flavonoid biosynthesis and the origin of the flavonoid skeleton
The Origin of the Flavonoid Skeleton

Also 6’-deoxychalcone can be produced; its synthesis is thought to involve an additional reduction step catalyzed by polyketide reductase (EC. 1.1.1.-).
Chalcone synthase from some plant species, such as barley (Hordeum vulgare), accepts as substrates also caffeoil-CoA, feruloil-CoA and cinnamoyl-CoA.
It is the most abundant enzyme of the phenylpropanoid pathway, probably because it has a low catalytic activity, and, in fact, is considered to be the rate-limiting enzyme in flavonoid biosynthesis.
As for phenylalanine ammonia lyase, chalcone synthase gene expression is under the control of both internal and external factors. In some plants, one or two isoenzymes are found, while in others up to 9.
Chalcone synthase belongs to polyketide synthase group, present in bacteria, fungi and plants. These enzymes are able to catalyze the production of polyketide chains through sequential condensations of acetate units provided by malonyl-CoA units. They also includes stilbene synthase (EC 2.3.1.146), which catalyzes the formation of resveratrol, a non flavonoid polyphenol compound that has attracted much interest because of its potential health benefits.
Generally, chalcones do not accumulate in plants because naringenin chalcone is converted to (2S)-naringenin, a flavanone, in the reaction catalyzed by chalcone isomerase (EC 5.5.1.6).
The enzyme, the first of the flavonoid biosynthesis to be discovered, catalyzes a stereospecific isomerization and closes the C ring. Two types of chalcone isomerases are known, called type I and II. Type I enzymes use only 6′-hydroxychalcone substrates, like naringenin chalcone, while type II, prevalent in legumes, use both 6′-hydroxy- and 6′-deoxychalcone substrates.
It should be noted that with 6′-hydroxychalcones, isomerization can also occur nonenzymically to form a racemic mixture, both in vitro and in vivo, enough to allow a moderate synthesis of anthocyanins. On the contrary, under physiological conditions 6′-deoxychalcones are stable, and so the activity of type II chalcone isomerases is required to form flavanones.
The enzyme increases the rate of the reaction of 107 fold compared to the spontaneous reaction, but with a higher kinetics for the 6′-hydroxychalcones than 6′-deoxychalcones. Finally, it produces (2S)-flavanones, which are the biosynthetically required enantiomers.
As other enzymes in flavonoid biosynthesis, also chalcone isomerase gene expression is subject to strict control. And, as phenylalanine ammonia lyase and chalcone synthase, it is induced by elicitors.
In the reaction catalysed by flavanone-3β-hydroxylase (EC 1.14.11.9), (2S)-flavanones undergo a stereospecific isomerization that converts them into the respective (2R,3R)-dihydroflavonols. In particular, naringenin is converted into dihydrokaempferol.
The enzyme is a cytosolic non-heme-dependent dioxygenase, dependent on Fe2+ and 2-oxoglutarate, and therefore belonging to the family of 2-oxoglutarate-dependent dioxygenase (which distinguishes them from the other hydroxylases of the flavonoid biosynthetic pathway which are cytochrome P450 enzymes).

Naringenin chalcone, (2S)-naringenin, and dihydrokaempferol are central intermediates in flavonoid biosynthesis, since they act as branch-point compounds from which the synthesis of distinct flavonoid subclasses can occur. For example, directly or indirectly:

Not all of these side metabolic pathways are present in every plant species, or are active within each tissue type of a given plant. Like enzymes previously seen, the activity of those involved in these “side-routes” is subject to strict control, resulting in a tissue-specific profile of flavonoid compounds. For example, grape seeds, flesh and skin have distinct anthocyanin, catechin, flavonol and condensed tannin profiles, whose synthesis and accumulation are strictly and temporally coordinated during the ripening process.

References

Andersen Ø.M., Markham K.R. Flavonoids: chemistry, biochemistry, and applications. CRC Press Taylor & Francis Group, 2006

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

Heldt H-W. Plant biochemistry – 3th Edition. Elsevier Academic Press, 2005

Vogt T. Phenylpropanoid biosynthesis. Mol Plant 2010;3(1):2-20. doi:10.1093/mp/ssp106

Wink M. Biochemistry of plant secondary metabolism – 2nd Edition. Annual plant reviews (v. 40), Wiley J. & Sons, Inc., Publication, 2010

Lignans: structure, metabolism, benefits, and foods

Lignans are a subgroup of non-flavonoid polyphenols.
They are widely distributed in the plant kingdom, being present in more than 55 plant families, where they act as antioxidants and defense molecules against pathogenic fungi and bacteria.
In humans, epidemiological and physiological studies have shown that they can exert positive effects in the prevention of lifestyle-related diseases, such as type II diabetes and cancer. For example, an increased dietary intake of these polyphenols correlates with a reduction in the occurrence of certain types of estrogen-related tumors, such as breast cancer in postmenopausal women.
In addition, some lignans have also aroused pharmacological interest. Examples are:

  • podophyllotoxin, obtained from plants of the genus Podophyllum (Berberidaceae family); it is a mitotic toxin whose derivatives have been used as chemotherapeutic agents;
  • arctigenin and tracheologin, obtained from tropical climbing plants; they have antiviral properties and have been tested in the search for a drug to treat AIDS .

CONTENTS

Chemical structure of lignans

Their basic chemical structure consists of two phenylpropane units linked by a C-C bond between the central atoms of the respective side chains (position 8 or β), also called β-β’ bond. 3-3′, 8-O-4′, or 8-3′ bonds are observed less frequently; in these cases the dimers are called neolignans. Hence, their chemical structure is referred to as (C6-C3)2, and they are included in the phenylpropanoid group, as well as their precursors: the hydroxycinnamic acids (see below).

Skeletal formula of phenylpropanoid unit of lignans
Fig. 1 – Phenylpropanoid unit

Based on their carbon skeleton, cyclization pattern, and the way in which oxygen is incorporated in the molecule skeleton, they can be divided into 8 subgroups: furans, furofurans, dibenzylbutanes, dibenzylbutyrolactones, dibenzocyclooctadienes, dibenzylbutyrolactols, aryltetralins and arylnaphthalenes. Furthermore, there is considerable variability regarding the oxidation level of both the propyl side chains and the aromatic rings.
They are not present in the free form in nature, but linked to other molecules, mainly as glycosylated derivatives.
Among the most common lignans, secoisolariciresinol (the most abundant one), lariciresinol, pinoresinol, matairesinol and 7-hydroxymatairesinol are found.

Note: They occur not only as dimers but also as more complex oligomers, such as dilignans and sesquilignans.

Synthesis of lignans

In this section, we will examine the synthesis of some of the most common lignans.
The pathway starts from 3 of the 4 most common dietary hydroxycinnamic acids: p-coumaric acid, sinapic acid, and ferulic acid (caffeic acid is not a precursor of this subgroup of polyphenols). Therefore, they arise from the shikimic acid pathway, via phenylalanine.

Synthesis pathways for lignans
Fig. 2 – Lignan Biosynthesis

The first three reactions reduce the carboxylic group of the hydroxycinnamates to alcohol group, with formation of the corresponding alcohols, called monolignols, that is, p-coumaric alcohol, sinapyl alcohol and coniferyl alcohol. These molecules also enter the pathway of lignin biosynthesis.

  • The first step, which leads to the activation of the hydroxycinnamic acids, is catalysed by hydroxycinnamate:CoA ligases, commonly called p-coumarate:CoA ligases (EC 6.2.1.12), with formation of the corresponding hydroxycinnamate-CoAs, namely, feruloil-CoA, p- coumaroyl-CoA and sinapil-CoA.
  • In the second step, a NADPH-dependent cinnamoyl-CoA: oxidoreductase, also called cinnamoyl-CoA reductase (EC1.2.1.44) catalyzes the formation of the corresponding aldehydes, and the release of coenzyme A.
  • In the last step, a NADPH-dependent cinnamyl alcohol dehydrogenase, also called monolignol dehydrogenase (EC 1.1.1.195), catalyzes the reduction of the aldehyde group to an alcohol group, with the formation of the aforementioned monolignols.

The next step, the dimerization of monolignols, involves the intervention of stereoselective mechanisms, or, more precisely, enantioselective mechanisms. In fact, most of the plant lignans exists as (+)- or (-)-enantiomers, whose relative amounts can vary from species to species, but also in different organs on the same plant, depending on the type of reactions involved.
The dimerization can occur through enzymatic reactions involving laccases (EC 1.10.3.2). These enzymes catalyze the formation of radicals that, dimerizing, form a racemic mixture. However, this does not explain how the racemic mixtures found in plants are formed. The most accepted mechanism to explain the stereospecific synthesis involves the action of the laccase and of a protein able to direct the synthesis toward one or the other of the two enantiomeric forms: the dirigent protein. The reaction scheme might be: the enzyme catalyzes the synthesis of phenylpropanoid radicals that are orientated in such a way to obtain the desired stereospecific coupling by the dirigent protein.

Skeletal formula of the lignan (-)-matairesinol
Fig. 3 – (-)-Matairesinol

For example, pinoresinol synthase, consisting of laccase and dirigent protein, catalyzes the stereospecific synthesis of (+)-pinoresinol from two units of coniferyl alcohol. (+)-Pinoresinol, in two consecutive stereospecific reactions catalyzed by NADPH-dependent pinoresinol/lariciresinol reductase (EC 1.23.1.2), is first reduced to (+)-lariciresinol, and then to (-)-secoisolariciresinol. (-)-Secoisolariciresinol, in the reaction catalyzed by NAD(P)-dependent secoisolariciresinol dehydrogenase (EC 1.1.1.331) is oxidized to (-)-matairesinol.

Metabolism by human gut microbiota

Their importance to human health is due largely to their metabolism by gut microbiota, which carries out deglycosylations, para-dehydroxylations, and meta-demethylations without enantiomeric inversion. Indeed, this metabolization leads to the formation molecules with a modest estrogen-like activity (phytoestrogens), a situation similar to that observed with some isoflavones, such as those of soybean, some coumarins, and some stilbenes. These active metabolites are the so-called “mammalian lignans or enterolignans”, such as the aglycones of enterodiol and enterolactone, formed from secoisolariciresinol and matairesinol, respectively.
Studies conducted on animals fed diets rich in lignans have shown their presence as intact molecules, in low concentrations, in serum, suggesting that they may be absorbed as such from the intestine. These molecules exhibit estrogen-independent actions, both in vivo and in vitro, such as inhibition of angiogenesis, reduction of diabetes, and suppression of tumor growth.
Note: The term “phytoestrogen” refers to molecules with estrogenic or antiandrogenic activity, at least in vitro.

Once absorbed, they enter the enterohepatic circulation, and, in the liver, may undergo phase II reactions and be sulfated or glucuronidated, and finally excreted in the urine.

Food sources

The richest dietary source is flaxseed (linseed), that contains mainly secoisolariciresinol, but also lariciresinol, pinoresinol and matairesinol in good quantity (for a total amount of more than 3.7 mg/100 g dry weight). They are also found in sesame seeds.

Skeletal formula of the lignan (-)-secoisolariciresinol
Fig. 4 – (-)-Secoisolariciresinol

Another important source is whole grains.
They are also present in other foods, but in concentrations from one hundred to one thousand times lower than those of flaxseed. Examples are:

  • beverages, generally more abundant in red wine, followed in descending order by black tea, soy milk and coffee;
  • fruits, such as apricots, pears, peaches, strawberries;
  • among vegetables, Brassicaceae, garlic, asparagus and carrots;
  • lentils and beans.

Their presence in grains and, to a lesser extent in red wine and fruit, makes them, at least in individuals who follow a Mediterranean-style eating pattern, the main source of phytoestrogens.

References

Andersen Ø.M., Markham K.R. Flavonoids: chemistry, biochemistry, and applications. CRC Press Taylor & Francis Group, 2006

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

Heldt H-W. Plant biochemistry – 3th Edition. Elsevier Academic Press, 2005

Manach C., Scalbert A., Morand C., Rémésy C., and Jime´nez L. Polyphenols: food sources and bioavailability. Am J Clin Nutr 2004;79(5):727-747. doi:10.1093/ajcn/79.5.727

Satake H, Koyama T., Bahabadi S.E., Matsumoto E., Ono E. and Murata J. Essences in metabolic engineering of lignan biosynthesis. Metabolites 2015;5:270-290. doi:10.3390/metabo5020270

Tsao R. Chemistry and biochemistry of dietary polyphenols. Nutrients 2010;2:1231-1246. doi:10.3390/nu2121231

van Duynhoven J., Vaughan E.E., Jacobs D.M., Kemperman R.A., van Velzen E.J.J, Gross G., Roger L.C., Possemiers S., Smilde A.K., Doré J., Westerhuis J.A.,and Van de Wiele T. Metabolic fate of polyphenols in the human superorganism. PNAS 2011;108(suppl. 1):4531-4538. doi:10.1073/pnas.1000098107

Wink M. Biochemistry of plant secondary metabolism – 2nd Edition. Annual plant reviews (v. 40), Wiley J. & Sons, Inc., Publication, 2010

Hydroxycinnamic acids: properties, synthesis, health benefits, natural sources

Hydroxycinnamic acids or hydroxycinnamates are phenolic compounds belonging to non-flavonoid polyphenols.
They are present in all parts of fruits and vegetables although the highest concentrations are found in the outer part of ripe fruits, concentrations that decrease during ripening, while the total amount increases as the size of the fruits increases.

Their dietary intake has been associated with the prevention of the development of chronic diseases such as:

  • cardiovascular disease;
  • cancer;
  • type-2 diabetes.

These effects do seem to be due not only to their high antioxidant activity (that depends upon the hydroxylation pattern of the aromatic ring, see below), but also to other mechanisms of action such as, e.g., the reduction of intestinal absorption of glucose or the modulation of secretion of some gut hormones.

CONTENTS

Chemical structure of hydroxycinnamic acids

Their basic structure is a benzene ring to which a three carbon chain is attached, structure that is referred to as C6-C3. Therefore they can be included in the phenylpropanoid group.

Basic skeleton structure of hydroxycinnamic acids, phenolic compounds belonging to non-flavonoid polyphenols
Basic Skeleton of Hydroxycinnamates

The main dietary hydroxycinnamates are:

  • caffeic acid or 3,4-dihydroxycinnamic acid;
  • ferulic acid or 4-hydroxy-3-methoxycinnamic acid;
  • sinapic acid or 4-hydroxy-3,5-dimethoxycinnamic acid;
  • p-coumaric acid or 4-coumaric acid or 4-hydroxycinnamic acid.

In nature, they are associated with other molecules to form, e.g., glycosylated derivatives or esters of tartaric acid, quinic acid, or shikimic acid. In addition, several hundreds of anthocyanins acylated with the aforementioned hydroxycinnamates have been identified (in descending order with p-coumaric acid, more than 150, caffeic acid, about 100, ferulic acid, about 60, and sinapic acid, about 25). They are rarely present in the free form, except in processed foods that have undergone fermentation, sterilization or freezing. For example, an overlong storage of blood orange fruits causes a massive hydrolysis of hydroxycinnamic derivatives to free acids, and this in turn could lead to the formation of malodorous compounds such as vinyl phenols, indicators of too advanced senescence of the fruit.

Synthesis of hydroxycinnamic acids

Hydroxycinnamate biosynthesis consists of a series of enzymatic reactions subsequent to that catalyzed by phenylalanine ammonia lyase (PAL).

Phenylalanine ⇄ trans-Cinnamic acid + NH3

This enzyme catalyzes the deamination of phenylalanine to yield trans-cinnamic acid, so linking the aromatic amino acid to the hydroxycinnamic acids and their activated forms.

Synthesis of hydroxycinnamic acids from phenylalanine
Biosynthesis of Hydroxycinnamates

In the first step, a hydroxyl group is attached at the 4-position of the aromatic ring of trans-cinnamic acid to form p-coumaric acid. The reaction catalysed by trans-cinnamate 4-monooxygenase (EC:1.14.14.91).

trans-Cinnamic acid + NADPH + H+ + O2 ⇄ p-Coumaric acid + NADP+ + H2O

The addition of a second hydroxyl group at the 3-position of the ring of p-coumaric acid leads to the formation of caffeic acid. The reaction catalysed by p-coumarate 3-hydroxylase (EC 1.14.13.-).

p-Coumaric acid + NADPH + H+ + O2 ⇄ Caffeic acid + NADP+ + H2O

The O-methylation of the hydroxyl group at the 3-position yields ferulic acid. The reaction catalyzed by caffeate 3-O-methyltransferase (EC:2.1.1.68).

Caffeic acid + SAM ⇄ Ferulic acid + SAH

Ferulic acid is converted into sinapic acid through two reactions:  a hydroxylation at the 5-position to form 5-hydroxy ferulic acid, in a reaction catalyzed by ferulate 5-hydroxylase (EC:1.14.-.-), and the subsequent O-methylation of the same hydroxyl group in a reaction catalyzed by caffeate 3-O-methyltransferase.

Ferulic acid + NADPH + H+ + O2 ⇄ 5-Hydroxy ferulic acid + NADP+ + H2O

5-Hydroxy ferulic acid + SA from M ⇄ Sinapic acid + SAH

Hydroxycinnamic acids are not present in high quantities since they are rapidly converted to glucose esters or coenzyme A (CoA) esters, in reactions catalyzed by O-glucosyltransferases and hydroxycinnamate:CoA ligases, respectively. These activated intermediates are branch points, being able to participate in a wide range of reactions such as condensation with malonyl-CoA to form flavonoids, or the NADPH-dependent reduction to form lignans (precursors of lignin).

The main hydroxycinnamic acids in foods

Kiwis, blueberries, plums, cherries, apples, pears, chicory, artichokes, carrots, lettuce, eggplant, wheat and coffee are among the richest sources.

Caffeic acid

It is generally, both in the free form and bound to other molecules, the most abundant hydroxycinnamic acid in vegetables and most of the fruits, where it represents between 75 and 100% of the hydroxycinnamates.
The richest sources are coffee (drink), carrots, lettuce, potatoes, even sweet ones, and berries such as blueberries, cranberries and blackberries.
Smaller quantities are present in grapes and grape-derived products, orange juice, apples, plums, peaches, and tomatoes.
Caffeic acid and quinic acid bind to form chlorogenic acid, present in many fruit and in high concentration in coffee.

Ferulic acid

It is the most abundant hydroxycinnamic acid in cereals, which are also its main dietary source.
In wheat grain, its content is between 0.8 and 2 g/kg dry weight, which represents up to 90% of the total polyphenols. It is found chiefly, up to 98% of the total content, in the aleurone layer and pericarp (that is, the outer parts of the grain), and therefore its content in wheat flours depends upon the degree of refining, while the main source is obviously the bran. The molecule is present mainly in the trans form, and esterified with arabinoxylans and hemicelluloses. And in fact, in wheat bran the soluble free form represents only about 10% of its total amount. Dimers were also found, which form bridge structures between chains of hemicellulose.
In fruits and vegetables, ferulic acid is much less common than caffeic acid. The main sources are asparagus, eggplant and broccoli; lower quantities are found in blackberries, blueberries, cranberries, apples, carrots, potatoes, beets, coffee and orange juice.

Sinapic acid

The highest amounts are found in citrus peel and seeds (in orange juice, the amount is much lower); appreciable quantities in Chinese cabbage and in some varieties of cranberries.

p-Coumaric acid

High amounts are present in eggplant, the richest source, broccoli and asparagus; other sources are sweet cherries, plums, blueberries, cranberries, citrus peel and seeds, and orange juice.

References

Andersen Ø.M., Markham K.R. Flavonoids: chemistry, biochemistry, and applications. CRC Press Taylor & Francis Group, 2006

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

Manach C., Scalbert A., Morand C., Rémésy C., and Jime´nez L. Polyphenols: food sources and bioavailability. Am J Clin Nutr 2004;79(5):727-747. doi:10.1093/ajcn/79.5.727

Preedy V.R. Coffee in health and disease prevention. Academic Press, 2014

Zhao Z.,  Moghadasian M.H. Bioavailability of hydroxycinnamates: a brief review of in vivo and in vitro studies. Phytochem Rev 2010;9(1):133-145. doi:10.1007/s11101-009-9145-5

Polyphenols from grapes and wines: content, biological activities, and benefits

The consumption of grapes and grape-derived products, particularly red wine but only at meals, has been associated with numerous health benefits, which include, in addition to the antioxidant/antiradical effect, also anti-inflammatory, cardioprotective, anticancer, antimicrobial, and neuroprotective activities.
Grapes contain many nutrients such as sugars, vitamins, minerals, fiber and phytochemicals. Among the latter, polyphenols from grapes are the most important compounds in determining the health effects of the fruit and derived products.
Indeed, grapes are among the fruits with highest content in polyphenols, whose composition is strongly influenced by several factors such as:

  • cultivar;
  • climate;
  • exposure to disease;
  • processing

Nowadays, the main species of grapes cultivated worldwide are: European grapes, Vitis vinifera, North American grapes, Vitis rotundifolia and Vitis labrusca, and French hybrids.
Note: Grapes are not a fruit but an infructescence, that is, an ensemble of fruits (berries): the bunch of grapes. In turn, it consists of a peduncle, a rachis, cap stems or pedicels, and berries.

CONTENTS

What are polyphenols from grapes and wines?

Polyphenols from red grapes and wine are significantly higher, both in quantity and variety, than in white ones. This, according to many researchers, would be the basis of the more health benefits related to the consumption of red grapes and wine than white grapes and derived products.
Polyphenols from grapes and wine are a complex mixture of flavonoid compounds, the most abundant group, and non-flavonoid compounds.
Among flavonoids, they are found:

Among non-flavonoid polyphenols:

Most of the flavonoids present in wine derive from the epidermal layer of the berry skin, while 60-70% of the total polyphenols are present in the grape seeds. It should be noted that more than 70% of grape polyphenols are not extracted and remain in the pomace.
The complex chemical interactions that occur between these compounds, and between them and the other compounds of different nature present in grapes and wine, are probably essential in determining both the quality of the grapes and wine and the broad spectrum of therapeutic effects of these foods.
In wine, the mixture of polyphenols play important functions being able to influence:

  • bitterness;
  • astringency;
  • red color, of which they are among the main responsible;
  • sensitivity to oxidation, being molecules easily oxidizable by atmospheric oxygen.

Finally, they act as preservatives and are the basis of long aging.

Anthocyanins

They are flavonoids widely distributed in fruits and vegetables.
They are primarily located in the berry skin (in the outer layers of the hypodermal tissue), to which they confer color, having a hue that varies from red to blue. In some varieties, called “teinturier”, they also accumulate in the flesh of the berry.
There is a close relationship between berry development and the biosynthesis of anthocyanins. The synthesis starts at veraison (when the berry stops growing and changes its color), causes a color change of the berry that turns purple, and reaches the maximum levels at complete ripening.
Among wine flavonoids, they are one of the most potent antioxidants.
Each grape species and cultivars has a unique composition of anthocyanins. Moreover, in grapes of Vitis vinifera, due to a mutation in the gene coding for 5-O-glucosyltransferase, mutation that determines the synthesis of an inactive enzyme, only 3-monoglucoside derivatives are synthesized, while in other species  the glycosylation at position 5 also occurs. Interestingly, 3-monoglucoside derivatives are more intensely colored than 3,5-diglucoside derivatives.

Skeletal formula of malvidin-3-glucoside, an anthocyanin
Fig. 1 – Malvidin-3-glucoside

In red grapes and wine, the most abundant anthocyanins are the 3-monoglucosides of malvidin (the most abundant one both in grapes and wine), petunidin, delphinidin, peonidin, and cyanidin. In turn, the hydroxyl group at position 6 of the glucose can be acylated with an acetyl, caffeic or coumaric group, acylation that further enhances the stability.
Anthocyanidins, namely the non-conjugated molecules, are not present in grapes and in wine, except as traces.
Anthocyanins are scarcely present in white grapes and wine.
The composition of anthocyanins in wine is highly influenced both by the type of cultivar and by processing techniques, since they are present in wine as a result of extraction by maceration/fermentation processes. For this reason, wines deriving from similar varieties of grapes can have very different anthocyanin compositions.
Together with proanthocyanidins, they are the most important polyphenols in contributing to some organoleptic properties of red wine, as they are primarily responsible for astringency, bitterness, chemical stability against oxidation, as well as of the color of the young wine. In this regard, it should be underscored that with time their concentration decreases, while the color is due more and more to the formation of polymeric pigments produced by condensation of anthocyanins both among themselves and with other molecules.
During wine aging, proanthocyanidins and anthocyanins react to produce more complex molecules that can  partially precipitate.

Flavanols or catechins

They are, together with condensed tannins, the most abundant flavonoids, representing up to 50% of the total polyphenols in white grapes and between 13% and 30% in red ones.
Their levels in wine depend on the type of cultivar.

Polyphenols from grapes: skeletal formula of catechin, a flavanol
Fig. 2 – Catechin

Typically, the most abundant flavanol in wine is catechin, but epicatechin and epicatechin-3-gallate are also present.

Proanthocyanidins or condensed tannins

Composed of catechin monomers, they are present in the berry skin, seeds and rachis of the bunch of grapes as:

  • dimers: the most common are procyanidins B1-B4, but also procyanidins B5-B8 can be present;
  • trimers: procyanidin C1 is the most abundant;
  • tetramers;
  • polymers, containing up to 8 monomers.
Skeletal formula of procyanidin C1, a proanthocyanidin
Fig. 3 – Procyanidin C1

Their levels in wine depend on the type of grape varieties and wine-making technology, and, like anthocyanins, are much more abundant in red wines, in particular in aged wines, compared to white ones.
In addition, as previously said, together with anthocyanins, condensed tannins are important in determining some organoleptic properties of the wine.

Flavonols

They are present in a large variety of fruit and vegetables, even if in low concentrations.
They are the third most abundant group of flavonoids from grapes, after proanthocyanidins and catechins.
They are mainly present in the outer epidermis of the berry skin, where they play a role both in providing protection against UV-A and UV-B radiations and in copigmentation together with anthocyanins.
Flavanol synthesis begins in the sprout; the highest concentration is reached a few weeks after veraison, then it decreases as the berry increases in size.
Their total amount is very variable, with the red varieties often richer than the white ones.
In grapes, they are present as 3-glucosides and their composition depends on the type of grapes and cultivar:

  • the derivatives of quercetin, kaempferol and isorhamnetin are found in white grapes;
  • the derivatives of myricetin, laricitrin and syringetin are found, together with the previous ones, only in red grapes, due to the lack of expression in white grapes of the gene coding for flavonoid-3′,5′-hydroxylase.
Polyphenols from grapes: skeletal formula of quercetin-3-glucoside, a flavonol
Fig. 4 – Quercetin-3-glucoside

In general, the 3-glucosides and 3-glucuronides of quercetin are the major flavonols in most of the grape varieties. Conversely, quercetin-3-rhamnoside and quercetin aglycone are the major flavonols in muscadine grapes.
In wine and grape juice, unlike grapes, they are also found as aglycones, as a result of the acid hydrolysis that occurs during processing and storage. They are present in wine in a variable amount, and the major molecules are the glycosides of quercetin and myricetin, which alone represent 20-50% of the total flavonols in red wine.

Hydroxycinnamates

Hydroxycinnamic acids are the main class of non-flavonoid polyphenols from grapes and the major polyphenols in white wine.
The most important are p-coumaric, caffeic, sinapic, and ferulic acids, present in wine as esters with tartaric acid.
They have antioxidant activity and in some white varieties of Vitis vinifera, together with flavonols, are the polyphenols mainly responsible for absorbing UV radiation in the berry.

Stilbenes

They are phytoalexins which are produced in low concentrations only by a few edible species, including grapevine (on the contrary, flavonoids are present in all higher plants).
Together with the other polyphenols from grapes and wine, also stilbenes, particularly resveratrol, have been associated with health benefits resulting from the consumption of wine.

Polyphenols from grapes: skeletal formula of trans-resveratrol, a stilbene
Fig. 5 – trans-Resveratrol

Their content increases from the veraison to the ripening of the berry, and is influenced by the type of cultivar, climate, wine-making technology, and fungal pressure.
The main stilbenes present in grapes and wine are:

  • cis- and trans-resveratrol (3,5,4′-trihydroxystilbene);
  • piceid or resveratrol-3-glucopyranoside and astringin or 3′-hydroxy-trans-piceid;
  • piceatannol;
  • dimers and oligomers of resveratrol, called viniferins, of which the most important are:

α-viniferin, a trimer;
β-viniferin, a cyclic tetramer;
γ-viniferin, a highly polymerized oligomer;
ε-viniferin, a cyclic dimer.

In grapes, other glycosylated and isomeric forms of resveratrol and piceatannol, such as resveratroloside, hopeaphenol, or resveratrol di- and tri-glucoside derivatives, have been found in trace amounts.
Glycosylation of stilbenes is important for the modulation of antifungal activity, protection from oxidative degradation, and storage of the wine.
The synthesis of dimers and oligomers of resveratrol, both in grapes and wine, represents a defense mechanism against exogenous attacks or, on the contrary, the result of the action of extracellular enzymes released from pathogens in an attempt to eliminate undesirable compounds.

Hydroxybenzoates

The hydroxybenzoic acid derivatives are a minor component in grapes and wine.
In grapes, gentisic, gallic, p-hydroxybenzoic and protocatechuic acids are the main ones.

Skeletal formula of gallic acid, an hydroxybenzoic acid
Fig. 6 – Gallic Acid

Unlike hydroxycinnamates, which are present in wine as esters with tartaric acid, they are found in their free form.
Together with flavonols, proanthocyanidins, catechins, and hydroxycinnamates they are among the responsible of astringency of wine.

References

Andersen Ø.M., Markham K.R. Flavonoids: chemistry, biochemistry, and applications. CRC Press Taylor & Francis Group, 2006

Basli A, Soulet S., Chaher N., Mérillon J.M., Chibane M., Monti J.P.,1 and Richard T. Wine polyphenols: potential agents in neuroprotection. Oxid Med Cell Longev 2012. doi:10.1155/2012/805762

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

Flamini R., Mattivi F.,  De Rosso M., Arapitsas P. and Bavaresco L. Advanced knowledge of three important classes of grape phenolics: anthocyanins, stilbenes and flavonols. Int J Mol Sci 2013;14:19651-19669. doi:10.3390/ijms141019651

Georgiev V., Ananga A. and Tsolova V. Recent advances and uses of grape flavonoids as nutraceuticals. Nutrients 2014;6: 391-415. doi:10.3390/nu6010391

Guilford J.M. and Pezzuto J.M. Wine and health: a review. Am J Enol Vitic 2011;62(4):471-486. doi:10.5344/ajev.2011.11013

He S., Sun C. and Pan Y. Red wine polyphenols for cancer prevention. Int J Mol Sci 2008;9:842-853. doi:10.3390/ijms9050842

Xia E-Q., Deng G-F., Guo Y-J. and Li H-B. Biological activities of polyphenols from grapes. Int J Mol Sci 2010;11:622-646. doi:10.3390/ijms11020622

Waterhouse A.L. Wine phenolics. Ann N Y Acad Sci 2002;957:21-36. doi:10.1111/j.1749-6632.2002.tb02903.x

Chemical composition of olive oil

From a chemical point of view, we can identify in the olive oil two fractions, depending on the behavior in the presence of heating and strong alkaline solutions (concentrated solutions of KOH or NaOH):

  • the saponifiable fraction, which represents 98-99% of the total weight, is composed of lipids that form soaps in the above conditions;
  • the unsaponifiable fraction, which represents the remaining 1-2% of the total weight, is composed of substances that fail to form soaps in the above conditions.

CONTENTS

Saponifiable fraction

It is composed of saturated fatty acids and unsaturated fatty acids, esterified almost entirely to glycerol to form triglycerides (or triacylglycerols). To a much lesser extent, diglycerides (or diacylglycerols), monoglycerides (monoacylglycerols), and free fatty acids are also found.
Unsaturated fatty acids make up 75 to 85% of the total fatty acids. Oleic (O) and linoleic (L) acids are the most abundant ones; palmitoleic, eptadecenoic, gadoleic and alpha-linolenic (Ln) acids are present in lower/trace amounts.

International Olive Oil Council (IOOC) requirements for fatty acids in olive oil
IOOC Requirements for Olive Oil

Oleic acid is the major fatty acid in olive oils. According to the rules laid down by the International Olive Oil Council (IOOC), its concentration must range from 55% to 83% of total fatty acids.
Linoleic acid is the most abundant polyunsaturated fatty acid in olive oil; its concentration must vary between 2.5% and 21% (IOOC). Because of its high degree of unsaturation, it is subject to oxidation; this means that an oil high in linoleic acid becomes rancid easily, and thus it may be stored for a shorter time.
In a Mediterranean-type diet, olive oil is the main source of fat: therefore, oleic acid, among monounsaturated fatty acids, and linoleic acid, among polyunsaturated fatty acids, are the most abundant fatty acids.
alpha-Linolenic acid must be present in very low amount, according to the IOOC standards ≤1%. It is an omega-3 polyunsaturated fatty acid, which may have health benefits. However, because of to its high degree of unsaturation (higher than that of linoleic acid), it is very susceptible to oxidation, and therefore it promotes rancidity of the olive oil that contains it.
Saturated fatty acids make up 15 to 25% of the total fatty acids.
Palmitic (P) (7.5-20%) and stearic (S) acids (0.5-5%) are the most abundant saturated fatty acids; myristic, heptadecanoic, arachidic, behenic and lignoceric acids may be present in trace amounts.

The presence of fatty acids that should be absent or present in amounts different than those found is a marker of adulteration with other vegetable oils. On this regard, particular attention is paid to myristic, arachidic, behenic, lignoceric, gadoleic and alpha-linolenic acids, whose limits are set by IOOC.

Fatty acid composition is influenced by several factors.

  • The climate.
  • The latitude.
  • The zone of production.
    Italian, Spanish and Greek olive oils are high in oleic acid and low in palmitic and linoleic acids, while Tunisian olive oils are high in palmitic and linoleic acids but lower in oleic acid. Therefore, oils can be divided into two groups:

one rich in oleic acid and low in palmitic and linoleic acids;
the other high in palmitic and linoleic acids and low in oleic acid.

  • The cultivar.
  • The degree of olive ripeness at the time of oil extraction.
    It should be noted that oleic acid is formed first in the fruit, and data seem to indicate a competitive relationship between oleic acid and palmitic, palmitoleic, and linoleic acids.

Triglycerides

As previously said, fatty acids in olive oil are almost entirely present as triglycerides.
In small percentage, they are also present as diglycerides, monoglycerides, and in free form.

Sterospecific numbering of triglycerides

During triglyceride biosynthesis, thanks to the presence of specific enzymes, only about 2% of glycerol binds palmitic acid in the sn-2 position (also the percentage of stearic acid in the sn-2 position is very low); for the most part, the sn-2 position is occupied by oleic acid.
On the contrary, if we consider oils that have undergone a nonenzymatic esterification, the percentage of palmitic acid in the sn-2 position increases significantly.
Note: sn = Stereospecific numbering

Among triglycerides present in significant proportions in olive oil, there are:

  • OOO: 40-59%;
  • POO: 12-20%;
  • OOL: 12.5-20%;
  • POL:  5.5-7%;
  • SOO: 3- 7%.

POP, POS, OLnL, OLnO, PLL, PLnO are present in smaller amounts.
Trilinolein (LLL) is a triglyceride that contains three molecules of linoleic acid. Its low content is an indicator of an oil of good quality.
Triglycerides containing three saturated fatty acids or three molecules of alpha-linolenic acid have not been reported.

Diglycerides and monoglycerides

Their presence is due to an incomplete synthesis and/or a partial hydrolysis of triglycerides.
The content of diglycerides in virgin olive oil ranges from 1% to 2.8%. 1,2-Diglycerides prevail in fresh olive oil, representing over 80% of the diglycerides. During oil storage, isomerization occurs with a progressive increase of the more stable 1-3 isomers, which after about 10 months become the major isomers.
Therefore, the ratio 1,2/1,3-diglycerides may be used as an indicator of the age of the oil.
Monoglycerides are present in amounts lower than diglycerides, <0.25%, with 1-monoglycerides far more abundant than 2-monoglycerides.

Unsaponifiable fractions

It is composed of a large number of different molecules, very important from a nutritional point of view, as they contribute significantly to the health effects of olive oil.
Furthermore, they are responsible for the stability and the taste of olive oil, and are also used to detect adulteration with other vegetable oils.
This fraction includes tocopherols, sterols, polyphenols, pigments, hydrocarbons, aromatic and aliphatic alcohol, triterpene acids, waxes, and minor constituents.
Their content is influenced by factors similar to those seen for fatty acid composition, such as:

  • the cultivar;
  • the degree of ripeness of the olive;
  • the zone of production;
  • the crop year and olive harvesting practices;
  • the storage time of olives;
  • the oil extraction process;
  • the storage conditions of the oil.

It should be noted that many of these compounds are not present in refined olive oils, as they are removed during the refining processes.

Polyphenols

They make up 18 to 37% of the unsaponifiable fraction.
They are a very heterogeneous group of molecules with nutritional and organoleptic properties  (for example, oleuropein and hydroxytyrosol give oil its bitter and pungent taste).
For a more extensive discussion, see: ” Polyphenols in olive oil: variability and composition.”

Hydrocarbons

They make up 30 to 50% of the unsaponifiable fraction.
Squalene and beta-carotene are the main molecules.
Squalene, isolated for the first time from shark liver, is the major constituent of the unsaponifiable fraction, and constitutes more than 90% of the hydrocarbons. Its concentration ranges from 200 to 7500 mg/kg of olive oil.

Skeletal formula of squalene, an hydrocarbon of the unsaponifiable fraction of olive oil

It is an intermediate in the biosynthesis of the four-ring structure of steroids, and it seems to be responsible of several health effects of olive oil.
In the hydrocarbon fraction of virgin olive oil, n-paraffins, diterpene and triterpene hydrocarbons, isoprenoidal polyolefins are also found.
Beta-carotene acts both as antioxidant, protecting oil during storage, and as dye (see below).

Sterols

They are important lipids of olive oil, and are:

  • linked to many health benefits for consumers;
  • important to the quality of the oil;
  • widely used for checking its genuineness.
    On this regard, it is to underline that sterols are species-specific molecules; for example, the presence of high concentrations of brassicasterol, a sterol typically found in Brassicaceae (Cruciferae) family, such as rapeseed, indicates adulteration of olive oil with canola oil.

Four classes of sterols are present in olive oil: common sterols, 4-methylsterols, triterpene alcohols, and triterpene dialcohols. Their content ranges from 1000 mg/kg, the minimum value required by the IOOC standard, to 2000 mg/kg. The lowest values are found in refined oils because of the refining processes may cause losses up to 25%.

Common sterols or 4α-desmethylsterols

Common sterols are present mainly in the free and esterified form; however they have been also found as lipoproteins and sterylglucosides.
The main molecules are beta-sitosterol, which makes up 75 to 90% of the total sterol, Δ5-avenasterol, 5 to  20%, and campesterol, 4%. Other components found in lower amounts or traces are, for example, stigmasterol, 2%, cholesterol, brassicasterol, and ergosterol.Skeletal formula of beta-sitosterol, a sterol of the unsaponifiable fraction of olive oil

4-Methylsterols

They are intermediates in the biosynthesis of sterols, and are present both in the free and esterified form. They are present in small amounts, much lower than those of common sterols and triterpene alcohols, varying between 50 and 360 mg/kg. The main molecules are obtusifoliol, cycloeucalenol, citrostadienol, and gramisterol.

Triterpene alcohols or 4,4-dimethylsterols

They are a complex class of sterols, present both in the free and esterified form. They are found in amounts ranging from 350 to 1500 mg/kg.
The main components are beta-amyrin, 24-methylenecycloartanol, cycloartenol, and butyrospermol; other molecules present in lower/trace amounts are, for example, cyclosadol, cyclobranol, germanicol, and dammaradienol.

Triterpene dialcohols

The main triterpene dialcohols found in olive oil are erythrodiol and uvaol.
Erythrodiol is present both in the free and esterified form; in virgin olive oil, its level varies between 19 and 69 mg/kg, and the free form is generally lower than 50 mg/kg.

Tocopherols

They make up 2 to 3% of the unsaponifiable fraction, and include vitamin E.
Of the eight E-vitamers, alpha-tocopherol represents about 90% of tocopherols in virgin olive oil. It is present in the free form and in very variable amount, but on average higher than 100 mg/kg of olive oil. Thanks to its in vivo antioxidant properties, its presence is a protective factor for health. Alpha-tocopherol concentration seems to be related to the high levels of chlorophylls and to the concomitant requirement for deactivation of singlet oxygen.
Beta-tocopherol, delta-tocopherol, and gamma-tocopherol are usually present in low amounts.

Pigments

In this group we find chlorophylls and carotenoids.
In olive oil, chlorophylls are present as phaeophytins, mainly  phaeophytin a (i.e. a chlorophyll from which magnesium has been removed and substituted with two hydrogen ions), and confer the characteristic green color to olive oil. They are photosensitizer molecules that contribute to the photooxidation of olive oil itself.
Beta-carotene and lutein are the main carotenoids in olive oil. Several xanthophylls are also present, such as antheraxanthin, beta-cryptoxanthin, luteoxanthin, mutatoxanthin, neoxanthin, and violaxanthin.
Olive oil’s color is the result of the presence of chlorophylls and carotenoids and of their green and yellow hues. Their presence is closely related.

Triterpene acids

They are important components of the olive, and are present in trace amounts in the oil.
Oleanolic and maslinic acids are the main triterpene acids in virgin olive oil: they are present in the olive husk, from which they are extracted in small amount during processing.

Aliphatic and aromatic alcohols

Fatty alcohols and diterpene alcohols are the most important ones.
Aliphatic alcohols have a number of carbon atoms between 20 and 30, and are located mostly inside the olive stones, from where they are partially extracted by milling.

Fatty alcohols

They are linear saturated alcohols with more than 16 carbon atoms.
They are found in the free and esterified form and are present, in virgin olive oil, in amount not generally higher than 250 mg/kg.
Docosanol (C22), tetracosanol (C24), hexacosanol (C26), and octacosanol (C28) are the main fatty alcohols in olive oil, with tetracosanol and hexacosanol present in larger amounts.
Waxes, which are minor constituents of olive oil, are esters of fatty alcohols with fatty acids, mainly of palmitic acid and oleic acid. They can be used as a criterion to discriminate between different types of oils; for example, they must be present in virgin and extra virgin olive oil at levels <150 mg/kg, according to the IOOC standards.

Diterpene alcohols

Geranylgeraniol and phytol are two acyclic diterpene alcohols, present in the free and esterified form. Among esters present in the wax fraction of extra virgin olive oil, oleate, eicosenoate , eicosanoate, docosanoate, and tetracosanoate have been found, mainly as phytyl derivatives.

Volatile compounds

More than 280 volatile compounds have been identified in olive oil, such as hydrocarbons, the most abundant fraction, alcohols, aldehydes, ketones, esters, acids, ethers and many others. However, only about 70 of them are present at levels higher than the perception threshold beyond which they may contribute to the aroma of virgin olive oil.

Minor components

Phospholipids are found among the minor components of olive oil; the main ones are phosphatidylserine, phosphatidylethanolamine, phosphatidylcholine, phosphatidylinositol.
In the unfiltered oils, trace amounts of proteins may be found.

References

Gunstone F.D. Vegetable oils in food technology: composition, properties and uses. 2th Edition. Wiley J. & Sons, Inc., Publication, 2011

Caponio F., Bilancia M.T., Pasqualone A., Sikorska E., Gomes T. Influence of the exposure to light on extra virgin olive oil quality during storage. Eur Food Res Technol 2005;221:92-98. doi:10.1007/s00217-004-1126-8

Servili M., Sordini B., Esposto S., Urbani S., Veneziani G., Di Maio I., Selvaggini R. and Taticchi A. Biological activities of phenolic compounds of extra virgin olive oil. Antioxidants 2014;3:1-23. doi:10.3390/antiox3010001

Gluten: definition, gliadins, glutenins, and containing grains

Gluten is not a single protein but a mixture of cereal proteins, about 80% of its dry weight (for example gliadins and glutenins in wheat grains), lipids, 5-7%, starch, 5-10%, water, 5-8%, and mineral substances, <2%.
It forms when components naturally present in the grain of cereals, the caryopsis, and in their flours, are joined together by means of mechanical stress in aqueous environment, i.e. during the formation of the dough.
The term is also related to the family of proteins that cause problems for celiac patients (see below).
Isolated for the first time in 1745 from wheat flour by the Italian chemist Jacopo Bartolomeo Beccari, it can be extracted from the dough by washing it gently under running water: starch, albumins and globulins, that are water-soluble, are washed out, and a sticky and elastic mass remains, precisely the gluten (it means glue in Latin).

CONTENTS

Cereals containing gluten

It is present in:

  • wheat, such as:

durum wheat (Triticum durum); groats and semolina for dry pasta making are obtained from it;
common wheat or bread wheat (Triticum aestivum), so called because it is used in bread and fresh pasta making, and in bakery products;

  • rye (Secale cereale);
  • barley (Hordeum vulgare);
  • spelt, in the three species:

einkorn (Triticun monococcum);
emmer (Triticum dicoccum Schrank);
spelta (Triticum spelta);

  • khorasan wheat (Triticum turanicum); a variety of it is Kamut®;
  • triticale (× Triticosecale Wittmack), which is a hybrid of rye and common wheat;
  • bulgur, which is whole durum wheat, sprouted and then processed;
  • seitan, which is not a cereal, but a wheat derivative, also defined by some as “gluten steak”.

Given that most of the dietary intake of gluten comes from wheat flour, of which about 700 million tons per year are harvested, representing about 30% of the global cereal production, the following discussion will focus on wheat gluten, and mainly on its proteins.

Note: The term gluten is also used to indicate the protein fraction that remains after removal of starch and soluble proteins from the dough obtained with corn flour: however, this “corn gluten” is “functionally” different from that obtained from wheat flour.

Cereal grain proteins

The study of cereal grain proteins, as seen, began with the work of Beccari. 150 years later, in 1924, the English chemist Osborne T.B., which can rightly be considered the father of plant protein chemistry, developed a classification based on their solubility in various solvents.
The classification, still in use today, divides plant proteins into 4 families.

  • Albumins, soluble in water.
  • Globulins, soluble in saline solutions; for example avenalin of oat.
  • Prolamins, soluble in 70% alcohol solution, but not in water or absolute alcohol.
    They include:

gliadins of wheat;
zein of corn;
avenin of oats;
hordein of barley;
secalin of rye.

They are the toxic fraction of gluten for celiac patients.

  • Glutelins, insoluble in water and neutral salt solutions, but soluble in acidic and basic solutions.
    They include glutenins of wheat.
Proteins found in cereal grains: albumins, globulins, prolamins, glutelins
Cereal Grain Proteins

Albumins and globulins are cytoplasmic proteins, often enzymes, rich in essential amino acids, such as lysine, tryptophan and methionine. They are found in the aleurone layer and embryo of the caryopsis.
Prolamins and glutelins are the storage proteins of cereal grains. They are rich in glutamine and proline, but very low in lysine, tryptophan and methionine. They are found in the endosperm, and are the vast majority of the proteins in the grains of wheat, corn, barley, oat, and rye.
Although Osborne classification is still widely used, it would be more appropriate to divide cereal grain proteins into three groups: structural and metabolic proteins, storage proteins, and defense proteins.

Wheat gluten proteins

Proteins represent 10-14% of the weight of the wheat caryopsis (about 80% of its weight consists of carbohydrates).
According to the Osborne classification, albumins and globulins represent 15-20% of the proteins, while prolamins and glutelins are the remaining 80-85%, composed respectively of gliadins, 30-40%, and glutenins, 40-50%. Therefore, and unlike prolamins and glutelins in the grains of other cereals, gliadins and glutenins are present in similar amounts, about 40% (see Fig. 2).

Gluten and other proteins found in wheat grains
Wheat Grain Proteins

Technologically, gliadins and glutenins are very important. Why?
These proteins are insoluble in water, and in the dough, that contains water, they bind to each other through a combination of intermolecular bonds, such as:

  • covalent bonds, i.e. disulfide bridges;
  • noncovalent bonds, such as hydrophobic interactions, van der Waals forces, hydrogen bonds, and ionic bonds.

Thanks to the formation of these intermolecular bonds, a three-dimensional lattice is formed. This structure entraps starch granules and carbon dioxide bubbles produced during leavening, and gives strength and elasticity to the dough, two properties of gluten widely exploited industrially.
In the usual diet of the European adult population, and in particular in Italian diet that is very rich in derivatives of wheat flour, gliadin and glutenin are the most abundant proteins, about 15 g per day. What does this mean? It means that gluten-free diet engages celiac patients both from a psychological and social point of view.

Note: The lipids of the gluten are strongly associated with the hydrophobic regions of gliadins and glutenins and, unlike what you can do with the flour, they are extracted with more difficulty (the lipid content of the gluten depends on the lipid content of the flour from which it was obtained).

Gliadins: extensibility and viscosity

Gliadins are hydrophobic monomeric prolamins, of globular nature and with low molecular weight. On the basis of electrophoretic mobility in low pH conditions, they are separated into the following types:

  • alpha/beta, and gamma, rich in sulfur, containing cysteines, that are involved in the formation of intramolecular disulfide bonds, and methionines;
  • omega, low in sulfur, given the almost total absence of cysteine and methionine.

They have a low nutritional value and are toxic to celiac patients because of the presence of particular amino acid sequences in the primary structure, such as proline-serine-glutamine-glutamine and glutamine-glutamine-glutamine-proline.
Gliadins are associated with each other and with glutenins through noncovalent interactions; thanks to that, they act as “plasticizers” in dough making. Indeed, they are responsible for viscosity and extensibility of gluten, whose three-dimensional lattice can deform, allowing the increase in volume of the dough as a result of gas production during leavening. This property is important in bread-making.
Their excess leads to the formation of a very extensible dough.

Glutenins: elasticity and toughness

Glutenins are polymeric proteins, that is, formed of multiple subunits, of fibrous nature, linked together by intermolecular disulfide bonds. The reduction of these bonds allows to divide them, by SDS-PAGE, into two groups.

  • High molecular weight (HMW) subunits, low in sulfur, that account for about 12% of total gluten proteins. The noncovalent bonds between them are responsible for the elasticity and tenacity of the gluten protein network, that is, of the viscoelastic properties of gluten, and so of the dough.
  • Low molecular weight (LMW) subunits, rich in sulfur (cysteine residues).
    These proteins form intermolecular disulfide bridges to each other and with HMW subunits, leading to the formation of a glutenin macropolymer.

Glutenins allow dough to hold its shape during mechanical (kneading) and not mechanical stresses (increase in volume due to both the leavening and the heat of cooking that increases the volume occupied by gases present) which is submitted. This property is important in pasta making.
If in excess, glutenins lead to the formation of a strong and rigid dough.

Properties of wheat gluten

From the nutritional point of view, gluten proteins do not have a high biological value, being low in lysine, an essential amino acid. Therefore, a gluten-free diet does not cause any important nutritional deficiencies.
On the other hand, it is of great importance in food industry: the combination, in aqueous solution, of gliadins and glutenins to form a three-dimensional lattice, provides viscoelastic properties, that is, extensibility-viscosity and elasticity-tenacity, to the dough, and then, a good structure to bread, pasta, and in general, to all foods made with wheat flour.
It has a high degree of palatability.
It has a high fermenting power in the small intestine.
It is an exorphin: some peptides produced from intestinal digestion of gluten proteins may have an effect in central nervous system.

Gluten-free cereals

The following is a list of gluten-free cereals, minor cereals, and pseudocereals used as foods.

  • Cereals

corn or maize (Zea mays)
rice (Oryza sativa)

  • Minor cereals
    They are defined “minor” not because they have a low nutritional value, but because they are grown in small areas and in lower quantities than wheat, rice and maize.

Fonio (Digitaria exilis)
Millet (Panicum miliaceum)
Panic (Panicum italicum)
Sorghum (Sorghum vulgare)
Teff (Eragrostis tef)
Teosinte; it is a group of four species of the genus Zea. They are plants that grow in Mexico (Sierra Madre), Guatemala and Venezuela.

  • Pseudocereals.
    They are so called because they combine in their botany and nutritional properties characteristics of cereals and legumes, therefore of another plant family.

Amaranth; the most common species are:

Amaranthus caudatus;
Amaranthus cruentus;
Amarantus hypochondriacus.

Buckwheat (Fagopyrum esculentum)
Quinoa (Chenopodium quinoa), a pseudocereal with excellent nutritional properties, containing fibers, iron, zinc and magnesium. It belongs to Chenopodiaceae family, such as beets.

  • Cassava, also known as tapioca, manioc, or yuca (Manihot useful). It is grown mainly in the south of the Sahara and South America. It is an edible root tuber from which tapioca starch is extracted.

It should be noted that naturally gluten-free foods may not be truly gluten-free after processing. Indeed, the use of derivatives of gliadins in processed foods, or contamination in the production chain may occur, and this is obviously important because even traces of gluten are harmful for celiac patients.

Oats and gluten

Oats (Avena sativa) is among the cereals that celiac patients can eat. Recent studies have shown that it is tolerated by celiac patients, adult and child, even in subjects with dermatitis herpetiformis. Obviously, oats must be certified as gluten-free (from contamination).

References

Beccari J.B. De Frumento. De bononiensi scientiarum et artium instituto atque Academia Commentarii, II. 1745:Part I.,122-127

Bender D.A. “Benders’ dictionary of nutrition and food technology”. 8th Edition. Woodhead Publishing. Oxford, 2006

Berdanier C.D., Dwyer J., Feldman E.B. Handbook of nutrition and food. 2th Edition. CRC Press. Taylor & Francis Group, 2007

Phillips G.O., Williams P.A. Handbook of food proteins. 1th Edition. Woodhead Publishing, 2011

Shewry P.R. and Halford N.G. Cereal seed storage proteins: structures, properties and role in grain utilization. J Exp Bot 2002:53(370);947-958. doi:10.1093/jexbot/53.370.947

Yildiz F. Advances in food biochemistry. CRC Press, 2009

Polyphenols in olive oil

Olive oil, which is obtained from the pressing of the olives, the fruits of olive tree (Olea europaea), is the main source of lipids in the Mediterranean diet, and a good source of polyphenols.
Polyphenols, natural antioxidants, are present in olive pulp and, following pressing, they pass into the oil.
Note: olives are also known as drupes or stone fruits.
The concentration of polyphenols in olive oil is the result of a complex interaction between various factors, both environmental and linked to the extraction process of the oil itself, such as:

  • the place of cultivation;
  • the cultivars (variety);
  • the level of ripeness of the olives at the time of harvesting.
    Their level usually decreases with over-ripening of the olives, although there are exceptions to this rule. For example, in warmer climates, olives produce oils richer in polyphenols, in spite of their faster maturation.
  • the climate;
  • the extraction process. In this regard, it is to underscore that the content of polyphenol in refined olive oil is not significant.

Any variation of the concentration of different polyphenols influence the taste, nutritional properties and stability of olive oil. For example, hydroxytyrosol and oleuropein (see below) give extra virgin olive oil a pungent and bitter taste.

CONTENTS

Key polyphenols in olive oil

Among polyphenols in olive oil, there are molecules with simple structure, such as phenolic acids and alcohols, and molecules with complex structure, such as flavonoids, secoiridoids, and lignans.

Flavonoids

Flavonoids include glycosides of flavonols (rutin, also known as quercetin-3-rutinoside), flavones (luteolin-7-glucoside), and anthocyanins (glycosides of delphinidin).

Phenolic acids and phenolic alcohols

Among phenolic acids, the first polyphenols with simple structure observed in olive oil, they are found:

  • hydroxybenzoic acids, such as, gallic, protocatechuic, and 4-hydroxybenzoic acids (all with C6-C1 structure).
  • hydroxycinnamic acids, such as caffeic, vanillin, syringic, p-coumaric, and o-coumaric acids (all with C6-C3 structure).

Among phenolic alcohols, the most abundant are hydroxytyrosol (also known as 3,4-dihydroxyphenyl-ethanol), and tyrosol [also known as 2-(4-hydroxyphenyl)-ethanol].

Hydroxytyrosol

Hydroxytyrosol can be present as:

  • simple phenol;
  • phenol esterified with elenolic acid, forming oleuropein and its aglycone;
  • part of the molecule verbascoside.
Hydroxytyrosol, a phenolic alcohol, and one of the polyphenols in olive oil
Hydroxytyrosol

It can also be present in different glycosidic forms, depending on the –OH group to which the glucoside, i.e. elenolic acid plus glucose, is bound.
It is one of the main polyphenols in olive oil, extra virgin olive oil, and olive vegetable water.
In nature, its concentration, such as that of tyrosol, increases during fruit ripening, in parallel with the hydrolysis of compounds with higher molecular weight, while the total content of phenolic molecules and alpha-tocopherol decreases. Therefore, it can be considered as an indicator of the degree of ripeness of the olives.
In fresh extra virgin olive oil, hydroxytyrosol is mostly present in esterified form, while in time, due to hydrolysis reactions, the non-esterified form becomes the predominant one.
Finally, the concentration of hydroxytyrosol is correlated with the stability of olive oil.

Secoiridoids

They are the polyphenols in olive oil with the more complex structure, and are produced from the secondary metabolism of terpenes.
They are glycosylated compounds and are characterized by the presence of elenolic acid in their structure (both in its aglyconic or glucosidic form). Elenolic acid is the molecule common to glycosidic secoiridoids of Oleaceae.
Unlike tocopherols, flavonoids, phenolic acids, and phenolic alcohols, that are found in many fruits and vegetables belonging to different botanical families, secoiridoids are present only in plants of the Oleaceae family.
Oleuropein, demethyloleuropein, ligstroside, and nuzenide are the main secoiridoids.
In particular, oleuropein and demethyloleuropein (as verbascoside) are abundant in the pulp, but they are also found in other parts of the fruit. Nuzenide is only present in the seeds.

Oleuropein

Oleuropein, the ester of hydroxytyrosol and elenolic acid, is the most important secoiridoid, and the main olive oil polyphenol.

Oleuropein, a secoiridoid, and one of the polyphenols in olive oil
Oleuropein

It is present in very high quantities in olive leaves, as also in all the constituent parts of the olive, including peel, pulp and kernel.
Oleuropein accumulates in olives during the growth phase, up to 14% of the net weight; when the fruit turns greener, its quantity reduces. Finally, when the olives turns dark brown, color due to the presence of anthocyanins, the reduction in its concentration becomes more evident.
It was also shown that its content is greater in green cultivars than in black ones.
During the reduction of oleuropein levels (and of the levels of other secoiridoids), an increase of compounds such as flavonoids, verbascosides, and simple phenols can be observed.
The reduction of its content is also accompanied by an increase in its secondary glycosylated products, that reach the highest values in black olives.

Lignans

Lignans, in particular (+)-1- acetoxypinoresinol and (+)-pinoresinol, are another group of polyphenols in olive oil.
(+)-pinoresinol is a common molecule in the lignin fraction of many plants, such as sesame (Sesamun indicum) and the seeds of the species Forsythia, belonging to the family Oleaceae. It has been also found in the olive kernel.
(+)-1- acetoxypinoresinol and (+)-1-hydroxypinoresinol, and their glycosides, have been found in the bark of the olive tree.

Examples of lignans, a class of pholyphenols, in olive oil
Lignans in Olive Oil

Lignans are not present in the pericarp of the olives, nor in leaves and sprigs that may accidentally be pressed with the olives.
Therefore, how they can pass into the olive oil becoming one of the main phenolic fractions is not yet known.
(+)-1- acetoxypinoresinol and (+)-pinoresinol are absent in seed oils, are virtually absent from refined virgin olive oil, while they may reach a concentration of 100 mg/kg in extra-virgin olive oil.
As seen for simple phenols and secoiridoids, there is considerable variation in their concentration among olive oils of various origin, variability probably related to differences between olive varieties, production areas, climate, and oil production techniques.

References

Cicerale S., Lucas L. and Keast R. Biological activities of phenolic compounds present in virgin olive oil. Int. J. Mol. Sci. 2010;11: 458-479. doi:10.3390/ijms11020458

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

Manach C., Scalbert A., Morand C., Rémésy C., and Jime´nez L. Polyphenols: food sources and bioavailability. Am J Clin Nutr 2004;79(5):727-747. doi:10.1093/ajcn/79.5.727

Owen R.W., Mier W., Giacosa A., Hull W.E., Spiegelhalder B. and Bartsch H. Identification of lignans as major components in the phenolic fraction. Clin Chem 2000;46:976-988.

Tripoli E., Giammanco M., Tabacchi G., Di Majo D., Giammanco S. and La Guardia M. The phenolic compounds of olive oil: structure, biological activity and beneficial effects on human health. Nutr Res Rev 2005:18;98-112. doi:10.1079/NRR200495

Calories burned, and water and minerals lost during running

Calorie, carbohydrate, fat, and protein expenditure, and water and mineral losses during runningDuring running, athletes burn calorie, and lose water and salts in amounts depending on various factors such as the technique, training level, environmental conditions, and physiological characteristics of each runner. The knowledge of these factors allows to plan an adequate diet both during workout  and recovery, with the aim of optimizing performance.
Below we will analyze the energy expenditure of runners engaged in workouts on various distances, the amounts of carbohydrates, lipids, and proteins oxidized to meet the energy requirements, and which minerals are lost in sweat.

CONTENTS

Energy expenditure during running

During running energy expenditure is equal to 0.85-1.05 kcal per kilogram per kilometer.
This range is due to the fact that athletes with a good technique spend less than those with a poor technique.
A 70 kilogram (154 pound) athlete has an energy expenditure per kilometer between:

70 x 0.85 x 1 = 59.5 kcal
and
70 x 1.05 x 1 = 73.5 kcal

The table shows the calculations to determine the energy expenditure of the athlete to run 10, 20, 30, and 40 kilometers.

Distance

Energy expenditure

10 km 0.85 x 70 x 10 = 595 kcal
1.05 x 70 x 10 = 735 kcal
20 km 0.85 x 70 x 20 = 1190 kcal
1.05 x 70 x 20 = 1470 kcal
30 km 0.85 x 70 x 30 = 1785 kcal
1.05 x 70 x 30 = 2205 kcal
40 km 0.85 x 70 x 40 = 2380 kcal
1.05 x 70 x 40 = 2940 kcal

Note: who has started running for a short time ago has an energy expenditure even higher than 1.05 kcal per kilogram per kilometer.

During running, the energy for muscle work derives from the oxidation of carbohydrates, lipids, and proteins. Carbohydrates and lipids are the main energy source, and their oxidation rate depends on the intensity of exercise: as it increases, the percentage of lipid oxidation decreases whereas that of carbohydrates increases, as summarized below.

Intensity Fuel
30% VO2max Mainly fats
40-60% VO2max Fats and carbohydrates
75% VO2max Mainly carbohydrates
80% VO2max Almost exclusively carbohydrates

Note: The failure to use the suitable fuel can promote fatigue and lead to overtraining.

Then, when running above the anaerobic threshold, the oxidation of carbohydrates can provide the entire energy requirement. At marathon pace, carbohydrates provide 60-70% of the energy requirement, whereas at lower pace they provide less than 50% of energy requirement.
Below, the amounts of carbohydrates, lipids, and proteins oxidized during workout are analyzed. During workout ,the energy expenditure is covered for about 60% by carbohydrates, for about 40% by lipids, whereas the residual percentage, between 3 and 5%, by proteins.

Carbohydrate oxidation during workout

For a 70 kilogram runner the amount of carbohydrates oxidized per kilometer is between:

(0.6 x 59.5) /4 = 8.9 g/km
and
(0.6 x 73.5) /4 = 11 g/km

Note: carbohydrates provide, on average, 4 kcal per gram.
The table shows the calculations to determine the amount of carbohydrates oxidized when the athlete runs 10, 20, 30, and 40 kilometers.

Distance Carbohydrate expenditure

10 km

[(0.85 x 70 x 10) x 0.6 ] / 4 = 89 g
[(1.05 x 70 x 10) x 0.6 ] / 4 = 110 g

20 km

[(0.85 x 70 x 20) x 0.6] / 4 = 179 g
[(1.05 x 70 x 20) x 0.6] / 4 = 221 g

30 km

[(0.85 x 70 x 30) x 0.6] / 4 = 268 g
[(1.05 x 70 x 30) x 0.6] / 4 = 331 g

40 km

[(0.85 x 70 x 40) x 0.6] / 4 = 357 g
[(1.05 x 70 x 40) x 0.6] / 4 = 441 g

Lipid oxidation during workout

By calculations similar to those for carbohydrates, we determine the amount of lipids oxidized per kilometer, which is between:

(0.4 x 59.5) / 9 = 2.6 g/km
and
(0.4 x 73.5) / 9 = 3.3 g/km

Note: lipids provide, on average, 9 kcal per gram.
The table shows the calculations to determine the amount of lipids oxidized when the athlete runs 10, 20, 30, and 40 kilometers.

Distance

Lipid expenditure

10 km [(0.85 x 70 x 10) x 0.4] / 9 = 26 g
[(1.05 x 70 x 10) x 0.4] / 9 = 33 g
20 km [(0.85 x 70 x 20) x 0.4] / 9 = 53 g
[(1.05 x 70 x 20) x 0.4] / 9 = 65 g
30 km [(0.85 x 70 x 30) x 0.4] / 9 = 79 g
((1.05 x 70 x 30) x 0.4] / 9 = 98 g
40 km [(0.85 x 70 x 40) x 0.4] / 9 = 106 g
[(1.05 x 70 x 40) x 0.4] / 9 = 131 g

Protein oxidation during workout

Protein requirements of adults are equal to 0.9 grams per kilogram of body weight, and, for a 70 kilogram athlete is:

70 x 0.9 = 63 g

During workout  the energy expenditure is covered for about 3-5% by protein oxidation.

The table shows the calculations to determine the amount of proteins oxidized when the athlete runs 10, 20, 30, and 40 kilometers, and proteins provide 3% of the energy requirement.

Distance

Protein expenditure (3%)

10 km [(0.85 x 70 x 10) x 0.03)] / 4 = 4.5 g
[(1.05 x 70 x 10) x 0.03)] / 4 = 5.5 g
20 km [(0.85 x 70 x 20) x 0.03)] / 4 = 8.9 g
[(1.05 x 70 x 20) x 0.03)] / 4 = 11 g
30 km [(0.85 x 70 x 30) x 0.03)] / 4 = 13.4 g
[(1.05 x 70 x 30) x 0.03)] / 4 = 16.5 g
40 km [(0.85 x 70 x 40) x 0.03)] /4 = 17.9 g
[(1.05 x 70 x 40) x 0.03)] /4 = 22.1 g

Note: proteins provide, on average, 4 kcal per gram.

For energy expenditure of 0.85 and 1.05 kcal per kilogram per kilometer, the average additional protein oxidation per kilogram to run 10, 20, 30, and 40 kilometers, rounded to the second decimal place, is:

  • 10 km: [(4.5 + 5.5) / 2] / 70 = 0.07 g
  • 20 km: [(4.5 + 5.5) / 2] / 70 = 0.14 g
  • 30 km: [(4.5 + 5.5) / 2] / 70 = 0.21 g
  • 40 km: [(4.5 + 5.5) / 2] / 70 = 0.29 g

Finally, adding the daily protein requirement of adults, the total protein requirement of a 70 kilogram runner, for the four distances, is:

  • 10 km: 0.07 + 0.9 = 0.97 g
  • 20 km: 0.14 + 0.9 = 1.04 g
  • 30 km: 0.21 + 0.9 = 1.11 g
  • 40 km: 0.29 + 0.9 = 1.19 g

By calculations similar to the previous ones, we determine the overall protein requirement when proteins provide 5% of the energy requirement.

  • 10 km: 0.12 + 0.9 = 1.02 g
  • 20 km: 0.24 + 0.9 = 1.14 g
  • 30 km: 0.36 + 0.9 = 1.26 g
  • 40 km: 0.48 + 0.9 = 1.38 g

Excluding athletes who run 30 kilometers or more every day, the values are slightly higher than 0.9 grams per kilogram of body weight.
In reality, the daily protein requirement is just slightly higher because a certain amount of nitrogen, hence proteins, is lost, as well as in the urine, also through sweating.

Water and minerals loss during running

Water losses depend on the amount of sweat produced, that depends on:

  • air temperature and humidity;
  • solar radiation.

The loss will be greater the higher these values are.
Finally, the amount of sweat produced is different from person to person.

Minerals lost in sweat are mostly:

  • sodium (Na+) and chlorine (Cl), about 1 gram per liter of sweat in heat acclimatized athletes;
  • potassium (K+), in an amount equal to about 15% of the sodium lost;
  • magnesium (Mg2+), in an amount equal to about 1% of the sodium lost.

The amount of minerals lost depends on how much sweat is produced, and it increases in non-heat acclimatized athletes.

The table shows the values, in grams per liter, of the minerals lost in sweat for non-heat and heat-acclimated athletes.

  Non-heat acclimated athletes

heat acclimated athetes

Sodium

1.38

0.92

Chlorine

1.5

1.00

Potassium

0.20

0.15

Magnesium

0.01

0.01

Total

3.09

2.08

Therefore, during physical activity, sodium is the mineral we need most of all.
After physical activity, runner, or who sweats heavily, tends to eat saltier food. This effect, known as selective hunger, was discovered, for sodium, in studies conducted on foundry workers. Probably, the selective hunger doesn’t not exist for potassium and magnesium.

References

Sawka M.N., Burke L.M., Eichner E.R., Maughan, R.J., Montain S.J., Stachenfeld N.S. American College of Sports Medicine position stand: exercise and fluid replacement. Med Sci Sport Exercise 2007;39(2):377-390. doi:10.1249/mss.0b013e31802ca597

Shirreffs S., Sawka M.N. Fluid and electrolyte needs for training, competition and recovery. J Sport Sci 2011;29:sup1, S39-S46. doi:10.1080/02640414.2011.614269

Digestion of starch and alpha-amylase

Amylose and amylopectin, the two families of homopolysaccharides constituting starch, during their biosynthesis within vegetable cells, are deposited in highly organized particles called granules.

alpha-amylase
alpha-Amylase

Granules have a partially crystalline structure and diameter ranging from 3 to 300 µm.
The access of the alpha-amylase (EC 3.2.1.1), the enzyme that catalyzes the breakdown of amylose and amylopectin into maltose, maltotriose, and alpha-dextrins or alpha-limit dextrins, to carbohydrates making up granules varies as a function of:

  • amylose-amylopectin ratio;
  • temperature and packaging of amylose and amylopectin;
  • granules-associated proteins;
  • presence of fibers.

Amylose-amylopectin ratio

Starch for foodstuff use is obtained from various sources, the most important of which are corn (normal, waxy or high amylose content), potatoes, rice, tapioca and wheat.
Depending on botanical origin, molecular weight, degree of branching, and amylose-amylopectin ratio will vary.
Generally, there is 20-30% amylose and 70-80% amylopectin, even if there are starches with high amylose or amylopectin content (e.g. waxy corn). These differences justify the existence of starches with different chemical-physical characteristics and, to a certain extent, different digestibility.

  • corn: 24% amylose, 76% amylopectin;
  • waxy corn: 0,8% amylose, 99.2% amylopectin;
  • Hylon VII corn: 70% amylose, 30% amylopectin;
  • potatoes: 20% amylose, 80% amylopectin;
  • rice: 18.5 amylose, 81.5% amylopectin;
  • tapioca: 16.7% amylose, 83.3% amylopectin;
  • wheat: 25% amylose, 75% amylopectin.

Temperature and packaging of amylose and amylopectin

The chains of amylose, and to a lesser extent ramifications of amylopectin, thanks to the formation of hydrogen bonds with neighboring molecules and within the same molecules, have the tendency to aggregate. For this reason, pure amylose and amylopectin are poorly soluble in water at below 55 °C (131°F), and are more resistant to alpha-amylase action (resistant starch).
However, in aqueous solution, these granules hydrate increasing in volume of about 10%.
Above 55°C (131°F), the partially crystalline structure is lost, granules absorb further water, swell and pass to a disorganized structure, that is, starch gelatinization occurs, by which starch assumes an amorphous structure more easily attachable by alpha-amylase.

Granules-associated proteins

In granules, starch is present in association with proteins, many of which are hydrophobic, that means with low affinity for water. This association have the effect to hinder the interaction, in the intestinal lumen, between alpha-amylase, a polar protein, and the carbohydrates making up starch granules.
The physical processes to which cereals undergo before being eaten, such as milling or heating for several minutes, change the relationship between starch and the associated proteins, making it more available to α-amylase action.

Fibers

Alpha-amylase activity may also be hindered by the presence of nondigestible polysaccharides, the fibers: cellulose, hemicellulose and pectin.

Conclusions

The presence of inhibitors, of both chemical and physical type, hinders starch digestion, even when pancreatic α-amylase secretion is normal. This means that a part of starch, ranging from 1% to 10%, may escape the action of the enzyme, being then metabolized by colonic bacteria.
Refined starch is instead hydrolyzed efficiently, even when there is an exocrine pancreatic insufficiency (EPI), condition in which alpha-amylase concentration in gut lumen may be reduced to 10% of the normal.

References

Belitz .H.-D., Grosch W., Schieberle P. “Food Chemistry” 4th ed. Springer, 2009

Bender D.A. “Benders’ Dictionary of Nutrition and Food Technology”. 8th Edition. Woodhead Publishing. Oxford, 2006

Osorio-Dıaz P., Bello-Perez L.A., Agama-Acevedo E., Vargas-Torres A., Tovar J., Paredes-Lopez O. In vitro digestibility and resistant starch content of some industrialized commercial beans (Phaseolus vulgaris L.). Food Chem 2002;78:333-7 doi:10.1016/S0308-8146(02)00117-6

Stipanuk M.H.. “Biochemical and physiological aspects of human nutrition” W.B. Saunders Company-An imprint of Elsevier Science, 2012

GLA and physiology and pathophysiology of the skin

gamma-Linolenic acid (GLA), an omega-6 PUFA, like its precursor linoleic acid (the most abundant polyunsaturated fatty acid in human skin epidermis, where it’s involved in the maintenance of the epidermal water barrier), plays important roles in the physiology and pathophysiology of the skin.
Studies conducted on humans revealed that gamma-linolenic acid:

  • improves skin moisture, firmness, roughness;
  • decreases transepidermal water loss (one of the abnormalities of the skin in essential fatty acid deficiency animals).
Skelatal formula of Prostaglandin E1, a derivative of gamma-linolenic acid, an omega-6 PUFA
Prostaglandin E1

Using guinea pig skin epidermis as a model of human epidermis (they are functionally similar), it was demonstrated that supplementation of animals with gamma-linolenic acid-rich foods results in a major production of PGE1 and 15-HETrE in the skin (as previously demonstrated in in vitro experiments).
Because these molecules have both anti-inflammatory/anti-proliferative properties supplementation of diet with gamma-linolenic acid acid-rich foods may be an adjuncts to standard therapy for inflammatory/proliferative skin disorders.

Supplemental sources of GLA

The main supplemental sources of gamma-linolenic acid are oils of the seeds of:

  • borage (20%-27% of the total fatty acids);
  • black currant (from 15% to 19% of the total fatty acids);
  • evening primrose (from 7% to 14% of the total fatty acids), and

Role of gamma-linolenic acid in lowering blood pressure

The relationship between dietary fatty acid intake and blood pressure mainly comes from studies conducted on genetically modified rats that spontaneously develops hypertension (a commonly used animal model for human hypertension).
In these studies many membrane abnormalities were seen so hypertension in rat model may be related to change in polyunsaturated fatty acid metabolism at cell membrane level.
About polyunsaturated fatty acids, several research teams have reported that gamma-linolenic acid reduces blood pressure in normal and genetically modified rats (greater effect) and it was purported by interfering with Renin-Angiotensin System (that promote vascular resistance and renal retention) altering the properties of the vascular smooth muscle cell membrane and so interfering with the action of angiotensin II.
Another possible mechanism of action of gamma-linolenic acid to lower blood pressure could be by its metabolite dihomo-gamma-linolenic acid: it may be incorporated in vascular smooth muscle cell membrane phospholipids, then released by the action of phospholipase A2 and transformed by COX-1 in PGE1 that induces vascular smooth muscle relaxation.

Role gamma-linolenic acid in treatment of rheumatoid arthritis

In a study conducted by Leventhal et al. on 1993 it was demonstrated the dietary intake of higher concentration of borage oil (about 1400 mg of gamma-linolenic acid/day) for 24 weeks resulted in clinically significant reductions in signs and symptoms of rheumatoid arthritis activity.
In a subsequent study by Zurier et al. on 1996 the dietary intake of an higher dose (about 2.8 g/day gamma-linolenic acid) for 6 months reduced, in a clinically relevant manner, signs and symptoms of the disease activity; patients who remained for 1 year on the 2.8 g/day dietary gamma-linolenic acid exhibited continued improvement in symptoms (the use of gamma-linolenic acid also at the above higher dose is well tolerated, with minimal deleterious effects). These data underscore that the daily amount and the duration of gamma-linolenic acid dietary intake do correlate with the clinical efficacy.

References

Akoh C.C. and Min D.B. “Food lipids: chemistry, nutrition, and biotechnology” 3th ed. 2008

Chow Ching K. “Fatty acids in foods and their health implication” 3th ed. 2008

Fan Y.Y. and Chapkin R.S. Importance of dietary γ-linolenic acid in human health and nutrition. J Nutr 1998;128:1411-1414. doi:10.1093/jn/128.9.1411

Leventhal L.J., Boyce E.G. and Zurier R.B. Treatment of rheumatoid arthritis with gammalinolenic acid. Ann Intern Med 1993 119:867-873. doi:10.7326/0003-4819-119-9-199311010-00001

Miller C.C. and Ziboh V.A. Gammalinolenic acid-enriched diet alters cutaneous eicosanoids. Biochem Biophys Res Commun 1988 154:967-974. doi:10.1016/0006-291X(88)90234-3

Zurier R.B., Rossetti R.G., Jacobson E.W., DeMarco D.M., Liu N.Y., Temming J.E., White B.M. and Laposata M. Gamma-linolenic acid treatment of rheumatoid arthritis. A randomized, placebocontrolled trial. Arthritis Rheum 1996 39:1808-1817. doi:10.1002/art.1780391106

Alcohol, blood pressure, and hypertension

Many studies have shown a direct, dose-dependent relationship between alcohol intake and blood pressure, particularly for intake above two drinks per day.
This relationship is independent of:

  • age;
  • salt intake;
  • obesity;
  • finally, it persists regardless of beverage type.

Furthermore, heavy consumption of alcoholic beverages for long periods of time is one of the factors predisposing to hypertension: from 5 to 7% of hypertension cases is due to an excessive alcohol consumption.
A meta-analysis of 15 randomized controlled trials has shown that decreasing alcoholic beverage intake intake has therapeutic benefit to hypertensive and normotensive with similar systolic and diastolic blood pressure reductions (in hypertensive reduction occurs within weeks).

CONTENTS

Alcohol intake and prevention of hypertension

Guidelines on the primary prevention of hypertension recommend that alcohol (ethanol) consumption in most men, in absence of other contra, should be less than 28 g/day, the limit in which it may reduce coronary heart disease risk.
Relationship Between Ethanol Intake and HypertensionThe consumption limited to these quantities must be obtained by intake of drinks with low ethanol content, preferably at meals (drinking even lightly to moderately outside of meals increases the probability to have hypertension). This means no more than 680 ml or 24 oz of regular beer or 280 ml or 10 oz of wine (12% ethanol), especially in hypertension; for women and thinner subjects consumption should be halved1.
To avoid intake of drinks with high ethanol content even though the total ethanol content not exceeding 28 g/day.

Relationship between ethanol intake and blood pressure

Anyway, uncertainty remains regarding benefits or risks attributable to light-to-moderate alcoholic beverage intake on the risk of hypertension.
In a study published on April 2008, the authors examined the association between ethanol intake and the risk of developing hypertension in 28848 women from “The Women’s Health Study” and 13455 men from the “Physicians’ Health Study”, (the follow-up lasted respectively for 10.9 and 21.8 years). The study confirms that heavy ethanol intake (exceeding 2 drinks/day) increases hypertension risk in both men and women but, surprisingly, found that the association between light-to-moderate alcohol intake (up to 2 drinks/day) and the risk of developing hypertension is different in women and men. Women have a potential reduced risk of hypertension from a light-to-moderate ethanol consumption with a J-shaped association2; men have no benefits of light-to-moderate ethanol consumption but an increased risk of hypertension.
However, guidelines for the primary prevention of hypertension limit alcohol consumption to less 2 drinks/day in men and less 1 drink/day in thinner subjects and women.

1. A standard drink contains approximately 14 g of ethanol i.e. a 340 ml or 12 oz of regular beer, 140 ml or 5 oz wine (12% alcohol), or 42 ml or 1,5 oz of distilled spirits (inadvisable).

2. Many studies have shown a J-shaped relationship between ethanol intake and blood pressure. Light drinker (no more than 28 g of ethanol/day) have lower blood pressure than teetotalers; instead, who consumes more than 28 g ethanol/day have higher blood pressure than non drinker. So alcohol is a vasodilator at low doses but a vasoconstrictor at higher doses.

References

Pickering T.G. New guidelines on diet and blood pressure. Hypertension 2006;47:135-136. doi:10.1161/01.HYP.0000202417.57909.26

Sesso H.D., Cook N.R., Buring J.E., Manson J.E. and Gaziano J.M. Alcohol consumption and the risk of hypertension in women and men. Hypertension 2008;51:1080-1087. doi:10.1161/HYPERTENSIONAHA.107.104968

Writing Group of the PREMIER Collaborative Research Group. Effects of comprehensive lifestyle modification on blood pressure control: main results of the PREMIER Clinical Trial. JAMA 2003;289:2083-2093. doi:10.1001/jama.289.16.2083

World Health Organization, International Society of Hypertension Writing Group. 2003 World Health Organization (WHO)/International Society of Hypertension (ISH) statement on management of hypertension. Guidelines and recommendations. J Hyperten 2003;21:1983-1992.

Overweight, physical activity, and blood pressure

Body weight, especially overweight and obesity, is a determinant of blood pressure at all age; in fact:

  • it has been estimated that the risk of developing elevated blood pressure is two to six time higher in overweight than in normal-weight individuals;
  • there is a linear correlation between blood pressure and body weight or body mass index (BMI) (a BMI greater than 27, i.e. overweight or obesity, is correlated with increased blood pressure): even when dietary sodium intake is held constant, the correlation between change in weight and change in blood pressure is linear;
  • 60% of hypertensives are more than 20% overweight;
  • centripetal distribution of body fat (waist circumference greater than 34 inches in women and 39 inches in man), also associated with insulin resistance, is more important determinant of blood pressure elevation than that peripherally located in both man and women;
  • it has been shown that weight loss, both in hypertensive and normotensive individual, can reduce blood pressure and reductions occur before, and without, attainment of a desirable body weight.

In view of the difficulties of sustaining weight loss, efforts to prevent weight gain among those who have normal body weight are critically important.

How to calculate BMI

BMI is total body weight, expressed in kilograms [kg] or pounds [lb], divided by the height squared, expressed in meters or inches (in.).
It can be calculated using the following equations:

BMI = weight [kg]/height2 [m] or
BMI = (weight [lb.]/heigth2 [in.]) x 705

BMI is a good indication of body fat because most of the weight differential among adults is due to body fat; its major flaw is that some muscular individuals may be classified as obese even if they are not.
A healthy BMI is between 18 to 24,9.
Overweight is considered to be between 25 to 29,9.
Obesity is categorized by BMI according to three grades:

  • 30 to 34,9 I grade obesity;
  • 35 to 40 II grade obesity:
  • 40 and above III grade obesity.

Physical activity, overweight and blood pressure

Maintaining a high level of physical activity is a critical factor in sustaining weight loss.
In addition to the effect on body weight, activity and exercise in itself reduce the rise in blood pressure.
Physical activity to help control overweight and high blood pressurePhysical activity produces a fall in systolic blood pressure and diastolic blood pressure; so, increasing physical activity of low to moderate intensity to 30 to 45 minutes 3-4 days/week up to 1 hour nearly every day, as recommended by World Health Organization, is important for the primary prevention of hypertension.
Less active persons are 30% to 50% more likely to develop hypertension than active ones.
Remember: a rolling stone gathers no moss!

References

Writing Group of the PREMIER Collaborative Research Group. Effects of comprehensive lifestyle modification on blood pressure control: main results of the PREMIER Clinical Trial. JAMA 2003;289:2083-2093. doi:10.1001/jama.289.16.2083

World Health Organization, International Society of Hypertension Writing Group. 2003 World Health Organization (WHO)/International Society of Hypertension (ISH) statement on management of hypertension. Guidelines and recommendations. J Hyperten 2003;21:1983-92.

Trans fatty acids (TFA): structure, sources, health effects, examples

Trans fatty acids (TFA) or trans-unsaturated fatty acids or trans fats are unsaturated fatty acids , a subclass of lipids, with at least one a double bond in the trans or E configuration.
Carbon-carbon double bonds show planar conformation, and so they can be considered as plains from whose opposite sides carbon chain attaches and continues. “The entry” and “the exit” of the carbon chain from the plain may occur on the same side of the plain, and in this case double bond is defined in cis or Z configuration, or on opposite side, and in that case it is defined in trans configuration.

Examples of C18:1 cis/trans isomers, and of a saturated fatty acid
Fig. 1 – C18:1 cis/trans Isomers

Unsaturated fatty acids most commonly have their double bonds in cis configuration; the other, less common configuration is trans.
Cis bond causes a bend in the fatty acid chain, whereas the geometry of trans bond straightens the fatty acid chain, imparting a structure more similar to that of saturated fatty acids.

CONTENTS

Properties of fats rich in trans fatty acids

Below, some distinctive characteristics of the fats rich in trans fats, that make them particularly suited for the production of margarines and vegetable shortening used in home and commercial cooking, and manufacturing processes.

  • Bent molecules can’t pack together easily, but linear ones can do it.
    This means that trans fatty acids contribute, together with the geometrically similar saturated fatty acids, to the hardness of the fats in which they are, giving them a higher melting point.
    Heightening the melting point of fats means that it is possible to convert them from liquid form to semi-solids and solids at room temperature.
    Note: trans fats tend to be less solid than saturated fatty acids.
  • They have:

a melting point, consistency and “mouth feel” similar to those of butter;
a long shelf life at room temperature;
a flavor stability.

  • They are stable during frying.

Sources of trans fatty acids

Dietary TFA come from different sources briefly reviewed below.

  • In industrialized countries, greater part of the consumed trans fatty acids, in USA about 80 percent of the total, are produced industrially, in varying amounts, during partial hydrogenation of edible oils containing unsaturated fatty acids (see below).
  • They are produced at home during frying with vegetable oils containing unsaturated fatty acids.
  • They come from bacterial transformation of unsaturated fatty acids ingested by ruminants in their rumen (see below).
  • Another natural source is represented by some plant species, such as leeks, peas, lettuce and spinach, that contain trans-3-hexadecenoic acid, and rapeseed oil, that contains brassidic acid (22:1∆13t) and gondoic acid (20:1∆11t). In these sources trans fatty acids are present in small amounts.
  • Very small amounts, less than 2 percent, are formed during deodorization of vegetable oils, a process necessary in the refining of edible oils. During this process trans fatty acids with more than one double bond are formed in small amounts. These isomers are also present in fried foods and in considerable amounts in some partially hydrogenated vegetable oils (see below).

Industrial trans fatty acids

Hydrogenation is a chemical reaction in which hydrogen atoms react, in the presence of a catalyst, with a molecule.
The hydrogenation of unsaturated fatty acids involves the addition of hydrogen atoms to double bonds on the carbon chains of fatty acids. The reaction occurs in presence of metal catalyst and hydrogen, and is favored by heating vegetable oils containing unsaturated fatty acids.

Partial hydrogenation of vegetable oils

The process of hydrogenation was first discovered in 1897 by French Nobel prize in Chemistry, jointly with fellow Frenchman Victor Grignard, Paul Sabatier using a nickel catalyst.
Partially hydrogenated vegetable oils were developed in 1903 by a German chemist, Wilhelm Normann, who files British patent on “Process for converting unsaturated fatty acids or their glycerides into saturated compounds”. The term trans fatty acids or trans fats appeared for the first time in the Remark column of the 5th edition of the “Standard Tables of Food Composition” in Japan.
During partial hydrogenation, an incomplete saturation of the unsaturated sites on the carbon chains of unsaturated fatty acids occurs. For example, with regard to fish oil, trans fatty acid content in non-hydrogenated oils and in highly hydrogenated oils is 0.5 and 3.6%, respectively, whereas in partially hydrogenated oils is 30%.

The cis to trans isomerization of oleic acid to vaccenic acid
Fig. 2 – From Oleic Acid to Vaccenic Acid

But, most importantly, some of the remaining cis double bonds may be moved in their positions on the carbon chain, producing geometrical and positional isomers, that is, double bonds can be modified in both conformation and position.
Below, other changes that occur during partial hydrogenation are listed.

Partially hydrogenated vegetable oils were developed for the production of vegetable fats, a cheaper alternative to animal fats. In fact, through hydrogenation, oils such as soybean, safflower and cottonseed oils, which are rich in unsaturated fatty acids, are converted into semi-solid fats (see above).
The first hydrogenated oil was cottonseed oil in USA in 1911 to produce vegetable shortening.
In the 1930’s, partial hydrogenation became popular with the development of margarine.
Currently, per year in USA, 6-8 billion tons of hydrogenated vegetable oil are produced.

Ruminant trans fatty acids

Ruminant trans fats are produced by bacteria in the rumen of the animals, for example cows, sheep and goats, using as a substrate a proportion of the relatively small amounts of unsaturated fatty acids present in their feedstuffs, that is, feed, plants and herbs. And, considering an animal that lives at least a year, and has the opportunity to graze and/or eat hay, there is a season variability in unsaturated fatty acids intake, and trans fats produced. In fact, in summer and spring, pasture plants and herbs may contain more unsaturated fatty acids than the winter feed supply.
Then, TFA are present at low levels in meat and full fat dairy products, typically <5% of total fatty acids, and are located in the sn-1 and sn-3 positions of the triacylglycerols, whereas in margarines and other industrially hydrogenated products they appear to be concentrated in the sn-2 position of the triacylglycerols.
Ruminant trans fatty acids are mainly monounsaturated fatty acids, with 16 to 18 carbon atoms, and constitute a small percentage of the trans fatty acids in the diet (see below).

Isomers of dietary trans fatty acids

The most important cluster of trans fatty acids is trans-C18:1 isomers, that is, fatty acids containing 18 carbon atoms plus one double bond, whose position varies between Δ6 and Δ16 carbon atoms. In both sources, the most common isomers are those with double bonds between positions Δ9 and Δ11.
However, even if these molecules are present both in industrial and ruminant TFA, there is a considerable quantitative difference. For example, vaccenic acid (C18:1 Δ11t) represents over 60 percent of the trans-C18:1 isomers in ruminant trans fatty acids, whereas in industrial ones elaidic acid (C18:1Δ9t) comprises 15-20 percent and C18:1 Δ10t and vaccenic acid over 20 percent each others.

trans-C18:1 fatty acid isomers, the most important cluster of trans fatty acids
Fig. 3 – trans-C18:1 Isomers

Trans fatty acids: effects on human health

Ruminant trans fatty acids, in amounts actually consumed in diets, are not harmful for human health (see below).
Conversely, consumption of industrial trans fats has neither apparent benefit nor intrinsic value, above their caloric contribution, and, from human health standpoint they are only harmful, having adverse effects on:

  • serum lipid levels;
  • endothelial cells;
  • systemic inflammation;
  • other risk factors for cardiovascular disease.

Moreover, they are positively associated with the risk of coronary heart disease (CHD), and sudden death from cardiac causes and diabetes.

Note: further in the text, we will refer to industrial trans fatty acids as trans fats or trans fatty acids.

Trans fatty acids: effects at plasmatic level

Low-density lipoprotein cholesterol (LDL-C) and high-density lipoprotein cholesterol (HDL-C) plasma levels are well-documented risk markers for the development of coronary heart disease (CHD).

  • High LDL-C levels are associated with an increased incidence of ischemic heart disease.
  • High HDL-C levels are associated with a reduced incidence of the risk.

For this reason, the ratio between total cholesterol level and HDL-C is often used as a combined risk marker for these two components in relation to the development of heart disease: the higher the ratio, the higher the risk.

TFA, as previously said, have adverse effects on serum lipids.
These effects have been evaluated in numerous controlled dietary trials by isocaloric replacement of saturated fatty acids or cis-unsaturated fatty acids with trans fats. It was demonstrated that such replacement:

  • raises LDL-C levels;
  • lowers HDL-C levels, in contrast to saturated fatty acids that increase HDL-C levels when used as replacement in similar study;
  • increases the ratio of total cholesterol to HDL-C, approximately twice that for saturated fatty acids, and, on the basis of this effect alone, trans fatty acids has been estimated to cause about 6% of coronary events in the USA.

Furthermore, trans fats:

  • produce a deleterious increase in small, dense LDL-C subfractions, that is associated with a marked increased in the risk of CHD, even in the presence of relatively normal LDL-C;
  • increase the blood levels of triglycerides, and this is an independent risk factor for CHD;
  • increase levels of Lp(a)lipoprotein, another important coronary risk factor.

But on 2004 prospective studies have shown that the relation between the intake of trans fatty acids and the incidence of CHD is greater than that predicted by changes in serum lipid levels alone. This suggests that trans fats influence other risk factors for CHD, such as inflammation and endothelial-cell dysfunction.

Trans fatty acids, inflammation and endothelial-cell dysfunction

The role of inflammation in atherosclerosis, and consequently in CHD, is burgeoned in the last decade.
Interleukin-6, C-reactive protein (CRP), and an increased activity of tumor necrosis factor (TNF) system are markers of inflammation.
In women greater intake of trans fatty acids is associated with increased activity of TNF system, and in those with a higher body mass index with increased levels of interleukin-6 and CRP. For example, the difference in CRP seen with an average intake of trans fats of 2.1% of the total daily energy intake, as compared with 0.9%, correspond to an increased risk of cardiovascular disease of 30%. Similar results have been reported in patients with established heart disease, in randomized, controlled trials, in in vitro studies, and in studies in which it has been analyzed membrane levels of trans fatty acids, a biomarker of their dietary intake.
So, trans fats promote inflammation, and their inflammatory effects may account at least in part for their effects on CHD that, as seen above, are greater than would be predicted by effects on serum lipoproteins alone.
Attention: the presence of inflammation is an independent risk factor not only for CHD but also for insulin resistance, diabetes, dyslipidemia, and heart failure.

Another site of action of TFA may be endothelial function.
Several studies have suggested the association between greater intake of trans fats and increased levels of circulating biomarkers of endothelial dysfunction, such as E-selectin, sICAM-1, and sVCAM-1.

Other effects of trans fatty acids

In vitro studies have demonstrate that trans fats affect lipid metabolism through several pathways.

  • They alter secretion, lipid composition, and size of apolipoprotein B-100 (apo B-100).
  • They increase cellular accumulation and secretion of free cholesterol and cholesterol esters by hepatocytes.
  • They alter expression in adipocytes of genes for peroxisome proliferator-activated receptor-γ (PPAR- γ), lipoprotein lipase, and resistin, proteins having a central roles in the metabolism of fatty acids and glucose.

Industrial trans fatty acids and CHD

Industrial trans fats are independent cardiovascular risk factor.
Since the early 1990s attention has been focused on the effect of trans fatty acids on plasma lipid and lipoprotein concentrations (see above).
Furthermore, four major prospective studies covering about 140,000 subjects, monitored for 6-14 years, have all found positive epidemiological evidence relating their levels in the diet, assessed with the aid of a detailed questionnaire on the composition of the diet, to the risk of CHD. These four studies are:

  • “The Health Professionals Follow-up study” (2005);
  • “The Alpha-Tocopherol Beta-Carotene Cancer Prevention Study” (1997);
  • “The Nurses’ Health Study” (2005);
  • “The Zutphen Elderly Study” (2001).

These studies cover such different populations that the results very probably hold true for the populations as a whole.
A meta-analysis of these studies have shown that a 2% increase in energy intake from industrial TFA was associated with a 23% increase in the incidence of CHD. The relative risk of heart disease was 1.36 in “The Health Professionals Follow-up Study”, 1.14 in “The Alpha-Tocopherol Beta-Carotene Cancer Prevention Study”; 1.93 (1.43-2.61) in “The Nurses’ Health Study”, and 1.28 (1.01-1.61) in “The Zutphen Elderly Study”.
So,  there is a substantially increased risk even at low levels of intake: 2% of total energy intake, for a 2,000 Kcal diet is 40 Kcal or about 4-5 g of fat corresponding to a teaspoonful of fat!
Moreover, in three of the studies, the association between the intake of industrial trans fats and the risk of CHD was stronger than a corresponding association between the intake of saturated fatty acids and the risk of heart disease. In “The Zutphen Elderly Study”, this association was not investigated.
Because of the adverse effects of industrial trans fatty acids, for the same authors are unethical conducting randomized long-term trials to test their effects on the incidence of CHD.
So, avoidance of industrial trans fats, or a consumption of less 0.5% of total daily energy intake is necessary to avoid their adverse effects, far stronger on average than those of food contaminants or pesticide residues.

Further evidence
A study conducted in an Australia population with a first heart attack and no preceding history of CHD or hyperlipidemia has showed a positive association between levels of trans fatty acids in adipose tissue and the risk of nonfatal myocardial infarction.
It was shown that adipose tissue C18:1Δ7t, found in both animal and vegetable fats, was an independent predictor of a first myocardial infarction, that is, its adipose tissue level is still a predictor for heart disease after adjustment for total cholesterol. Again, it appears that only a minor part of the negative effects of trans fats occurs via plasma lipoproteins.
During the course of this study, mid-1996, TFA were eliminated from margarines sold in Australia (see below). This was a unique opportunity to investigate the temporal relationship between trans fat intake and their adipose tissue levels. It was demonstrated that trans fats disappear from adipose tissue of both case-patients and controls with a rate about 15% of total trans fats/y.
Another study conduct in Costa Rica have found a positive association between myocardial infarction and trans fatty acids.
Interestingly, in a larger, community-based case-control study, levels of trans fats in red blood cell membranes were associated, after adjustment for other risk factors, with an increase in the risk of sudden cardiac death. Moreover, the increased risk appeared to be related to trans-C18:2 levels, that were associated with a tripling of the risk, but not with cell membrane levels of trans-C18:1,  the major trans fatty acids in foods (see above).

Trans fatty acids and diabetes

In a prospective study covering 84,204 female nurses, from “The Nurses’ Health Study”, aged 34–59 y, analyzed from the 1980 to 1996, with no cancer, diabetes, or cardiovascular disease at base line, the intake of trans fatty acids was significantly related to the risk of developing type 2 diabetes. And, after adjustment for other risk factors trans fat intake was positively associated with the incidence of diabetes with a risk up to 39% greater.
Data from controlled intervention studies showed that TFA could impair insulin sensitivity in subjects with insulin resistance and type 2 diabetes (saturated fatty acids do the analogous response, with no significant difference between TFA and them) more than unsaturated fatty acids, in particular the isomer of conjugated linoleic acid (CLA) trans-10, cis-12-CLA. Be careful because some dietary supplements contain CLA isomers and may be diabetogenic and proatherogenic in insulin-resistant subjects.

No significant effect was seen in insulin sensitivity of lean, healthy subjects.

Ruminant trans fatty acids and the risk of CHD

Four prospective studies have evaluated the relation between the intake of ruminant trans fatty acids and the risk of CHD: no significant association was identified.
In another study published on 2008 was analyzed data from four Danish cohort studies that cover 3,686 adults enrolled between 1974 and 1993, and followed for a median of 18 years. In Denmark, consumption of dairy products is relatively high and the range of ruminant trans fat intake is relatively broad, up to 1.1% of energy. Conversely, in the other countries, ruminant trans fatty acid consumption for most people is substantially lower than 1% of energy, in USA about 0.5% of energy. After adjustment for other risk factors, no significant associations between ruminant TFA consumption and incidence of CHD were found, confirming, in a population with relatively high intake of ruminant trans fatty acids, conclusions of four previous prospective studies.
So ruminant trans fats, in amounts actually consumed in diets, do not raise CHD risk.
The absence of risk of CHD with trans fats from ruminants as compared with industrial trans fatty acids  may be due to a lower intake. In the USA, greater part of trans fats have industrial origin (see above); moreover trans fat levels in milk and meats are relatively low, 1 to 8% of total fats.
The absence of a higher risk of CHD may be due also to the presence of different isomers. Ruminant and industrial sources share many common isomers, but there are some quantitative difference (see fig. 4):

  • vaccenic acid level is higher in ruminant fats, 30-50% of trans isomers;
  • trans-C18:2 isomers, present in deodorized and fried vegetable oils, as well as in some partially hydrogenated vegetable oils, are not present in appreciable amounts in ruminants fats.

Finally other, still unknown, potentially protective factors could outweigh harmful effects of ruminant trans fats.

Trans fatty acids: legislation regulating their content

USA
Until 1985 no adverse effects of trans fatty acids on human health was demonstrated, and in 1975 a Procter & Gamble study showed no effect of trans fats on cholesterol.
Their use in fast food preparation grew up from 1980’s, when the role of dietary saturated fats in increasing cardiac risk began clear. Then, it was led a successful campaign to get McDonald’s to switch from beef tallow to vegetable oil for frying its French fries. Meanwhile, studies began to raise concerns about their effects on health. On 1985 Food and Drug Administration (FDA) concluded that TFA and oleic acid affected serum cholesterol level similarly, but from the second half of 1985 their harmful began clear, and the final proof came from both controlled feeding trials and prospective epidemiologic studies.
On 2003 FDA ruled that food labels, for conventional foods and supplements, show their content beginning January 1, 2006. Notably, this ruling was the first substantive change to food labeling since the requirement for per-serving food labels information was added in 1990.
On 2005 the US Department of Agriculture made a minimized intake of trans fatty acids a key recommendation of the new food-pyramid guidelines.
On 2006 American Heart Association recommended to limit their intake to 1% of daily calorie consumption, and suggested food manufacturers and restaurants switch to other fats.
On 2006 New York City Board of Health announced trans fat ban in its 40,000 restaurants within July 1, 2008, followed by the state of California in 2010-2011.

Australia
After June 1996 they were eliminated from margarine sold in Australia, that before contributed about 50% of their dietary intake.

Europe
On March 11, 2003 the Danish government, after a debate started in 1994 and two new reports in 2001 and 2003, decided to phase out the use of industrial trans fats in food before the end of 2003. Two years later, however, the European Commission (EC) asked Denmark to withdraw this law, which was not accepted on the European Community level, unfortunately. However, in 2007, EC decided to closes its infringement procedure against Denmark because of increasing scientific evidence of the danger of this type of fatty acids.
The Danish example was followed by Austria and Switzerland in 2009, Iceland, Norway, and Hungary in 2011, and most recently, Estonia and Georgia in 2014. So, about 10% of the European Union population, about 500 million people, lives in countries where it is illegal to sell food high in industrial trans fats.
Governments of other European Union countries instead rely on the willingness of food producers to reduce trans fatty acid content in their products. This strategy has proved effective only for Western European countries (see below).

Canada
Canada is considering legislation to eliminate them from food supplies, and, in 2005, ruled that pre-packaged food labels show their content.

Therefore, with the exception of the countries where the use of trans fats in the food industry was banned, the only way to reduce their intake in the other countries is consumer’s decision to choose foods free in such fatty acids, avoiding those known containing them, and always reading nutrition facts and ingredients because they may come from margarine, vegetable oil and frying. Indeed, for example in the USA, the producers of foods that contain less than 0.5 g of industrial trans fatty acids per serving can list their content as 0 on the packaging. This content is low but if a consumer eats multiple servings, he consumes substantial amount of them.

Be careful: food labels are not obligatory in restaurants, bakeries, and many other retail food outlets.

Trans fatty acids and food reformulation

Public health organizations, including the World Health Organization in September 2006, have recommended reducing the consumption of industrial trans fatty acids; only in USA the near elimination of these fatty acids might avoid between 72,000 and 280,000 of the 1.2 million of CHD events every year.
Food manufacturers and restaurants may reduce industrial TFA use choosing alternatives to partially hydrogenated oils.
In Denmark, their elimination (see above) from vegetable oils did not increase consumption of saturated fatty acids because they were mostly replaced with cis-unsaturated fatty acids. Moreover, there were no noticeable effects for the consumer: neither increase in the cost nor reduction in availability and quality of foods.
In 2009, Stender et al. have shown that industrial trans fatty acids in food such as French fries, cookies, cakes, and microwave-oven popcorn purchased in USA, South Africa, and many European Country can be replaced, at similar prices, with a mixture of saturated, monounsaturated, and polyunsaturated fatty acids. Such substitution has even greater nutritional benefit than one-to-one substitution of industrial trans fats with saturated fatty acids alone. However, be careful because only in French fries with low industrial trans fats the percentage of saturate fatty acids remains constant, whereas in cookies and cakes is in average +33 percentage points and microwave-oven popcorn +24 percentage points: saturated fatty acids are less dangerous than industrial trans fats but more than mono- and polyunsaturated fatty acids.
The same research group, analyzing some popular foods in Europe, purchased in supermarkets, even of the same supermarket chain, and fast food, namely, McDonald’s and Kentucky Fried Chicken (KFC), from 2005 to 2014, showed that their TFA content was reduced or even absent in several Western European countries while remaining high in Eastern and Southeastern Europe.
In 2010 Mozaffarian et al. evaluated  the levels of industrial trans fats and saturated fatty acids in major brand-name U.S. supermarket and restaurant foods after reformulation to reduce industrial trans fatty acid content, in two time: from 1993 through 2006 and from 2008 through 2009. They found a generally reduction in industrial trans fat content without any substantial or equivalent increase in saturated fatty acid content.

Foods high in trans fatty acids: examples and values

Many foods high in trans fats are popularly consumed worldwide.
In USA greater part of these fatty acids comes from partially hydrogenated vegetable oils, with an average consumption from this source that has been constant since the 1960′s.
It should be noted that the following trans fatty acid values must be interpreted with caution because, as previously said, many fast food establishments, restaurants and industries may have changed, or had to change the type of fat used for frying and cooking since the analysis were done.
The reported values, unless otherwise specified, refer to percentage in trans fatty acids/ 100 g of fatty acids.

Margarine

Among foods with trans fats, stick or hard margarine had the highest percentage of them, but levels of these fatty acids have declined as improved technology allowed the production of softer margarines which have become popular. But there are difference in trans fatty acid content of margarine from different countries. Below some examples.

  • The highest content, 13-16.5%, is found in soft margarine from Iceland, Norway, and the UK.
  • Less content is found in Italy, Germany, Finland, and Greece, 5.1%, 4.8%, 3.2%, and 2.9% respectively).
  • In Portugal, The Netherlands, Belgium, Denmark, France, Spain, and Sweden margarine trans fat content is less than 2%.

USA and Canada lag behind Europe, but in the USA, with the advent of trans fat labeling of foods and the greater knowledge of the risk associated with their consumption by the buyers, change is occurring. For this reason, at now, in the USA margarine is considered to be a minor contributor to the intake of TFA, whereas the major sources are commercially baked and fast food products like cake, cookies, wafer, snack crackers, chicken nuggets, French fries or microwave-oven popcorn (see below).

Vegetable shortenings

Trans fatty acid content of vegetable shortenings ranges from 6% to 50%, and varies in different country: in Germany, Austria and New Zealand it is less than France or USA.
However, like margarines, their trans fat content is decreasing. In Germany it decreased from 12% in 1994 to 6% in 1999, in Denmark is 7% (1996) while in New Zealand is about 6% (1997).

Vegetable oils

At now, non-hydrogenated vegetable oils for salad and cooking contain no or only small amounts of trans fats.
Processing of these oils can produce minimal level of them, ranged from 0.05g/100 food for extra virgin oil to 2.42 g/100 g food for canola oil. So, their contribution to trans fat content of the current food supply is very little.
One exception is represented by Pakistani hydrogenated vegetable oils whose TFA content ranges from 14% to 34%.

Prepared soups

Among foods with trans fats, prepared soups contain significant amount of them, ranging from 10% of beef bouillon to 35% of onion cream. So, they contribute great amount of such fatty acids to the diet if frequently consumed.

Processed foods

Thanks to their properties (see above), trans fatty acids are used in many processed foods as cookies, cakes, croissants, pastries and other baked goods. And, baked goods are the greatest source of these fats in the North American diet. Of course, their trans fat content depends on the type of fat used in processing.

Sauces

Mayonnaise, salad dressings and other sauces contain only small or no-amounts of trans fats.

Human milk and infant foods

Trans fat content of human milk reflects the trans fat content of maternal diet in the previous day, is comprised between 1 and 7%, and is decreasing from 7.1% in 1998 to 4.6% in 2005/2006.
Infant formulas have trans fat values on average 0.1%-4.5%, with a brand up to 15.7%.
Baby foods contain greater than 5% of trans fats.

Fast foods and restaurant’s foods

Vegetable shortenings high in trans fats are used as frying fats, so fast foods and many restaurant’s foods may contain relatively large amounts of them. Foods are fried pies, French fries, chicken nuggets, hamburgers, fried fish as well as fried chicken.
In articles published by Stender et al. from 2006 to 2009, it is showed that for French fries and chicken nuggets their content varies largely from nation to nation, but also within the same fast food chain in the same country, and even in the same city, because of the cooking oil used. For example, oil used in USA and Peru outlets of a famous fast food chain contained 23-24% of trans fats, whereas oil used in many European countries of the same fast food chain contained about 10%, with some countries, such as Denmark, as low as 5% and 1%.
And, considering a meal of French fries and chicken nuggets, in serving size of 171 and 160 g respectively, purchased at McDonald‘s in New York City, it contained over 10 g of TFA, while if purchased at KFC in Hungary they were almost 25 g.
Below, again from the work of Stender et al. it can see a cross-country comparison of trans fat contents of chicken nuggets and French fries purchased at McDonald ‘s or KFC.

Chicken nuggets and French fries from McDonald’s:

  • less than 1 g only if the meals were purchased in Denmark;
  • 1-5 g in Portugal, the Netherlands, Russia, Czech Republic, or Spain;
  • 5-10 g in the United States, Peru, UK, South Africa, Poland, Finland, France, Italy, Norway, Spain, Sweden, Germany, or Hungary.

Chicken and French fries from KFC:

  • less than 2 g if the meals were purchased UK (Aberdeen), Denmark, Russia, or Germany (Wiesbaden);
  • 2-5 in Germany (Hamburg), France, UK (London or Glasgow), Spain, or Portugal;
  • 5-10 in the Bahamas, South Africa, or USA;
  • 10-25 g in Hungary, Poland, Peru, or Czech Republic.

References

Akoh C.C. and Min D.B. Food lipids: chemistry, nutrition, and biotechnology. 3rd Edition. CRC Press Taylor & Francis Group, 2008

Ascherio A., Katan M.B., Zock P.L., Stampfer M.J., Willett W.C. Trans fatty acids and coronary heart disease. N Engl J Med 1999;340:1994-1998. doi:10.1056/NEJM199906243402511

Ascherio A., Rimm E.B., Giovannucci E.L., Spiegelman D., Stampfer M., Willett W.C. Dietary fat and risk of coronary heart disease in men: cohort follow up study in the United States. BMJ 1996; 313:84-90. doi:10.1080/17482970601069094

Asp N-G. Fatty acids in focus – the good and the bad ones. Scand J Food Nutr 2006;50:155-160. doi:10.1080/17482970601069094

Baylin A., Kabagambe E.K., Ascherio A., Spiegelman D., Campos H. High 18:2 trans-fatty acids in adipose tissue are associated with increased risk of nonfatal acute myocardial infarction in Costa Rican adults. J Nutr 2003;133:1186-1191. doi:10.1093/jn/133.4.1186

Chow C.K. Fatty acids in foods and their health implication. 3rd Edition. CRC Press Taylor & Francis Group, 2008.

Clifton P.M., Keogh J.B., Noakes M. Trans fatty acids in adipose tissue and the food supply are associated with myocardial infarction. J Nutr 2004;134:874-879. doi:10.1093/jn/134.4.874

Costa N., Cruz R., Graça P., Breda J., and Casal S. Trans fatty acids in the Portuguese food market. Food Control 2016;64:128-134. doi:10.1016/j.foodcont.2015.12.010

Eckel R.H., Borra S., Lichtenstein A.H., Yin-Piazza D.Y. Understanding the Complexity of Trans fatty acid reduction in the American diet. American Heart Association trans fat conference 2006 report of the trans fat conference planning group. Circulation 2007;115:2231-2246. doi:10.1161/CIRCULATIONAHA.106.181947

Hu F.B., Manson J.E., Stampfer M.J., Colditz G., Liu S., Solomon C.G., and Willett W.C. Diet, lifestyle, and the risk of type 2 diabetes mellitus in women. N Engl J Med 2001;345:790-797. doi:10.1056/NEJMoa010492

Hu F.B., Willett W.C. Optimal diet for prevention of coronary heart disease JAMA 2002;288:2569-2578. doi:10.1001/jama.288.20.2569

Lemaitre R.N., King I.B., Raghunathan T.E., Pearce R.M., Weinmann S., Knopp R.H., Copass M.K., Cobb L.A., Siscovick D.S. Cell membrane trans-fatty acids and the risk of primary cardiac arrest. Circulation 2002;105:697-701. doi:10.1161/hc0602.103583

Lemaitre R.N, King I.B, Mozaffarian D., Sootodehnia N., Siscovick D.S. Trans-fatty acids and sudden cardiac death. Atheroscler Suppl 2006; 7(2):13-5. doi:10.1016/j.atherosclerosissup.2006.04.003

Lichtenstein A.H. Dietary fat, carbohydrate, and protein: effects on plasma lipoprotein patterns J. Lipid Res. 2006;47:1661-1667. doi:10.1194/jlr.R600019-JLR200

Lichtenstein A.H., Ausman L., Jalbert S.M., Schaefer E.J. Effect of different forms of dietary hydrogenated fats on serum lipoprotein cholesterol levels. N Engl J Med 1999;340:1933-1940. doi:10.1056/NEJM199906243402501

Lopez-Garcia E., Schulze M.B., Meigs J.B., Manson J.E, Rifai N., Stampfer M.J., Willett W.C. and Hu F.B. Consumption of trans fatty acids is related to plasma biomarkers of inflammation and endothelial dysfunction. J Nutr 2005;135:562-566. doi:10.1093/jn/135.3.562

Masanori S. Trans Fatty Acids: Properties, Benefits and Risks J Health Sci 2002;48(1):7-13.

Mensink R.P., Katan M.B. Effect of dietary trans fatty acids on high-density and low-density lipoprotein cholesterol levels in healthy subjects. N Engl J Med 1990;323:439-445. doi:10.1056/NEJM199008163230703

Mozaffarian D. Commentary: Ruminant trans fatty acids and coronary heart disease-cause for concern? Int J Epidemiol 2008;37(1):182-184. doi:10.1093/ije/dym263

Mozaffarian D., Jacobson M.F., Greenstein J.S. Food Reformulations to reduce trans fatty acids. N Eng J Med 2010;362:2037-2039. doi:https://doi.org/10.1056/NEJMc1001841

Mozaffarian D., Katan M.B., Ascherio A., Stampfer M.J., Willett W.C. Trans fatty acids and cardiovascular disease. N Engl J Med 2006;354:1601-1613. doi:10.1056/NEJMra054035

Mozaffarian D., Pischon T., Hankinson S.E., Joshipura K., Willett W.C., and Rimm E.B. Dietary intake of trans fatty acids and systemic inflammation in women. Am J Clin Nutr 2004;79:606-612. doi:https://doi.org/10.1093/ajcn/79.4.606

Oh K., Hu F.B., Manson J.E., Stampfer M.J., Willett W.C. Dietary fat intake and risk of coronary heart disease in women: 20 years of follow-up of the Nurses’ Health Study. Am J Epidemiol 2005;161(7):672-679. doi:10.1093/aje/kwi085

Okie S.  New York to Trans Fats: You’re Out! N Engl J Med 2007;356:2017-2021. doi:10.1056/NEJMp078058

Oomen C.M., Ocke M.C., Feskens E.J., van Erp-Baart M.A., Kok F.J., Kromhout D. Association between trans fatty acid intake and 10-year risk of coronary heart disease in the Zutphen Elderly Study: a prospective population-based study. Lancet 2001; 357(9258):746-751. doi:10.1016/S0140-6736(00)04166-0

Pietinen P., Ascherio A., Korhonen P., Hartman A.M., Willett W.C., Albanes D., VirtamO J.. Intake of fatty acids and risk of coronary heart disease in a cohort of Finnish men: the Alpha-Tocopherol, Beta-Carotene Cancer Prevention Study. Am J Epidemiol 1997;145(10):876-887. doi:10.1093/oxfordjournals.aje.a009047

Risérus U. Trans fatty acids, insulin sensitivity and type 2 diabetes. Scand J Food Nutr 2006;50(4):161-165. doi:10.1080/17482970601133114

Salmerón J., Hu F.B., Manson J.E., Stampfer M.J., Colditz G.A., Rimm E.B., and Willett W.C. Dietary fat intake and risk of type 2 diabetes in women. Am J Clin Nutr 2001;73:1019-1026 doi:10.1093/ajcn/73.6.1019

Stender S., Astrup A.,and Dyerberg J. A trans European Union difference in the decline in trans fatty acids in popular foods: a market basket investigation. BMJ Open 2012;2(5):e000859. doi:10.1136/bmjopen-2012-000859

Stender S., Astrup A., and Dyerberg J. Artificial trans fat in popular foods in 2012 and in 2014: a market basket investigation in six European countries. BMJ Open 2016;6(3):e010673. doi:10.1136/bmjopen-2015-010673

Stender S., Astrup A.,and Dyerberg J. Tracing artificial trans fat in popular foods in Europe: a market basket investigation. BMJ Open 2014;4(5):e005218. doi:10.1136/bmjopen-2014-005218

Stender S., Astrup A., Dyerberg J. What went in when trans went out?. N Engl J Med 2009;361:314-316. doi:10.1056/NEJMc0903380

Stender S., Dyerberg J. The influence of trans fatty acids on health. Fourth edition. The Danish Nutrition Council; publ. no. 34, 2003.

Stender S., Dyerberg J., Astrup A. Consumer protection through a legislative ban on industrially produced trans fatty acids in Denmark. Scand J Food Nutr 2006;50(4):155-160. doi:10.1080/17482970601069458

Stender S., Dyerberg J., Astrup A. High levels of trans fat in popular fast foods. N Engl J Med 2006;354:1650-1652. doi:10.1056/NEJMc052959

Willett W., Mozaffarian D. Ruminant or industrial sources of trans fatty acids: public health issue or food label skirmish? Am J Clin Nutr 2008;87(3): 515-516. doi:10.1093/ajcn/87.3.515

Processing, properties and health benefits of black tea

Black tea, like the other types of tea, is an infusion of dried and processed leaves of Camellia sinensis, a shrub belonging to the Theaceae family.
Unlike what happens during green tea production, during black tea production the almost complete oxidation of the substances contained in the leaves occurs, particularly catechins, polyphenols of the flavonoid group.
The color of the processed leaves is dark, whereas the beverage is brown-red in color.
Black tea is prepared with one tea bag per person, or one teaspoon per person in case of loose tea leaves, with an infusion time of 3-4 minutes in water at 95-100 °C.
Tea is a beverage with ancient origins, dating back to almost 4,000 years ago. It is one of the most-consumed beverages, particularly in Asia, where the favorite tea is green tea, especially in Japan and China, whereas black tea is preferred by Western populations and at global level, accounting for about 78% of the tea consumed.
The oxidation of the compounds present in the leaves during processing reduces the potential beneficial effects ascribed to the polyphenols initially present.

CONTENTS

How black tea is made

All the types of teas are produced from fresh leaves of Camellia sinensis. During harvesting, young leaves are preferred, as the older ones are considered inferior in quality.
The processing that leads to the production of loose dried tea leaves ready for brewing black tea proceeds through four steps: withering, rolling, oxidation, and drying. Such processing leads to the near complete oxidation of the substances present, particularly catechins.
Withering is the process by which the water present in the leaves, about 75% of the leaf’s weight, is removed, thus causing sap concentration. Withering, which makes the next step easier, can be achieved in three different ways:

  • by exposing leaves to sunlight;
  • by appropriately heating the rooms where the leaves are stored;
  • by machineries that ventilate the leaves.

The rolling step follows the withering of the leaves, and, breaking the leaf tissues, facilitates the outflow of lymph thus promoting the subsequent oxidation of polyphenols.
The oxidation step is also improperly called fermentation. In this step, the oxidation by atmospheric oxygen and polyphenol oxidase (EC 1.10.3.1) of 90-95% of the polyphenols occurs, accompanied by other changes that color the leaves with a red color. Temperature, typically 25°C, pH, relative humidity, 95% or more, ventilation, and duration are crucial factors, too. This step is crucial in determining the quality of the tea, as it gives it its organoleptic characteristics and composition in polyphenols, quite different from those of green tea, which is produced in such a way as to minimize oxidation processes.
Note that caffeine content does not vary significantly.
Drying is the last step. It is carried out at a temperature of 80-90 °C for about 20-25 minutes. The high temperature inactivates polyphenol oxidase, and then stops enzymatic oxidation processes.

Thearubigins and theaflavins

The oxidative processes that occur during black tea production affect monomeric and gallate catechins, to a lesser extent catechins glycosides, especially myricetin, and non-flavonoid compounds, such as teagallin, and leads to the formation of complex polyphenols such as thearubigins, theaflavins and theaflavic acids.
Thearubigins, brownish in color, are polymers of catechins not yet well characterized and the major polyphenols in black tea, accounting for about 20% of the dry leaf weight. They contribute to the richness in taste and color.
Theaflavins, red-orange in color, are dimers of catechins and account for about 3-5% of the dry leaf weight. They contribute to the astringent and brisk taste, as well as the red-orange in color.
The main theaflavins are:

  • theaflavin 3-gallate;
  • theaflavin 3′-gallate;
  • theaflavin 3,3’-digallate, the most abundant.

Skeletal formulas of theaflavins, dimers of catechins present in black tea

Health benefits of black tea

The health benefits of black tea are largely due to its complex polyphenols, thearubigins and theaflavins, being catechins largely oxidized during leaf processing.
Here are three examples.

  • Theaflavins have been highlighted as having antiviral activity which, similarly to catechins, appears to be particularly effective against positive single-stranded RNA viruses. These viruses also include SARS-CoV-1 and SARS-CoV-2, viruses belonging to the Coronaviridae family.
    Like catechins, the galloyl groups appear to be important for the antiviral activity of theaflavins.
  • The phytochemicals present in black tea, like those in green tea, seem to be able to reduce the glycemic index of starchy foods. The effect appears to be due to the inhibition of the activity of pancreatic alpha-amylase and other digestive enzymes, and to the direct interaction with starch, that would reduce the surface available to enzyme activity. The inhibition is greater on gluten-free foods; this seems to be due to the action of gluten on complex polyphenols that would not be able to interact with the polysaccharide. For more information, see the article on tea polyphenols.
  • Thearubigins and theaflavins seem to have anticariogenic effects due to the inhibitory action on salivary and bacterial amylase, and seem to be more  effective than green tea catechins.
    Moreover, black tea seems to be able to inhibit acid production in the oral cavity.

References

Asil M.H., Rabiei B., Ansari R.H. Optimal fermentation time and temperature to improve biochemical composition and sensory characteristics of black tea. Aust J Crop Sci 2012;6(3):550-8.

Kan L., Capuano E., Fogliano V., Oliviero T. and Verkerk R. Tea polyphenols as a strategy to control starch digestion in bread: the effects of polyphenol type and gluten. Food Funct 2020;11:5933-5943. doi:10.1039/D0FO01145B

Kuhnert N. Unraveling the structure of the black tea thearubigins. Arch Biochem Biophys 2010;501(1):37-51. doi:10.1016/j.abb.2010.04.013

Li S., Lo C-Y., Pan M-H., Lai C-S. and Ho C-T. Black tea: chemical analysis and stability. Food Funct 2013;4:10-18. doi:10.1039/C2FO30093A

Mhatre S., Srivastava T., Naik S., Patravale V. Antiviral activity of green tea and black tea polyphenols in prophylaxis and treatment of COVID-19: a review. Phytomedicine 2020;153286. doi:10.1016/j.phymed.2020.153286

Menet M-C., Sang S., Yang C.S., Ho C-T., and Rosen R.T. Analysis of theaflavins and thearubigins from black tea extract by MALDI-TOF mass spectrometry. J Agric Food Chem 2004;52:2455-61. doi:10.1021/jf035427e

Sharma V.K., Bhattacharya A., Kumar A. and Sharma H.K. Health benefits of tea consumption. Trop J Pharm Res 2007;6(3):785-792.

Relationship between potassium intake, blood pressure and hypertension

High dietary potassium (K+) intakes and blood pressure are inversely related: animal studies, observational epidemiological studies, clinical trials, and meta-analyses of these trials support this.
Furthermore, the prevalence of hypertension tends to be lower in populations with high K+ intakes than in those with low intakes.
Finally, an increase in potassium intake (2.5-3.9 g/d) reduces blood pressure in normotensive and hypertensive, and to a greater extent in blacks than in whites.

Diet high in potassium, blood pressure, and ictus

Controlled feeding studies, such as “The Dietary Approaches to Stop Hypertension (DASH) Study” and “OmniHeart Trial”,  have highlighted the role of a good potassium intake, along with other minerals and fiber, in blood pressure reduction.
These studies have shown that a dietary pattern rich in fruits, vegetables, and low-fat dairy products, with whole grains, poultry, fish and nuts but poor in fats, red meat, sweets, and sugar-containing beverages reduces blood pressure. And such dietary patterns are characterized by foods high in potassium, as well as magnesium, calcium and fiber, but poor in total fat, saturated fat and cholesterol. The best result on lowering blood pressure are with black participants than white participants.
In another study, a systematic review of the literature and meta-analyses has been conducted on potassium intake in apparently healthy adults and children without renal impairment. The study showed that, in adult with hypertension, an increased potassium intake reduced systolic blood pressure by 3.49 mm Hg and diastolic blood pressure by 1.96 mm Hg. No effect was seen in adult without hypertension and in children. In addition, there was no effect of increased potassium intake on blood lipids, or catecholamine concentrations in adults, whereas an inverse statistically significant association was seen between its intake and the risk of incident stroke. Hence, this study suggests that, in people without impaired renal function, increased potassium intake is potentially beneficial for the prevention and control of elevated blood pressure and stroke.

Potassium, sodium and blood pressure

The effects of potassium on blood pressure depend on the concurrent intake of sodium and vice versa:

  • an increased intake of K+ has:

a greater blood pressure-lowering effect when sodium intake is high;

a lesser blood pressure-lowering effect when sodium intake is low;

  • on the other hand, the blood pressure reduction from a lowered sodium intake is greatest when potassium intake is low.

An high K+ intake also increases urinary excretion of sodium, the so-called natriuretic effect.
In the generally healthy population with normal kidney function the recommended potassium intake level is 3.1 g/day. But, in the presence of impaired urinary potassium excretion, a K+ intake less than 3.1 g/day (120 mmol/d) is appropriate, because of adverse cardiac effects (arrhythmias) from hyperkalemia, that is, blood potassium level higher than normal.

Mediterranean Diet and K+ intake

As already pointed out, the best strategy to increase K+ intake is to consume legumes, and fruits and vegetables in season, i.e. foods high in  potassium, that is also accompanied by a variety of other nutrients. No supplements are needed.Potassium
Therefore, it is sufficient to follow a  Mediterranean dietary pattern, for:

  • meet the daily requirements of the mineral;
  • consume K+ intake in adequate amounts to ensure its blood pressure-lowering effect.

Potassium content in some foods

High content: >250 mg/100 g of product

  • Dried legumes (chickpeas, beans, lentils, peas and soybeans) and fresh beans;
  • garlic, chard, cauliflower, cabbage, Brussels sprouts, broccoli, artichokes, cardoons, fennel, mushrooms, potatoes, tomatoes, spinach, zucchini;
  • avocados, apricots, bananas, fresh and dried chestnuts, watermelon, kiwi, melon, hazelnuts;
  • sweet dried fruits (apricots, dates, figs, prunes, raisins etc..) and oily dried fruits (peanuts, almonds, walnuts, pine nuts, pistachios, etc.);
  • oat flour, whole wheat flour and spelt;
  • ketchup;
  • roasted coffee;
  • milk powder (also rich sodium);
  • yeast;
  • cocoa powder.

Medium content: 150-250 mg/100 g of product

  • asparagus, beets, carrots, chicory, green beans, fresh broad beans, endive, lettuce, peppers, fresh peas, tomatoes, leeks, radishes, celery, tomato and carrot juice, pumpkin;
  • pineapple, oranges, raspberries, blueberries, loquats, pears, peaches, grapefruit, grapes;
  • meat and fish products, both fresh and preserved (the latter, however, should be avoided because of their high sodium content).

Note: cooking methods tend to reduce the K+ content of the food.
To reduce potassium loss, avoid boiling in plenty of water, for more than an hour, vegetables cut into small pieces (this increases the “exchange area” with water).

References

Aburto N.J., Hanson S., Gutierrez H., Hooper .L, Elliott P., Cappuccio F.P. Effect of increased potassium intake on cardiovascular risk factors and disease: systematic review and meta-analyses. BMJ 2013;346:f1378. doi:10.1136/bmj.f1378

Appel L.J., Brands M.W., Daniels S.R., Karanja N., Elmer P.J. and Sacks F.M. Dietary approaches to prevent and treat HTN: a scientific statement from the American Heart Association. Hypertension 2006;47:296-308. doi:10.1161/01.HYP.0000202568.01167.B6

Cappuccio F.P. and MacGregor G.A. Does potassium supplementation lower blood pressure? A metaanalysis of published trials. J Hyperten 1991;9:465-473.

Geleijnse J.M., Witteman J.C., den Breeijen J.H., Hofman A., de Jong P., Pols H.A. and Grobbee D.E. Dietary electrolyte intake and blood pressure in older subjects: the Rotterdam Study. J Hyperten 1996;14:73741.

Matlou S.M., Isles C.G. and Higgs A. Potassium supplementation in Blacks with mild to moderate essential hypertension. J Hyperten 1986;4:61-64.

Pickering T.G. New guidelines on diet and blood pressure. Hypertension 2006;47:135-136. doi:10.1161/01.HYP.0000202417.57909.26

Rose G. Desirability of changing potassium intake in the community. In: Whelton P.K., Whelton A.K. and Walker W.G. eds. Potassium in cardiovascular and renal disease. Marcel Dekker, New York 1986;411-416

Writing Group of the PREMIER Collaborative Research Group. Effects of comprehensive lifestyle modification on blood pressure control: main results of the PREMIER Clinical Trial. JAMA 2003;289:2083-2093. doi:10.1001/jama.289.16.2083

World Health Organization, International Society of Hypertension Writing Group. 2003 World Health Organization (WHO)/ISH statement on management of HTN. Guidelines and recommendations. J Hyperten 2003;21:1983-1992.

Processing, properties and benefits of green tea

Green tea, like the other types of tea, is an infusion of dried and processed leaves of Camellia sinensis, a member of the Theaceae family.
The processing of the leaves leading to the product ready for use is such as to minimize the oxidation of the compounds contained in them, particularly phytochemicals such as catechins, which are polyphenols belonging to the class of flavonoids and the most responsible for the health benefits of green tea.
Having undergone no significant chemical modifications, leaves retain green color, whereas the beverage, prepared with one tea bag per person, or in case of loose tea, one teaspoon per person, for an infusion time of about 3 minutes in water at 75 °C, is golden yellow in color.
Some organoleptic properties of green tea, such as the flavor, that is more delicate and lighter than that of black tea, and the health properties, which have always been recognized in East Asia cultures, depend on leaf processing.
Only recently scientists started studying the health benefits of tea consumption, highlighting its role in preventing many diseases, such as cardiovascular diseases and some types of cancer.
It has been shown that tea polyphenols, particularly catechins, are able to activate intracellular signaling pathways by binding to membrane receptors and/or entering the cell and binding to cytoplasmic, mitochondrial or nuclear receptors. Then, depending on the cell type, they activate or inhibit some cellular processes.
Given the high consumption of tea in the world, even small effects on health could have significant effects on public health.

CONTENTS

Camellia sinensis

Camellia sinensis is an evergreen plant native to South, East, and Southeast Asia, which is now cultivated in at least 30 countries, mostly in tropical or subtropical climates, although some varieties grow in Cornwall and Washington State.
In nature, Camellia sinensis can grow up to 15-20 meters (49-65 ft), whereas in plantations it is pruned to less than 1,5 meters to facilitate leaf harvesting.
It can grow up to altitude of 1,500-2,000 meters (4,900-6,550 ft), and many of the high-quality teas are produced from such crops, as the plant grows slowly and the leaves acquire a better flavor.
The most cultivated varieties, of the four known, are Camellia sinensis var. sinensis, native to China, and Camellia sinensis var. assamica, native to India.
The different types of tea are produced from fresh leaves. Young leaves are preferred over older leaves that are considered to be inferior in quality.
Fresh leaves are rich in water-soluble polyphenols, especially catechins and glycosylated catechins. The major catechins in green tea are epigallocatechin-3-gallate or EGCG, the most active, epigallocatechin, epicatechin 3-gallate, epicatechin. Catechin, gallocatechin, catechin gallate, and gallocatechin gallate are also present, although in lower amount.

Skeletal formula of gallocatechin gallate, one of the catechins found in green tea
Gallocatechin gallate

These polyphenols account for 30%-42% of the dry leaf weight. Caffeine accounts for 1,5-4,5% of the dry leaf weight.
In addition to leaf processing, the organoleptic properties of the beverage are influenced by cultivar, characteristics of the soil where the plant grown up, methods of cultivation, altitude, climate, and time of year in which leaf harvest occurs.

How green tea is made

The differences in leaf processing, which lead to the different types of tea ready for consumption, cause different degrees of oxidation of the compounds present in them, especially catechins.
During green tea manufacturing, oxidative processes, both enzymatic and chemical, are minimized. After harvesting, leaves are exposed to sunlight for 2-3 hours and withered/dried. Then, the processing proceeds through three steps:

  • heat treatment;
  • rolling;
  • drying.

Heat treatment, short and gentle, is crucial for the quality and properties of the beverage. It can done either with a steam, the traditional Japanese method, or by dry cooking in hot pans, that is similar to a roasting method and is the traditional Chinese method. Heat treatment inactivates enzymes and then prevents the enzymatic oxidation processes, particularly those involving polyphenols. It also removes the grassy smell, and evaporates, in the case of the traditional Chinese method, part of the water of the leaf, which constitutes about 75% of its weight, making it softer, thus facilitating the next step.
Heat treatment is followed by the rolling step, that facilitates the subsequent drying step and, destroying the leaf tissue, favors the release of aromas, thus improving the quality of the product.
The drying, the last step, improves the appearance of beverage and leads to the production of new compounds.

Health benefits of green tea

In East Asia cultures, mainly in China and Japan, tea drinking has always been associated with health benefits. Below is a brief review of the results of epidemiological and laboratory studies that have analyzed the effects that green tea consumption can play in preventing many diseases. EGCG, which is the most abundant catechin in green tea accounting for about 60% of the polyphenols present in dried leaves, seems to play the main role.
At the molecular level, the galloyl groups at positions 3 and/or 3′ appear to be essential for many of the effects exerted by catechins.

Cardiovascular disease

Cardiovascular disease is the main cause of deaths worldwide, particularly in low- and middle-income countries, with an estimate of about 17 million deaths in 2008 that could increase up to 23.3 million by 2030.
Daily tea consumption, especially green tea, seems to be associated with a reduced risk of developing cardiovascular disease, such as hypertension and stroke.
Among the proposed mechanisms, the improved bioactivity of the endothelium-derived vasodilator nitric oxide, due to the action of tea polyphenols that could enhance nitric oxide synthesis and/or decrease its breakdown by superoxide anions, seem to be important.


Cancer

Several epidemiological and laboratory studies have shown encouraging results with respect to the preventive role of tea consumption, especially green tea, against the development of some cancers such as those of the oral cavity, digestive tract, and lung among those who have never smoked.
Tea polyphenols seem to act not only as antioxidants, but also as compounds that, directly, can influence gene expression and various metabolic pathways.

Antiviral activity

Recent studies have highlighted antiviral effects of catechins, particularly EGCG of green tea and theaflavins of black tea, especially against positive single-stranded RNA viruses, to which the Coronaviridae family, and then SARS-CoV-1 and SARS-CoV-2, belongs.
The antiviral properties of EGCG appear to be due to its structural characteristics, namely, the presence of pyrogallic and galloyl groups.

Starch digestion

In vitro studies have shown that green tea and black tea polyphenols can reduce the glycemic index of starchy foods. Hence, they could be useful in controlling their glycemic index in vivo. This effect seems to be due to the inhibition of pancreatic alpha-amylase and other digestive enzymes, and to a direct interaction between starch and phytochemicals that would reduce the surface area of starch granules available for enzyme activity. Green tea appears to be equally effective against both gluten-containing foods, against which black tea appears less effective, and gluten-free foods. For more information, see the article on tea polyphenols.

Weight loss

During weight loss and weight-loss maintenance it is important to keep as constant as possible the daily energy expenditure.
Since the 90s, it has been proposed that green tea, by virtue of its content of caffeine and catechins could be of help for:

  • maintaining, or even increasing, daily energy expenditure;
  • increasing fat oxidation.

In addition to these potential lipolytic and thermogenic effects, catechins and caffeine could act on other targets, such as intestinal absorption of fat and energy intake, perhaps through their effect on gut microbiota and gene expression.
And products for weight loss and weight maintenance based on green tea extracts have been marketed. It should be noted that these products contain catechins and caffeine in much higher amount than beverage.
How much truth is there in green tea’s fat burning effect?
The issue seems to have been resolved by a meta-analysis of 15 studies on weight loss and intake of these products. The study showed that green tea-based products induces, in overweight and obese adults, a weight loss that is:

  • not statistically significant;
  • very small;
  • probably not clinically important.

Hence, on the basis of scientific evidence, green tea does not seem to be helpful in fat loss nor in weight maintenance.

Anticariogenic activity

Animal and in vitro studies have shown that tea, and particularly its polyphenols, seem to possess:

  • antibacterial activity against cariogenic bacteria, such as Streptococcus mutans, as in the case of  green tea EGCG;
  • inhibitory action on salivary and bacterial amylase, in which black tea thearubigins and theaflavins are more effective than green tea catechins;
  • inhibitory action on acid production in the oral cavity.

All these properties make green tea and black tea beverages with potential anticariogenic activity.

References

Arab L., Khan F., and Lam H. Tea consumption and cardiovascular disease risk. Am J Clin Nutr 2013;98:1651S-1659S. doi:10.3945/ajcn.113.059345

Clifford M.N., van der Hooft J.J.J., and Crozier A. Human studies on the absorption, distribution, metabolism, and excretion of tea polyphenols. Am J Clin Nutr 2013;98:1619S-1630S. doi:10.3945/ajcn.113.058958

Dwyer J.T. and Peterson J. Tea and flavonoids: where we are, where to go next. Am J Clin Nutr 2013;98:1611S-1618S. doi:10.3945/ajcn.113.059584

Goenka P., Sarawgi A., Karun V., Nigam A.G., Dutta S., Marwah N. Camellia sinensis (Tea): implications and role in preventing dental decay. Phcog Rev 2013;7:152-156. doi:10.4103/0973-7847.120515

Green R.J., Murphy A.S., Schulz B., Watkins B.A. and Ferruzzi M.G. Common tea formulations modulate in vitro digestive recovery of green tea catechins. Mol Nutr Food Res 2007;51(9):1152-1162. doi:10.1002/mnfr.200700086

Grassi D., Desideri G., Di Giosia P., De Feo M., Fellini E., Cheli P., Ferri L., and Ferri C. Tea, flavonoids, and cardiovascular health: endothelial protection. Am J Clin Nutr 2013;98:1660S-1666S. doi:10.3945/ajcn.113.058313

Huang W-Y., Lin Y-R., Ho R-F., Liu H-Y., and Lin Y-S. Effects of water solutions on extracting green tea leaves. Sci World J 2013;Article ID 368350. doi:10.1155/2013/368350

Hursel R. and Westerterp-Plantenga M.S. Catechin- and caffeine-rich teas for control of body weight in humans. Am J Clin Nutr 2013;98:1682S-1693S. doi:10.3945/ajcn.113.058396

Hursel R., Viechtbauer W. and Westerterp-Plantenga M.S. The effects of green tea on weight loss and weight maintenance: a meta-analysis. Int J Obesity 2009;33:956-961. doi:10.1038/ijo.2009.135

Jurgens T.M., Whelan A.M., Killian L., Doucette S., Kirk S., Foy E. Green tea for weight loss and weight maintenance in overweight or obese adults. Editorial group: Cochrane Metabolic and Endocrine Disorders Group. 2012:12 Art. No.: CD008650. doi:10.1002/14651858.CD008650.pub2

Lambert J.D. Does tea prevent cancer? Evidence from laboratory and human intervention studies. Am J Clin Nutr 2013;98:1667S-1675S. doi:10.3945/ajcn.113.059352

Lorenz M. Cellular targets for the beneficial actions of tea polyphenols. Am J Clin Nutr 2013;98:1642S-1650S. doi:10.3945/ajcn.113.058230

Mathur A., Gopalakrishnan D., Mehta V., Rizwan S.A., Shetiya S.H., Bagwe S. Efficacy of green tea-based mouthwashes on dental plaque and gingival inflammation: a systematic review and meta-analysis. Indian J Dent Res 2018;29(2):225-232. doi:10.4103/ijdr.IJDR_493_17

Mhatre S., Srivastava T., Naik S., Patravale V. Antiviral activity of green tea and black tea polyphenols in prophylaxis and treatment of COVID-19: a review. Phytomedicine 2020;153286. doi:10.1016/j.phymed.2020.153286

Sharma V.K., Bhattacharya A., Kumar A. and Sharma H.K. Health benefits of tea consumption. Trop J Pharm Res 2007;6(3):785-792.

Yang Y-C., Lu F-H., Wu J-S., Wu C-H., Chang C-J. The protective effect of habitual tea consumption on hypertension. Arch Intern Med 2004;164:1534-1540. doi:10.1001/archinte.164.14.1534

Yuan J-M. Cancer prevention by green tea: evidence from epidemiologic studies. Am J Clin Nutr 2013;98:1676S-1681S. doi:10.3945/ajcn.113.058271

Xu J., Xu Z., Zheng W. A review of the antiviral role of green tea catechins. Molecules 2017;22(8):1337. doi:10.3390/molecules22081337

Blood pressure, hypertension and dietary sodium

A high sodium (Na+) intake (the main source is salt or sodium chloride NaCl) contributes to blood pressure raise, and hypertension development.
Many epidemiologic studies, animal studies, migration studies, clinical trials, and meta-analyses of trials support this, with the final evidence from rigorously controlled, dose-response trials. Furthermore, in primitive society Na+ intake is very low and people experience very low hypertension, and the blood pressure increase with age does not occur.
Probably, sodium intake effect sizes are to be underestimated!

CONTENTS

Recommended daily intake

Sodium’s physiologic requires are very low; in fact, the minimum recommended Na+ intake for maintain life is 250 mg/day (Note: iodized salt is an important source of dietary iodine in the United States and worldwide).
An Americans consumes the mineral in great excess of physiologic requires: despite the guidelines from the Departments of Agriculture and Health and Human Services, during the period from 2005 through 2006 the average salt intake in USA is of 10.4 g/day for the average man and 7.3 for the average woman, amount in excess regarding preceding years.
A study published on February 2010 on “The New England Journal of Medicine” have shown that “A population-wide reduction in dietary salt of 3 g per day (1200 mg of Na+ per day) is projected to reduce the annual number of new cases of coronary heart disease (CHD) by 60,000 to 120,000, stroke by 32,000 to 66,000, and myocardial infarction by 54,000 to 99,000 and to reduce the annual number of deaths from any cause by 44,000 to 92,000″ (Bibbins-Domingo et all., see References). These benefits are similar in magnitude to those from:

  • a 50% reduction in tobacco use;
  • a 5% reduction in body mass index among obese adults;
  • a reduction in cholesterol levels.

These benefits regard all adult group age, black and nonblack, male and female. The benefits for black are greater than nonblack, in both sex and all age group. It’s estimated an annual savings of $10 billion to 24 $ billion in health care costs.
Clinical trials have also documented that a reduced Na+ intake can lower blood pressure in the setting of antihypertensive medication, and can facilitate hypertension control.
But, in USA dietary salt intake is on the rise!
So, it is recommended, to prevent hypertension development, a reduction in its intake and, in view of the available food supply and the currently daily Na+ intake, a reasonable recommendation is an upper limit of 2.3 g/day (5.8 g/day of salt).
How achieves this level? It can be achieved:

  • cooking with as little salt as possible;
  • refraining from adding salt at the table;
  • avoiding highly salted, processed foods.

Food sources of sodium

They include:

  • salt used at the table: up to 20% of the daily salt intake;
  • salt or sodium compounds added during preparation or processing foods: between 35 to 80% of the daily sodium intake comes from processed foods.A major source of sodium is salt, or sodium chlorideWhich foods are?
    Processed, smoked or cured meat and fish e.g. sliced salami, sausage, salt pork, tuna fish in oil etc.; meat extracts and sauce, salted snack, soy sauce, barbecue sauce, commercial salad dressing; prepackage frozen foods; canned soup, canned legumes; cheese etc.
    There are also many sodium-containing additives as disodium phosphate (e.g. in cereals, ice cream, cheese), monosodium glutamate (i.e. meat, soup, condiments), sodium alginate (e.g. in ice creams), sodium benzoate (e.g. in fruit juice), sodium hydroxide (e.g. in pretzels, cocoa product), sodium propionate (e.g. in bread), sodium sulfite (e.g. in dried fruit), sodium pectinate (e.g. syrups, ice creams, jam), sodium caseinate (e.g. ice creams and other frozen products) and sodium bicarbonate (e.g. baking powder, tomato soup, confections).
    So pay attention to ingredients!
  • Inherent sodium of foods. Generally low in fresh foods.

The blood pressure response to lower dietary Na+ intake is heterogeneous with individuals having greater or lesser degrees of blood pressure reduction. Usually the effect of reduction tend to be greater in blacks, middle-aged and older persons, and individuals with hypertension, diabetes or chronic kidney disease.
Furthermore genetic and dietary factors influence the response to sodium reduction.

Diet modifies response of blood pressure to sodium

Some components of the diet may modify response of blood pressure to sodium.

  • A high dietary intake of calcium and potassium rich foods, such as fruit, vegetable, legumes (e.g. Mediterranean diet), and low-fat dairy products (e.g. DASH diet), may prevent or attenuate the rise in blood pressure for a given increase in sodium intake.
  • Some evidences, seen primarily in animal model, suggest that high dietary intake of sucrose may potentiate salt sensitivity of blood pressure.

Note: high Na+ intake can contribute to osteoporosis: they result in an increase in renal calcium excretion, particularly if daily calcium intakes are low.

References

Appel L.J., Brands M.W., Daniels S.R., Karanja N., Elmer P.J. and Sacks F.M. Dietary approaches to prevent and treat HTN: a scientific statement from the American Heart Association. Hypertension 2006;47:296-308. doi:10.1161/01.HYP.0000202568.01167.B6

Bibbins-Domingo K., Chertow G.M., Coxson P.G., Moran A., Lightwood J.M., Pletcher M.J., and Goldman L. Projected effect of dietary salt reductions on future cardiovascular disease. N Engl J Med 2010;362:590-599. doi:10.1056/NEJMoa0907355

Cappuccio FP. Overview and evaluation of national policies, dietary recommendtions and programmes around the world aiming at reducing salt intake in the population. World Health Organization. Reducing salt intake in populations: report of a WHO forum and technical meeting. WHO Geneva 2007;1-60.

Chen J, Gu D., Jaquish C.E., Chen C., Rao D.C., Liu D., Hixson J.E., Lee Hamm L., Gu C.C., Whelton P.K. and He J. for the GenSalt Collaborative Research Group. Association Between Blood Pressure Responses to the Cold Pressor Test and Dietary Sodium Intervention in a Chinese Population. Arch Intern Med. 2008;168:1740-1746. doi:10.1001/archinte.168.16.1740

Denton D.,  Weisinger R., Mundy N.I., Wickings E.J., Dixson A., Moisson P., Pingard A.M., Shade R., Carey D., Ardaillou R., Paillard F., Chapman J., Thillet J. & Michel J.B. The effect of increased salt intake on blood pressure of chimpanzees. Nature Med 1995;10:1009-1016. doi:10.1038/nm1095-1009

Ford E.S., Ajani U.A., Croft J.B., Critchley J.A., Labarthe D.R., Kottke T.E., Giles W.H, and Capewell S. Explaining the decrease in U.S. deaths from coronary disease, 1980-2000. N Engl J Med 2007;356:2388-2398. doi:10.1056/NEJMsa053935

Geleijnse J.M., Witteman J.C., den Breeijen J.H., Hofman A., de Jong P., Pols H.A. and Grobbee D.E. Dietary electrolyte intake and blood pressure in older subjects: the Rotterdam Study. J Hyperten 1996;14:73741.

Harlan W.R. and Harlan L.C. Blood pressure and calcium and magnesium intake. In: Laragh J.H., Brenner B.M., eds. Hypertension: pathophysiology, diagnosis and management. 2end ed. New York: Raven Press 1995;1143-1154

Holmes E., Loo R.L., Stamler J., Bictash M., Yap I.K.S., Chan Q., Ebbels T., De Iorio M., Brown I.J., Veselkov K.A., Daviglus M.L., Kesteloot H., Ueshima H., Zhao L., Nicholson J.K. and Elliott P. Human metabolic phenotype diversity and its association with diet and blood pressure. Nature 2008;453:396-400. doi:10.1038/nature06882

Pickering T.G. New guidelines on diet and blood pressure. Hypertension 2006;47:135-136. doi:10.1161/01.HYP.0000202417.57909.26

Simpson F.O. Blood pressure and sodium intake. In: Laragh J.H., Brenner B.M. eds. Hypertension: pathophysiology, diagnosis and management. 2end ed. New York: Raven Press 1995;273-281

Strazzullo P., D’Elia L., Kandala N. and Cappuccio F.P. Salt intake, stroke, and cardiovascular disease: meta-analysis of prospective studies. BMJ 2009;339:b4567. doi:10.1136/bmj.b4567

Tzoulaki I., Brown I.J., Chan Q., Van Horn L., Ueshima H., Zhao L., Stamler J., Elliott P., for the International Collaborative Research Group on Macro-/Micronutrients and Blood Pressure. Relation of iron and red meat intake to blood pressure: cross sectional epidemiological study. BMJ 2008;337:a258. doi:10.1136/bmj.a258

Weinberger M.H. The effects of sodium on blood pressure in humans. In: Laragh JH, Brenner BM, eds. Hypertension: pathophysiology, diagnosis and management. 2end ed. New York: Raven Press 1995;2703-2714.

Writing Group of the PREMIER Collaborative Research Group. Effects of comprehensive lifestyle modification on blood pressure control: main results of the PREMIER Clinical Trial. JAMA 2003;289:2083-2093. doi:10.1001/jama.289.16.2083

World Health Organization, International Society of Hypertension Writing Group. 2003 World Health Organization (WHO)/International Society of Hypertension (ISH) statement on management of hypertension. Guidelines and recommendations. J Hyperten 2003;21:1983-1992.

Foods high in anthocyanins, their absorption and metabolism

Together with catechins and proanthocyanidins, anthocyanins and their oxidation products are the most abundant flavonoids in the human diet.
Examples of anthocyanin rich foods are:

  • certain varieties of grains, such as some types of pigmented rice (e.g. black rice) and maize (purple corn);
  • in certain varieties of root and leafy vegetables such as aubergine, red cabbage, red onions and radishes, beans;
  • but especially in red fruits.

Example of anthocyanin rich food
Anthocyanins are also present in red wine; as the wine ages, they are transformed into various complex molecules.
Anthocyanin content in vegetables and fruits is generally proportional to their color: it increases during maturation, and it reaches values up to 4 g/kg fresh weight (FW) in cranberries and black currants.
These polyphenols are found primarily in the skin, except for some red fruits, such as cherries and red berries (e.g. strawberries), in which they are present both in the skin and flesh.
Glycosides of cyanidin are the most common anthocyanins in foods.

CONTENTS

Anthocyanin rich fruits

  • Berries are the main source of anthocyanins, with values ranging between 67 and 950 mg/100 g FW.
  • Other fruits, such as red grapes, cherries and plums, have content ranging between 2 and 150 mg/100 g FW.
  • Finally, in fruits such as nectarines, peaches, and some types of apples and pears, anthocyanins are poorly present, with a content of less than 10 mg/100 g FW.

Cranberries, besides their very high content of anthocyanins, are one of the rare food that contain glycosides of the six most commonly anthocyanidins present in foods: pelargonidin, delphinidin, cyanidin, petunidin, peonidin, and malvidin. The main anthocyanins are the 3-O-arabinosides and 3-O-galactosides of peonidin and cyanidin. A total of 13 anthocyanins have been detected, mainly 3-O-monoglycosides.

Anthocyanin absorption

Until recently, it was believed that anthocyanins, together with proanthocyanidins and gallic acid ester derivatives of catechins, were the least well-absorbed polyphenols, with a time of appearance in the plasma consistent with the absorption in the stomach and small intestine. Indeed, some studies have shown that their bioavailability has been underestimated since, probably, all of their metabolites have not been yet identified.
In this regard, it should be underlined that only a small part of the food anthocyanins is absorbed in their glycated forms or as hydrolysis products in which the sugar moiety has been removed. Therefore, a large amount of these ingested polyphenols enters the colon, where they can also suffer methylation, sulphatation, glucuronidation and oxidation reactions.

Anthocyanins and colonic microbiota

Few studies have examined the metabolism of anthocyanins by the gut microbiota in the colon.
Within two hours, it seems that all the anthocyanins lose their sugar moieties, thus producing anthocyanidins.
Anthocyanidins are chemically unstable in the neutral pH of the colon. They can be metabolized by colonic microbiota or chemically degraded producing a set of new molecules that have not yet fully identified, but which include phenolic acids such as gallic acid, syringic acid, protocatechuic acid, vanillic acid and phloroglucinol (1,3,5-trihydroxybenzene). These molecules, thanks to their higher microbial and chemical stability, might be the main responsible for the antioxidant activities and the other physiological effects that have been observed in vivo and attributed to anthocyanins.

References

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

de Pascual-Teresa S., Moreno D.A. and García-Viguera C. Flavanols and anthocyanins in cardiovascular health: a review of current evidence. Int J Mol Sci 2010;11:1679-1703. doi:10.3390/ijms11041679

Escribano-Bailòn M.T., Santos-Buelga C., Rivas-Gonzalo J.C. Anthocyanins in cereals. J Chromatogr A 2004:1054;129-141. doi:10.1016/j.chroma.2004.08.152

Han X., Shen T. and Lou H. Dietary polyphenols and their biological significance. Int J Mol Sci 2007;9:950-988. doi:10.3390/i8090950

Manach C., Scalbert A., Morand C., Rémésy C., and Jime´nez L. Polyphenols: food sources and bioavailability. Am J Clin Nutr 2004;79(5):727-747. doi:10.1093/ajcn/79.5.727

Tsao R. Chemistry and biochemistry of dietary polyphenols. Nutrients 2010;2:1231-1246. doi:10.3390/nu2121231

Tea polyphenols: preventive effects and mechanism of actions

The leaves of the tea plant, Camellia sinensis, are rich in compounds with many biological activities, ranging from preventing the development of chronic diseases to reducing the glycemic index of starchy foods.
More than 4000 different molecules have been found in the beverage, of which about one third of these are polyphenols, the most important phytochemicals in determining the nutritional values and health benefits of the tea.
Tea polyphenols are mostly flavonoids. Examples are catechins in green tea, among which epigallocatechin-3-gallate or EGCG is the most important and abundant, and thearubigins and theaflavins in black tea. Their galloyl groups at positions 3 and/or 3′ appear to be particularly important for their effects.

Skeletal formula of epigallocatechin gallate, a catechin, and one of the tea polyphenols
Epigallocatechin Gallate

Other bioactive compounds present in tea leaves are:

  • alkaloids, such as caffeine, theophylline and theobromine;
  • amino acids, and among them, theanine or R-glutamylethylamide, that is also a brain neurotransmitter and one of the most important amino acids in green tea;
  • proteins;
  • carbohydrates;
  • chlorophyll;
  • volatile organic molecules, that contribute to the aroma of the beverage;
  • fluoride, aluminum and trace elements.

CONTENTS

Biological activities of polyphenols

Polyphenols, both in vivo and in vitro, have a broad spectrum of biological activities, such as:

  • antioxidant and prooxidant properties;
  • a protective role against the development of diabetes, hyperlipidemia, and various types of tumors;
  • inhibition of inflammation;
  • antiviral activities;
  • anticariogenic activity.

Hence, there has been a growing interest in recent years toward the possible preventive effects of tea against many diseases, particularly cardiovascular disease, for example in the development and progression of atherosclerosis.

Mechanisms of action of tea polyphenols

Currently, knowledge is accumulating on the effects of tea polyphenols at cellular and molecular level.
It seems, at least in vitro, that catechins, and theaflavins and thearubigins are the compounds responsible for the physiological effects and health benefits of green tea and black tea, respectively.
Among the molecular mechanisms by which tea polyphenols seem to exert their effects, it has been observed, after binding to specific cell membrane receptors, a change in the activity of various protein kinases that then phosphorylate target proteins, such as transcription factors, that, in turn, translocate into the nucleus and modify the gene expression. This appears to be the mechanism of action of EGCG, and the mechanism proposed for thearubigins, polymeric polyphenols that, due to their large dimensions, may not be able to cross the plasma membrane.
In addition, some polyphenols could be able to cross the plasma membrane, then binding to specific cytoplasmic, mitochondrial or nuclear targets.
And, depending on the cell type and their amount, tea polyphenols can activate or inhibit certain cellular processes.

Tea polyphenols and starch digestion

Tea polyphenols exert an inhibitory effect on starch digestion.
In vitro studies have shown that green tea extracts, which contain monomeric polyphenols, have an equal inhibitory effect on starch digestibility of wheat bread and gluten-free bread, whereas black tea extracts, rich in tannins, namely, polymeric polyphenols, are less effective against wheat bread. Therefore, it seems that the inhibitory effect of tannins is negatively influenced by gluten, whereas gluten has a lower inhibitory effect on monomeric polyphenols.
The inhibitory effect of these phytochemicals has been attributed to various molecular mechanisms briefly described below.

  • A competitive inhibition on pancreatic alpha-amylase. The galloyl groups are thought  to be important for this effect.
  • The inhibition of other digestive enzymes present in the gastrointestinal tract.
  • The direct interaction with starch. Tea polyphenols can interact with starch granules through hydrogen bonds and hydrophobic forces, thus reducing the available surface to react with digestive enzymes.
  • Conversely, gluten could reduce the amount of polyphenols able to interact with starch and therefore able to inhibit its digestion.

Tea polyphenols could represent a means for controlling the glycemic index of starchy foods. However, it should be emphasized that, for example in the case of bread, to achieve an inhibitory effect, 100 g of bread must be co-digested with 2.5 cups of green tea or 2 cups of black tea.

References

Arab L., Khan F., and Lam H. Tea consumption and cardiovascular disease risk. Am J Clin Nutr 2013;98:1651S-1659S. doi:10.3945/ajcn.113.059345

Dwyer J.T. and Peterson J. Tea and flavonoids: where we are, where to go next. Am J Clin Nutr 2013;98:1611S-1618S. doi:10.3945/ajcn.113.059584

Grassi D., Desideri G., Di Giosia P., De Feo M., Fellini E., Cheli P., Ferri L., and Ferri C. Tea, flavonoids, and cardiovascular health: endothelial protection. Am J Clin Nutr 2013;98:1660S-1666S. doi:10.3945/ajcn.113.058313

Kan L., Capuano E., Fogliano V., Oliviero T. and Verkerk R. Tea polyphenols as a strategy to control starch digestion in bread: the effects of polyphenol type and gluten. Food Funct 2020;11:5933-5943. doi: 10.1039/D0FO01145B

Lambert J.D. Does tea prevent cancer? Evidence from laboratory and human intervention studies. Am J Clin Nutr 2013;98:1667S-1675S. doi:10.3945/ajcn.113.059352

Lenore Arab L., Khan F., and Lam H. Tea consumption and cardiovascular disease risk. Am J Clin Nutr 2013;98:1651S-1659S. doi:10.3945/ajcn.113.059345

Lorenz M. Cellular targets for the beneficial actions of tea polyphenols. Am J Clin Nutr 2013;98:1642S-1650S. doi:10.3945/ajcn.113.058230

Mathur A., Gopalakrishnan D., Mehta V., Rizwan S.A., Shetiya S.H., Bagwe S. Efficacy of green tea-based mouthwashes on dental plaque and gingival inflammation: a systematic review and meta-analysis. Indian J Dent Res 2018;29(2):225-232. doi:10.4103/ijdr.IJDR_493_17

Mhatre S., Srivastava T., Naik S., Patravale V. Antiviral activity of green tea and black tea polyphenols in prophylaxis and treatment of COVID-19: a review. Phytomedicine 2020;153286. doi:10.1016/j.phymed.2020.153286

Sharma V.K., Bhattacharya A., Kumar A. and Sharma H.K. Health benefits of tea consumption. Trop J Pharm Res 2007;6(3):785-792.

Yuan J-M. Cancer prevention by green tea: evidence from epidemiologic studies. Am J Clin Nutr 2013;98:1676S-1681S. doi:10.3945/ajcn.113.058271

Xu J., Xu Z., Zheng W. A review of the antiviral role of green tea catechins. Molecules 2017;22(8):1337. doi:10.3390/molecules22081337

Isoflavones: chemical structure, foods and health effects

Isoflavones are colorless polyphenols belonging to the flavonoid family.
Unlike the majority of the other flavonoids, they have a restricted taxonomic distribution, being present almost exclusively in the Leguminosae or Fabaceae plant family, mainly in soy.
Since legumes, soy in primis, are a major part of the diet in many cultures, these flavonoids may have a great impact on human health.
They are also present in beans and broad beans, but in much lower concentrations than those found in soy and soy products.
Also red clover or meadow clover (Trifolium pratense), another member of Leguminosae family, is a good source.
Currently, they are not found in fruits and vegetables.

Together with phenolic acids, such as caffeic acid and gallic acid, and quercetin glycosides, they are the most well-absorbed polyphenols, followed by flavanones and catechins (but not gallocatechins).

In plants, some isoflavones have antimicrobial activity and are synthesized in response to attacks by bacteria or fungi; thus they act as phytoalexins.

CONTENTS

Chemical structure of isoflavones

While most flavonoids have B ring attached to position 2 of C ring, isoflavones have B ring attached to position 3 of C ring.

Basic skeleton structure of isoflavones, polyphenols belonging to the flavonoid family
Basic Skeleton of Isoflavones

Even if they are not steroids, they have structural similarities to estrogens, particularly estradiol. This confers them pseudohormonal properties, such as the ability to bind estrogen receptors; therefore, they are classified as phytoestrogens or plant estrogens. The benefits often ascribed to soy and soy products (e.g. tofu) are believed to result from the ability of isoflavones to act as estrogen mimics .
It should be underlined that the binding to estrogen receptors seems to lose strength with time, therefore their potential efficacy should not be overestimated.
In foods, they are present in four forms:

  • aglycone;
  • 7-O-glucoside;
  • 6′-O-acetyl-7-O-glucoside;
  • 6′-O-malonyl-7-O-glucoside.

Soy isoflavones: genistein, daidzein and glycitein

Soy and soy products, such as soy milk, tofu, tempeh and miso, are the main source of isoflavones in the human diet.
The isoflavone content of soy and soy products varies greatly as a function of growing conditions, geographic zone, and processing; for example, in soy it ranges between 580 and 3800mg/kg fresh weight, while in soy milk it range between 30 and 175 mg/L. The most abundant isoflavones in soy and soy products are genistein, daidzein and glycitein, generally present in a concentration ratio of 1:1:0,2.; biochanin A and formononetin are other isoflavones present in less concentrations.

Basic skeleton structure of isoflavones genistein, daidzein, glycitein, biochanin A, formononetin
Isoflavones

The 6′-O-malonyl derivatives have a bitter, unpleasant, and astringent taste; therefore they give a bad flavor to the food in which they are contained. However, being sensitive to temperature, they are often hydrolyzed to glycosides during processing, such as the production of soy milk.
The fermentation processes needed for the preparation of certain foods, such as tempeh and miso, determines in turn the hydrolysis of glycosides to aglycones, i.e. the sugar-free molecule.
Isoflavone glycosides present in soy and soy products can also be deglycosylated by β-glucosidases in the small intestine.
The aglycones are very resistant to heat.
Although many compounds present in the diet are converted by intestinal bacteria to less active molecules, other compounds are converted to molecules with increased biological activity. This is the case of isoflavones, but also of prenylflavonoids from hops (Humulus lupulus), and lignans, that are other phytoestrogens.

Phytoestrogens and menopause

Vasomotor symptoms, such as night sweats and hot flashes, and bone loss are very common in perimenopause, also called menopausal transition, and menopause. Hormone replacement therapy (HRT) has proved to be a highly effective treatment for the prevention of menopausal bone loss and vasomotor symptoms.
The use of alternative therapies based on phytoestrogens is increased as a result of the publication of the “Women’s Health Initiative” study, that suggests that hormone replacement therapy could lead to more risks than benefits, in particular an increased risk of developing some chronic diseases.
Soy isoflavones are among the most used phytoestrogens by menopausal women, often taken in the form of isoflavone fortified foods or isoflavone supplements.
However, many studies have highlighted the lack of efficacy of soy isoflavones, and red clover isoflavones, even in large doses, in the prevention of vasomotor symptoms (hot flushes and night sweats) and bone loss during menopause.

References

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

Han X., Shen T. and Lou H. Dietary polyphenols and their biological significance. Int J Mol Sci 2007;9:950-988. doi:10.3390/i8090950

Lagari V.S., Levis S. Phytoestrogens for menopausal bone loss and climacteric symptoms. J Steroid Biochem Mol Biol 2014;139:294-301 doi:10.1016/j.jsbmb.2012.12.002

Lethaby A., Marjoribanks J., Kronenberg F., Roberts H., Eden J., Brown J. Phytoestrogens for menopausal vasomotor symptom. Cochrane Database of Systematic Reviews 2013, Issue 12. Art. No.: CD001395. doi:10.1002/14651858.CD001395.pub4

Levis S., Strickman-Stein N., Ganjei-Azar P., Xu P., Doerge D.R., Krischer J. Soy isoflavones in the prevention of menopausal bone loss and menopausal symptoms: a randomized, double-blind trial. Arch Intern Med 2011:8;171(15):1363-1369 doi:10.1001/archinternmed.2011.330

Manach C., Scalbert A., Morand C., Rémésy C., and Jime´nez L. Polyphenols: food sources and bioavailability. Am J Clin Nutr 2004;79(5):727-747. doi:10.1093/ajcn/79.5.727

Tsao R. Chemistry and biochemistry of dietary polyphenols. Nutrients 2010;2:1231-1246. doi:10.3390/nu2121231

Proanthocyanidins and procyanidins: food sources

The interest on proanthocyanidins, and their content in foods, has increased as a result of the discovery, due to clinical and laboratory studies, of their anti-infectious, anti-inflammatory, cardioprotective and anticarcinogenic properties. These protective effects have been attributed to their ability to:

  • act as free radical scavenger;
  • inhibit lipid peroxidation;
  • act on various protein targets within the cell, modulating their activity.

Proanthocyanidins in different foods vary greatly in terms of:

  • total content;
  • distribution of oligomers and polymers;
  • constituent catechin units and bonds between units.

In some foods, such as black beans and cashew nuts, only dimers are present, called A-type procyanidins and B-type procyanidins, whereas in most of the foods proanthocyanidins are found in a wide range of polymerizations, from 2 to 10 units or more.

Foods with the highest proanthocyanidin content are cinnamon and sorghum, which contain respectively about 8,000 and up to 4,000 mg/100 g of fresh weight (FW); grape seeds (Vitis vinifera) are another rich source, with a content of about 3,500 mg/100 g dry weight.
Other important sources are fruits and berries, some legumes (peas and beans), red wine and to a less extent beer, hazelnuts, pistachios, almonds, walnuts and cocoa.
The coffee is not a good source.
Proanthocyanidins are not detectable in the majority of vegetables; they have been found in small concentrations in Indian pumpkin. They are not detectable also in maize, rice and wheat, while there are present in barley.

CONTENTS

A-type procyanidins in foods

Although many food plants contain high amounts of proanthocyanidins, only a few, such as plums, avocados, peanuts or cinnamon, contain A-type procyanidins, and none in amounts equal to cranberries (Vacciniun macrocarpon).

Procyanidins: skeletal formula of procyanidin A2

Note: A-type procyanidins exhibit, in vitro, a capacity of inhibition of P-fimbriated Escherichia coli adhesion to uroepithelial cells greater than B-type procyanidins (adhesion represents the initial step of urogenital infections).

B-type procyanidins in foods

B-type procyanidins, consisting of catechin and/or epicatechin as constituent units, are the exclusive proanthocyanidins in at least 20 kinds of foods including blueberries (Vaccinium myrtillus), blackberries, marion berries, choke berries, grape seeds, apples, peaches, pears, nectarines, kiwi, mango, dates, bananas, Indian pumpkin, sorghum, barley, black eye peas, beans blacks, walnuts and cashews.

Proanthocyanidins in fruits

In the Western diet, fruit is the most important source of proanthocyanidins.

  • The major sources are some berries (blueberries, cranberries, and black currant) and plums (prunes), with a content of about 200 mg/100 g FW.
  • Intermediate sources are apples, chokeberries, strawberries, and green and red grapes (60-90 mg/100 g FW).
  • In other fruits the content is less than 40 mg/100 g FW.

In fruit, the most common proanthocyanidins are tetramers, hexamers, and polymers.
Good sources of proanthocyanidins are also some fruit juices.

Proanthocyanidins in grape seeds

A particularly rich source of proanthocyanidins is the seeds of grape.
Proanthocyanidins in grape seeds are only B-type procyanidins, for the most part present in the form of dimers, trimers and highly polymerized oligomers.
Grape seed proanthocyanidins are potent antioxidants and free radical scavenger, being the more effective either than vitamin E and vitamin C (ascorbic acid).
In vivo and in vitro experiments support the idea that proanthocyanidins, and in particular those from grape seeds, can act as anti-carcinogenic agents; it seems that they are involved, in cancer cells, in:

  • reduction of cell proliferation;
  • increase of apoptosis;
  • cell cycle arrest;
  • modulation of the expression and activity of NF-kB and NF-kB target genes.

References

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

Gu L., Kelm M.A., Hammerstone J.F., Beecher G., Holden J., Haytowitz D., Gebhardt S., and Prior R.L. Concentrations of proanthocyanidins in common foods and estimations of normal consumption. J Nutr 2004;134(3):613-617. doi:10.1093/jn/134.3.613

Han X., Shen T. and Lou H. Dietary polyphenols and their biological significance. Int J Mol Sci 2007;9:950-988. doi:10.3390/i8090950

Manach C., Scalbert A., Morand C., Rémésy C., and Jime´nez L. Polyphenols: food sources and bioavailability. Am J Clin Nutr 2004;79(5):727-747. doi:10.1093/ajcn/79.5.727

Nandakumar V., Singh T., and Katiyar S.K. Multi-targeted prevention and therapy of cancer by proanthocyanidins. Cancer Lett 2008;269(2):378-387. doi:10.1016/j.canlet.2008.03.049

Ottaviani J.I., Kwik-Uribe C., Keen C.L., and Schroeter H. Intake of dietary procyanidins does not contribute to the pool of circulating flavanols in humans. Am J Clin Nutr 2012;95:851-858. doi:10.3945/ajcn.111.028340

Santos-Buelga C. and Scalbert A. Proanthocyanidins and tannin-like compounds: nature, occurrence, dietary intake and effects on nutrition and health. J Sci Food Agr 2000;80(7):1094-1117. doi:10.1002/(SICI)1097-0010(20000515)80:7<1094::AID-JSFA569>3.0.CO;2-1

Tsao R. Chemistry and biochemistry of dietary polyphenols. Nutrients 2010;2:1231-1246. doi:10.3390/nu2121231

Wang Y.,Chung S., Song W.O., and Chun O.K. Estimation of daily proanthocyanidin intake and major food sources in the U.S. diet. J Nutr 2011;141(3):447-452. doi:10.3945/jn.110.133900

Flavonols: definition, structure, food sources

Flavonols are polyphenols belonging to the flavonoid family.
They are colorless molecules that accumulate mainly in the outer and aerial tissues, therefore skin and leaves, of fruit and vegetables, since their biosynthesis is stimulated by light. They are virtually absent in the flesh.
They are the most common flavonoids in fruit and vegetables, where they are generally present in relatively low concentrations.
Due to their widespread in nature and human diet, they should be taken into consideration when the positive effect on health associated with fruit and vegetable consumption is examined. Their effect is probably related to their ability to:

  • act as antioxidants;
  • act as anti-inflammatory agents;
  • act as anticancer factors;
  • regulate different cellular signaling pathways; an example is the action of quercetin, the most widespread flavonols, on the oxidative stress-induced MAPK activities.

CONTENTS

Chemical structure of flavonols

Chemically, these molecules differ from many other flavonoids since they have a double bond between positions 2 and 3 and an oxygen (a ketone group) in position 4 of the C ring, like flavones from which, however, they differ in the presence of a hydroxyl group at the position 3. Therefore, flavonol skeleton is a 3-hydroxyflavone.

3-Hydroxyflavone, the basic skeleton structure of flavonols, polyphenols belonging to the flavonoid family
3-Hydroxyflavone

The 3-hydroxyl group can link a sugar, that is, it can be glycosylated.
Like many other flavonoids, most of them is found in fruit and vegetables, and in plant-derived foods, in glycosylated form. The sugar associated with flavonols is often glucose or rhamnose, but other sugars may also be involved, such as:

  • galactose;
  • arabinose;
  • xylose;
  • glucuronic acid.

Flavonols are mainly represented by glycosides of:

  • quercetin;
  • kaempferol;
  • myricetin;
  • isorhamnetin.
Skeletal formulas of flavonols quercetin, kaempferol, myricetin, isorhamnetin
Flavonols

The most ubiquitous compounds are glycosylated derivatives of quercetin and kaempferol; in nature, these two molecules have respectively about 280 and 350 different glycosidic combinations.
Finally, it should be underlined that sugar moiety influences flavonol bioavailability.

Foods rich in flavonols

The major sources in human diet are:

  • fruit;
  • vegetables;
  • beverages such as red wine and tea.

In human diet, the richest source are capers, which contain up to 490 mg/100 g fresh weight (FW), but they are also abundant in onions, leeks, broccoli, curly kale, berries (e.g. blueberries), grapes and some herbs and spices, for example dill weed (Anethum graveolens). In these sources, their content ranges between 10 and 100 mg/100 g FW.
Even cocoa, green teablack tea, and red wine are good sources of flavonols. In wine, together with other polyphenols such as catechins, proanthocyanidins and low molecular weight polyphenols, they contribute to the astringency of the beverage.

Main flavonols in foods

The main flavonols in foods, listed in decreasing order of abundance, are quercetin, kaempferol, myricetin and ishoramnetin.

Quercetin

The richest sources of quercetin are capers, followed by onions, asparagus, lettuce and berries; in many other fruit and vegetables, it is present in smaller amounts, between 0.1 and 5 mg/100 g FW.
This flavonol is also present in cocoa and it could be one of its main protective agents against LDL oxidation.
Together with isoflavones, quercetin glycosides are the most well-absorbed polyphenols, followed by flavanones and catechins (on the contrary, gallic acid derivatives of catechins are among the least well absorbed polyphenols, together with anthocyanins and proanthocyanidins).

Kaempferol

Typical dietary sources of kaempferol include vegetables, such as spinach, kale and endive, with concentrations between 0.1 and 27 mg/100 g FW, and some spices such as chives, fennel and tarragon, with concentrations between 6.5 and 19 mg/100 g FW.
Fruit is a poor source of the molecule, with content down to 0.1 mg/100 g FW.

Myricetin

Myricetin is the third most abundant flavonol. It is found in some spices, such as oregano, parsley, and fennel, with concentrations between 2 and 20 mg/100 g FW, but also in tea, 0.5-1.6 mg/100 ml, and red wine, 0-9.7 mg/100 ml.
In fruit, it is only found in high concentrations in berries, while in most other fruit and vegetables it is present in a content of less than 0.2 mg/100 g FW.

Isorhamnetin

A fourth flavonol, less abundant than the previous ones, is isorhamnetin. It is only present in some foods such as some spices: chives, 5.0-8.5 mg/100 g FW, fennel, 9.3 mg/100 g FW, tarragon, 5 mg/100 g FW.
In fruit and vegetables it is only present in almonds, with a concentration between 1.2 and 10.3 mg/100 g FW, pears and onions.

References

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

Han X., Shen T. and Lou H. Dietary polyphenols and their biological significance. Int J Mol Sci 2007;9:950-988. doi:10.3390/i8090950

Manach C., Scalbert A., Morand C., Rémésy C., and Jime´nez L. Polyphenols: food sources and bioavailability. Am J Clin Nutr 2004;79(5):727-747. doi:10.1093/ajcn/79.5.727

Tsao R. Chemistry and biochemistry of dietary polyphenols. Nutrients 2010;2:1231-1246. doi:10.3390/nu2121231

Anthocyanins: definition, foods, and health benefits

Anthocyanins are a subgroup of flavonoids, therefore they are polyphenols, which give plants their distinctive colors.
They are water soluble pigments and are present in the vacuolar sap of the epidermal tissues of flowers and fruit.
They are responsible for the colors of the most of the petals, fruits and vegetables, and of some varieties of cereals such as black rice. In fact, they impart red, pink and purple to blue colors to berries, red apples, red grapes, cherries, and of many other fruits, red lettuce, red cabbage, onions or eggplant, but also red wine.
Together with carotenoids, they are responsible for autumn leaf color.
Finally, anthocyanins contribute to attract animals when a fruit is ready to eat or a flower is ready for pollination.

They are bioactive compounds found in plant foods that have a double interest for man:

  • the first one, a technological interest, due to their effects on the organoleptic characteristics of food products;
  • the other due to their healthy properties, being implicated in the protection against cardiovascular risk.
    In fact:

in vitro, they have an antioxidant activity, due to their ability to delocalize electrons and form resonance structures, and a protective role against oxidation of low density lipoproteins (LDL);

like other polyphenols, such as catechins, proanthocyanidins and other uncolored flavonoids, they can regulate different signaling pathways involved in cell growth, differentiation and survival.

CONTENTS

Chemical structure of anthocyanins

The basic chemical structure is flavylium cation (2-phenyl-1-benzopyrilium), which links hydroxyl (-OH) and/or methoxyl (-OCH3) groups, and one or more sugars.
The sugar-free molecule is called anthocyanidins.

Skeletal formula of the basic skeleton of anthocyanins: the flavylium cation or 2-phenyl-1-benzopyrilium
Flavylium Cation

Depending on the number and position of hydroxyl and methoxyl groups, various anthocyanidins have been described, and of these, six are commonly found in vegetables and fruits:

  • pelargonidin
  • cyaniding
  • delphinidin
  • petunidin
  • peonidin
  • malvidin
Skeletal formulas of different types of anthocyanins
Antocyanins

Anthocyanins, as most of the other flavonoids, are present in plants and plant foods in the form of glycosides, that is, linked to one or more sugar units.
The most common carbohydrates present in these natural pigments are:

The sugars are linked mainly to the C3 position as 3-monoglycosides, to the C3 and C5 positions as diglycosides (with the possible forms: 3-diglycosides, 3,5-diglycosides, and 3-diglycoside-5-monoglycosides). Glycosylations have been also found at C7, C3′ and C5′ positions.
The structure of these molecules is further complicated by the bond to the sugar unit of different acyl substituents such as:

  • aliphatic acids, such as acetic, malic, succinic and malonic acid;
  • cinnamic acids (aromatic substituents), such as sinapic, ferulic and p-coumaric acid;
  • finally, there are pigments with both aromatic and aliphatic substituents.

Furthermore, some anthocyanins have several acylated sugars in the molecule; these anthocyanins are sometimes called polyglycosides.

Depending on the type of hydroxylation, methoxylation and glycosylation patterns, and the different substituents linked to the sugar units, more than 500 different anthocyanins have been identified that are based on 31 anthocyanidins. Among these 31 monomers:

  • 30% are from cyanidin;
  • 22% are from delphinidin;
  • 18% are from pelargonidin.

Methylated derivatives of cyanidin, delphinidin and pelargonidin, namely peonidin, malvidin, and petunidin, all together represent 20% of the anthocyanins.
Therefore, up to 90% of the most frequently encountered anthocyanins are related to delphinidin, pelargonidin, cyanidin, and their methylated derivatives.

Role of pH

The color of these molecules is influenced by the pH of the vacuole where they are stored, ranging in color from:

  • red, under very acidic conditions;
  • to purple-blue, in intermediate pH conditions;
  • until yellow-green, in alkaline conditions.

In addition to the pH, the color of these flavonoids can be affected by the degree of hydroxylation or methylation pattern of the A and B rings, and by glycosylation pattern.
Finally, the color of certain plant pigments result from complexes between anthocyanins, flavones and metal ions.
It should be noted that anthocyanins are often used as pH indicators thanks to the differences in chemical structure that occur in response to changes in pH.

References

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

de Pascual-Teresa S., Moreno D.A. and García-Viguera C. Flavanols and anthocyanins in cardiovascular health: a review of current evidence. Int J Mol Sci 2010;11:1679-1703. doi:10.3390/ijms11041679

Escribano-Bailòn M.T., Santos-Buelga C., Rivas-Gonzalo J.C. Anthocyanins in cereals. J Chromatogr A 2004:1054;129-141. doi:10.1016/j.chroma.2004.08.152

Han X., Shen T. and Lou H. Dietary polyphenols and their biological significance. Int J Mol Sci 2007;9:950-988. doi:10.3390/i8090950

Manach C., Scalbert A., Morand C., Rémésy C., and Jime´nez L. Polyphenols: food sources and bioavailability. Am J Clin Nutr 2004;79(5):727-747. doi:10.1093/ajcn/79.5.727

Ottaviani J.I., Kwik-Uribe C., Keen C.L., and Schroeter H. Intake of dietary procyanidins does not contribute to the pool of circulating flavanols in humans. Am J Clin Nutr 2012;95:851-858. doi:10.3945/​ajcn.111.028340

Tsao R. Chemistry and biochemistry of dietary polyphenols. Nutrients 2010;2:1231-1246. doi:10.3390/nu2121231

Proanthocyanidins: structure and intestinal absorption

Proanthocyanidins or condensed tannins, also called pycnogenols and leukocyanidins, are polyphenolic compounds (in particular they are a flavonoid subgroup) widely distributed in the plant kingdom, second only to lignin as the most abundant phenol in nature.
They are present in high concentrations in various parts of the plants such as flowers, fruits, berries, seeds (e.g. in grape seeds), and bark (e.g. pine bark).

Together with anthocyanins and their oxidation products, and catechins, they are the most abundant flavonoids in human diet and it has been suggested that they constitute a significant fraction of the polyphenols ingested in the Western diet.
Therefore, condensed tannins should be taken into consideration when the epidemiological association between the intake of polyphenols, especially flavonoids, and chronic diseases are examined.

CONTENTS

Chemical structure of proanthocyanidins

Condensed tannins have a complex chemical structure being oligomers (dimers to pentamers) or polymers (six or more units, up to 60) of catechins or flavanols, which are joined by carbon-carbon bonds.

Basic skeleton structure of procyanidins
Basic Skeleton of Procyanidins

They may consist exclusively of:

  • (epi)catechin, and they are named procyanidins;
  • (epi)afzelechin, and they are named propelargonidins;
  • (epi)gallocatechin, and they are named prodelphinidins.

Propelargonidins and prodelphinidins are less common in nature and in foods than procyanidins.

Depending on the bonds between monomers, proanthocyanidins have a:

  • B-type structure, if the polymerization occurs via carbon-carbon bond between the position 8 of the terminal unit and the 4 of the extender (or C4-C6);
  • A-type structure, less frequent, if monomers are doubly linked via an ether bond C2-O-C7 or C2-O-C5 plus a B-type bond.

Procyanidins

The most common dimers are B-type procyanidins, B1 to B8, formed by catechin or epicatechin; in B1, B2, B3 and B-4 dimers, the two flavanol units are joined by a C4-C8 bond; in B5, B6, B7 and B8 dimers the two units are joined by C4-C6 bond.

Skeletal formulas of procyanidin B1, B2, B3, and B4
Procyanidins B1, B2, B3, and B4

Procyanidin C1 is a B-type trimer.

Procyanidin A-2 is an example of A-type procyanidin.

Intestinal absorption of proanthocyanidins

Condensed tannins are poorly absorbed from the intestine; together with anthocyanins and gallic acid ester derivatives of tea catechins, they are the least well-absorbed polyphenols.
It seems that low molecular weight oligomers (2-3 monomers) may be absorbed as such while polymers are not.
In the systemic circulation, dimers reach concentrations of two orders of magnitude lower than those of catechins.
It seems that condensed tannins with a degree of polymerization greater than three transit into the stomach and small intestine without significant modifications, and then, into the large intestine, they are catabolized by colonic microflora, with production of phenylpropionic, phenilvaleric and phenylacetic acids. These degradation products have been suggested to be the major metabolites of proanthocyanidins in healthy humans.

Procyanidins and catechins

It had been proposed that the catabolism of procyanidins in the gastrointestinal tract lead to the release of monomeric catechins, thus indirectly contributing to their systemic pool in humans. In recent years, it has been shown that this does not happen because procyanidins do not significantly contribute to:

  • the concentration of catechin metabolites in the systemic circulation;
  • the total catechin metabolites excreted in the urine;
  • finally, they do not significantly affect plasma metabolite profile derived from catechol-O-methyltransferase activity.

Therefore, analyzing the potential health benefits associated with the intake of foods containing these phytochemicals, catechins and procyanidins should be considered distinct classes of related compounds.

References

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

Gu L., Kelm M.A., Hammerstone J.F., Beecher G., Holden J., Haytowitz D., Gebhardt S., and Prior R.L. Concentrations of proanthocyanidins in common foods and estimations of normal consumption. J Nutr 2004;134(3):613-617. doi:10.1093/jn/134.3.613

Han X., Shen T. and Lou H. Dietary polyphenols and their biological significance. Int J Mol Sci 2007;9:950-988. doi:10.3390/i8090950

Manach C., Scalbert A., Morand C., Rémésy C., and Jime´nez L. Polyphenols: food sources and bioavailability. Am J Clin Nutr 2004;79(5):727-747. doi:10.1093/ajcn/79.5.727

Nandakumar V., Singh T., and Katiyar S.K. Multi-targeted prevention and therapy of cancer by proanthocyanidins. Cancer Lett 2008;269(2):378-387. doi:10.1016/j.canlet.2008.03.049

Ottaviani J.I., Kwik-Uribe C., Keen C.L., and Schroeter H. Intake of dietary procyanidins does not contribute to the pool of circulating flavanols in humans. Am J Clin Nutr 2012;95:851-858. doi:10.3945/ajcn.111.028340

Santos-Buelga C. and Scalbert A. Proanthocyanidins and tannin-like compounds: nature, occurrence, dietary intake and effects on nutrition and health. J Sci Food Agr 2000;80(7):1094-1117. doi:10.1002/(SICI)1097-0010(20000515)80:7<1094::AID-JSFA569>3.0.CO;2-1

Tsao R. Chemistry and biochemistry of dietary polyphenols. Nutrients 2010;2:1231-1246. doi:10.3390/nu2121231

Wang Y.,Chung S., Song W.O., and Chun O.K. Estimation of daily proanthocyanidin intake and major food sources in the U.S. diet. J Nutr 2011;141(3):447-452. doi:10.3945/jn.110.133900

Catechins: structure and food sources

Catechins or flavanols, with flavonols such as quercetin, and flavones such as luteolin, are a subgroup of flavonoids among the most widespread in nature.
Flavanols and proanthocyanidins, together with anthocyanins and their oxidation products, are the most abundant flavonoids in human diet.

CONTENTS

Chemical structure of catechins

Chemically they differ from many other flavonoids as:

  • they lack the double bond between positions 2 and 3 of the C ring;
  • they not have a keto group at position 4;
  • they have a hydroxyl group in position 3, and for this reason they are also called flavan-3-ols.
Basic skeleton structure of catechins, among the most abundant flavonoids in human diet
Basic Skeleton of Catechins

Another distinctive feature of flavan-3-ols is their ability to form oligomers (two to ten units) or polymers (eleven or more units, up to 60 units) called proanthocyanidins or condensed tannins.

Foods high in catechins

Flavanols commonly found in plant-derived food products are catechin, epicatechin, gallocatechin, epigallocatechin, and their gallic acid ester derivatives: catechin gallate, gallocatechin gallate, epicatechin gallate, and epigallocatechin gallate or EGCG.

Skeletal formulas of catechin, epicatechin, gallocatechin, epigallocatechin
Catechins

Flavanols present with higher frequency are catechin and epicatechin, which are also among the most common known flavonoids, and almost as popular as the related flavonol quercetin.
Cocoa and green tea are by far the richest sources in flavanols. In these foods the main flavonoids are catechin and epicatechin (cocoa is also a good source of epigallocatechin), but also their gallic acid ester derivatives, the gallocatechins.

Structural formulas of gallic acid ester derivatives of catechins
Gallic Acid Ester Derivatives of Catechins

However, they are also present in many fruits, especially in the skins of apples, blueberries (Vaccinium myrtillus) and grapes, in vegetables, red wine and beer, and peanuts.
As in many cases flavanols are present in the seeds or peels of fruits and vegetables, their intake may be limited by the fact that these parts are discarded during processing or while eaten.
Furthermore, in contrast to other flavonoids, catechins are not glycosylated in foods.
Proanthocyanidins, that is polymeric flavan-3-ols, are also commonly found in plant-derived food products. Their presence has been reported in the skin of peanuts and almonds, as in the berries.

Green and black tea

Green tea is an excellent source of flavonoids. The main flavonoids present in the leaves of the tea (as in cocoa beans) are catechin and epicatechin, monomeric flavanols, together with their gallate derivatives such as EGCG.
Epigallocatechin gallate is the most abundant catechin in green tea and it seems to have an important role in determining green tea benefits, as the reduction of:

  • vascular inflammation;
  • blood pressure;
  • concentration of oxidized LDL.

Black tea (fermented tea) contains fewer monomeric flavanols, as they are oxidized during fermentation of the leaves to more complex polyphenols such as theaflavins (theaflavin digallate, theaflavin-3-gallate, and theaflavin-3′-gallate, all dimers) and thearubigins (polymers).
Theaflavins and thearubigins are present only in the tea; their concentrations in brewed tea are between 50- and 100-folds lesser than in tea leaves.

It should be noted that tea epicatechins are remarkably stable to heat in acidic environment: at pH 5, only about 15% is degraded after seven hours in boiling water (therefore, adding lemon juice to brewed tea does not cause any reduction in their content).

Cocoa and cocoa products

Cocoa has the highest content of polyphenols and flavanols per serving, a concentration greater than those found in green tea and red wine. Most of the flavonoids present in cocoa beans and derived products, such as black chocolate, are catechin and epicatechin, monomeric flavanols, but also epigallocatechin, and their derivatives such as the gallocatechins; among polymers, proanthocyanidins are also important.

Fruits, vegetables, and legumes

Catechin and epicatechin are the main flavanols in fruits. They are found in many fruits in different concentrations, respectively, between 5-3 and 0.5-6 mg/100 g fresh weight.
On the contrary, gallocatechin, epicatechin gallate, epigallocatechin, and epigallocatechin gallate are present in various fruits such as red grapes, berries, apples, peaches and plums, but in very low concentrations, less than 1mg/100 g fresh weight.
Except for lentils and broad beans, few legumes and vegetables contain catechins, and in very low concentrations, less than 1.5 mg/100 g fresh weight.

References

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

de Pascual-Teresa S., Moreno D.A. and García-Viguera C. Flavanols and anthocyanins in cardiovascular health: a review of current evidence. Int J Mol Sci 2010;11:1679-1703. doi:10.3390/ijms11041679

Han X., Shen T. and Lou H. Dietary polyphenols and their biological significance. Int J Mol Sci 2007;9:950-988. doi:10.3390/i8090950

Manach C., Scalbert A., Morand C., Rémésy C., and Jime´nez L. Polyphenols: food sources and bioavailability. Am J Clin Nutr 2004;79(5):727-747. doi:10.1093/ajcn/79.5.727

Tsao R. Chemistry and biochemistry of dietary polyphenols. Nutrients 2010;2:1231-1246. doi:10.3390/nu2121231

Flavonoids: chimical structure, classification, and examples

Flavonoids are the most abundant polyphenols in human diet, representing about 2/3 of all those ones ingested. Like other phytochemicals, they are the products of secondary metabolism of plants and, currently, it is not possible to determine precisely their number, even if over 4000 have been identified.
In fruits and vegetables, they are usually found in the form of glycosides and sometimes as acylglycosides, while acylated, methylated and sulfate molecules are less frequent and in lower concentrations.
They are water-soluble and accumulate in cell vacuoles.

CONTENTS

Chemical structure of flavonoids

Their basic structure is a skeleton of diphenylpropane, namely, two benzene rings (ring A and B, see figure) linked by a three carbon chain that forms a closed pyran ring (heterocyclic ring containing oxygen, the C ring) with benzenic A ring. Therefore, their structure is also referred to as C6-C3-C6.

Basic skeleton structure of flavonoids, the most abundant polyphenols in human diet
Basic Skeleton of Flavonoids

In most cases, B ring is attached to position 2 of C ring, but it can also bind in position 3 or 4; this, together with the structural features of the ring B and the patterns of glycosylation and hydroxylation of the three rings, makes the flavonoids one of the larger and more diversified groups of phytochemicals, so not only of polyphenols, in nature.
Their biological activities, for example they are potent antioxidants, depend both on the structural characteristics and the pattern of glycosylation.

Classification of flavonoids

They can be subdivided into different subclasses depending on the carbon of the C ring on which B ring is attached, and the degree of unsaturation and oxidation of the C ring.
Flavonoids in which B ring is linked in position 3 of the ring C are called isoflavones; those in which B ring is linked in position 4, neoflavonoids, while those in which the B ring is linked in position 2 can be further subdivided into several subgroups on the basis of the structural features of the C ring. These subgroup are: flavones, flavonols, flavanones, flavanonols, flavanols or catechins and anthocyanins.
Finally, flavonoids with open C ring are called chalcones.

Basic skeleton structure of flavonoid subclasses
Flavonoid Subclasses

Flavones

They have a double bond between positions 2 and 3 and a ketone in position 4 of the C ring. Most flavones of vegetables and fruits has a hydroxyl group in position 5 of the A ring, while the hydroxylation in other positions, for the most part in position 7 of the A ring or 3′ and 4′ of the B ring may vary according to the taxonomic classification of the particular vegetable or fruit.
Glycosylation occurs primarily on position 5 and 7, methylation and acylation on the hydroxyl groups of the B ring.
Some flavones, such as nobiletin and tangeretin, are polymethoxylated.

Flavonols

Compared to flavones, they have a hydroxyl group in position 3 of the C ring, which may also be glycosylated. Again, like flavones, flavonols are very diverse in methylation and hydroxylation patterns as well, and, considering the different glycosylation patterns, they are perhaps the most common and largest subgroup of flavonoids in fruits and vegetables. For example, quercetin is present in many plant foods.

Flavanones

Flavanones, also called dihydroflavones, have the C ring saturated; therefore, unlike flavones, the double bond between positions 2 and 3 is saturated and this is the only structural difference between the two subgroups of flavonoids.
The flavanones can be multi-hydroxylated, and several hydroxyl groups can be glycosylated and/or methylated.
Some have unique patterns of substitution, for example, furanoflavanones, prenylated flavanones, pyranoflavanones or benzylated flavanones, giving a great number of substituted derivatives.
Over the past 15 years, the number of flavanones discovered is significantly increased.

Flavanonols

Flavanonols, also called dihydroflavonols, are the 3-hydroxy derivatives of flavanones; they are an highly diversified and multisubstituted subgroup.

Isoflavones

As anticipated, isoflavones are a subgroup of flavonoids in which the B ring is attached to position 3 of the C ring. They have structural similarities to estrogens, such as estradiol, and for this reason they are also called phytoestrogens.

Catechins

Catechins are also referred to flavan-3-ols as the hydroxyl group is almost always bound to position 3 of C ring; they are called flavanols as well.
Catechins have two chiral centers in the molecule, on positions 2 and 3, then four possible diastereoisomers. Epicatechin is the isomer with the cis configuration and catechin is the one with the trans configuration. Each of these configurations has two stereoisomers, namely, (+)-epicatechin and (-)-epicatechin, (+)-catechin and (-)-catechin.
(+)-Catechin and (-)-epicatechin are the two isomers most often present in edible plants.
Another important feature of flavanols, particularly of catechin and epicatechin, is the ability to form polymers, called proanthocyanidins or condensed tannins. The name “proanthocyanidins” is due to the fact that an acid-catalyzed cleavage produces anthocyanidins.
Proanthocyanidins typically contain 2 to 60 monomers of flavanols.
Monomeric and oligomeric flavanols (containing 2 to 7 monomers) are strong antioxidants.

Anthocyanidins

Chemically, anthocyanidins are flavylium cations and are generally present as chloride salts.
They are the only group of flavonoids that gives plants colors (all other flavonoids are colorless).
Anthocyanins are glycosides of anthocyanidins. Sugar units are bound mostly to position 3 of the C ring and they are often conjugated with phenolic acids, such as ferulic acid.
The color of the anthocyanins depends on the pH and also by methylation or acylation at the hydroxyl groups on the A and B rings.

Chalcones

Chalcones and dihydrochalcones are flavonoids with open structure; they are classified as flavonoids because they have similar synthetic pathways.

References

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

Han X., Shen T. and Lou H. Dietary polyphenols and their biological significance. Int J Mol Sci 2007;9:950-988. doi:10.3390/i8090950

Manach C., Scalbert A., Morand C., Rémésy C., and Jime´nez L. Polyphenols: food sources and bioavailability. Am J Clin Nutr 2004;79(5):727-747. doi:10.1093/ajcn/79.5.727

Panche A.N., Diwan A.D., and  Chandra S.R. Flavonoids: an overview. J Nutr Sci. 2016;5:e47. doi:10.1017/jns.2016.41

Tsao R. Chemistry and biochemistry of dietary polyphenols. Nutrients 2010;2:1231-1246. doi:10.3390/nu2121231

Polyphenols: chimical structure, classification, food sources

Polyphenols are one of the most important and certainly the most numerous among the groups of phytochemicals present in the plant kingdom.
Currently, over 8,000 phenolic structures have been identified, of which more than 4,000 belonging to the class of flavonoids, and several hundred occur in edible plants.
However, it is thought that the total content of polyphenols in plants is underestimated as many of the phenolic compounds present in fruits, vegetables and derivatives have not yet been identified, escaping the methods and techniques of analysis used, and the composition in polyphenols for most fruits and some varieties of cereals is not yet known.
They are present in many edible plants, both for men and animals, and it is thought to be their presence, along with that of other molecules such as carotenoids, vitamin C or vitamin E, the responsible for the healthy effects of fruits and vegetables.
In the human diet, they are the most abundant natural antioxidants, and the main sources are fruits, vegetables, whole grains, but also other types of foods and beverages derived from them, such as red wine, rich in resveratrol, the extra virgin olive oil, rich in hydroxytyrosol, chocolate or tea, in particularly green tea, rich in epigallocatechin gallate (EGCG).

CONTENTS

Chemical structure of polyphenols

The term polyphenols refers to a wide variety of molecules that can be divided into many subclasses, subdivisions that can be made on the basis of their origin, biological function, or chemical structure.
Chemically, they are compounds with structural phenolic features, which can be associated with different organic acids and carbohydrates.

Ball-and-stick model of phenol

In plants, the most part of them are linked to sugars, and therefore they are in the form of glycosides. Carbohydrates and organic acids can be bound in different positions on polyphenol skeletons.
Among polyphenols, there are simple molecules, such as phenolic acids, or complex structures such as proanthocyanidins, that are highly polymerized molecules.

Classification

They can be classified into different classes, according to the number of phenolic rings in their structure, the structural elements that bind these rings each others, and the substituents linked to the rings. Therefore, two main groups can then be identified: the flavonoid group and the non-flavonoid group.
Flavonoids share a structure formed by two aromatic rings, indicated as A and B, linked together by three carbon atoms forming an oxygenated heterocycle, the C ring; they can be further subdivided into six main subclasses, as a function of the type of heterocycle (the C ring) that is involved:

Non-flavonoids can be subdivided into:

  • simple phenols
  • phenolic acids
  • benzoic aldehydes
  • hydrolyzable tannins
  • acetophenones and phenylacetic acids
  • hydroxycinnamic acids
  • coumarins
  • benzophenones
  • xanthones
  • stilbenes;
  • lignans
  • secoiridoids

Variability of polyphenol content of plants and plant products

Although several classes of phenolic molecules, such as quercetin (a flavonol, see figure), are present in most plant foods (tea, wine, cereals, legumes, fruits, fruit juices, etc.), other classes are found only in a particular type of food (e.g. flavanones in citrus, isoflavones in soya, phloridzin in apples, etc.).
However, it is common that different types of polyphenols are in the same product; for example, apples contain flavanols, chlorogenic acid, hydroxycinnamic acids, glycosides of phloretin, glycosides of quercetin and anthocyanins.
The polyphenol composition may also be influenced by other parameters such as environmental factors, the degree of ripeness at harvest time, household or industrial processing, storage, and plant variety. From currently available data, it seems that the fruits with the highest content of polyphenols are strawberries, lychees and grapes, and the vegetables are artichokes, parsley and brussels sprouts. Melons and avocados have the lowest concentrations.

References

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

Han X., Shen T. and Lou H. Dietary polyphenols and their biological significance. Int J Mol Sci 2007;9:950-988. doi:10.3390/i8090950

Manach C., Scalbert A., Morand C., Rémésy C., and Jime´nez L. Polyphenols: food sources and bioavailability. Am J Clin Nutr 2004;79(5):727-747. doi:10.1093/ajcn/79.5.727

Tsao R. Chemistry and biochemistry of dietary polyphenols. Nutrients 2010;2:1231-1246. doi:10.3390/nu2121231

Human health and carotenoids

Carotenoids belong to the category of bioactive compounds taken up with diet, that is, molecules able to provide protection against many diseases such as cardiovascular diseases, cancer and macular degeneration. They are also important for the proper functioning of the immune system.
Among the mechanisms that seem to be at the basis of their human health-promoting effects have been reported (Olson, 1999, see References):

  • the capability to quench singlet oxygen (see above);
  • the scavenging of peroxyl radicals and reactive nitrogen species;
  • the modulation of carcinogen metabolism;
  • the inhibition of cell proliferation;
  • the enhancement of the immune response;
  • a filtering action of blue light;
  • the enhancement of cell differentiation;
  • stimulation of cell-to-cell communication

Carotenoids and antioxidant activity

Carotenoids, with the adaptation of organisms to aerobic environment, and therefore to the presence of oxygen, have offered protection against oxidative damage from free radicals, particularly by singlet oxygen, a powerful oxidizing agent (see also below).
Carotenoids stabilize singlet oxygen acting both chemical and physical point of view:

  • chemical action involves the union between the two molecules;
  • in physical action, the radical transfers its excitation energy to the carotenoid. The result is a low energy free radical and an excited carotenoid; later, the energy acquired by the carotenoid is released as heat to the environment, and the molecule, that remains intact, is ready to carry out another cycle of stabilization of singlet oxygen, and so on.
Human health and carotenoids
Fig. 1 – Free Radical

The capability of carotenoids to quench singlet oxygen is due to the conjugated double-bond system present in the molecule, and the maximum protection is given by those molecules that have nine or more double bonds (moreover, the presence of oxygen in the molecule, as in xanthophylls, seems to have a role).
Carotenoids are involved not only in singlet oxygen quenching, but also in the scavenging of other reactive species both of oxygen, as peroxyl radicals (therefore contributing to the reduction of lipid peroxidation) and nitrogen. These reactive molecules are generated during the aerobic metabolism but also in the pathological processes.

Lycopene, xanthophylls and human health

Lycopene, a carotene, canthaxanthin and astaxanthin, two xanthophylls present in foods of animal origin, are better antioxidants than beta-carotene but also than zeaxanthin that, with lutein, is involved in prevention of age-related macular degeneration.
Lycopene, in addition to act on oxygen free radicals, acts as antioxidant also on the radicals of vitamin C and vitamin E, that are generated during the antioxidant processes in which these vitamins are involved, “repairing them”.
Finally, lycopene exerts its antioxidant action also indirectly, inducing the synthesis of enzymes involved in the protection against the action of oxygen free radicals and other electrophilic species; these enzymes are quinone reductase, glutathione S-transferase and superoxide dismutase (they are part of the enzymatic antioxidant system).

Vitamin A and human health

Vitamin A, whose deficiency affects annually more than 100 million children worldwide, causing more than a million deaths and half million cases of blindness, is a well-known carotenoid derivative with many biological actions, being essential for reproduction, growth, vision, immune function and general human health.
In the human diet, the major sources of vitamin A are the preformed vitamin, which is found in foods of animal origins (meat, milk, eggs, etc), and provitamin A carotenoids, present in fruits and vegetables. In economically deprived countries, fruits and vegetables are the main source of vitamin A being less expensive than food of animal origin.
Of the more than 750 different carotenoids identified in natural sources, only about 50 have provitamin A activity, and among these, beta-carotene (precisely, all-trans-beta-carotene isomer) is the main precursor of the vitamin A.
Among the other carotenoids precursors of vitamin A, alpha-carotene, gamma-carotene, beta-cryptoxanthin, alpha-cryptoxanthin, and beta-carotene-5,6-epoxide have about half the bioactivity of beta-carotene.

Human health and vitamin A
Fig. 2 – Provitamin A Activity

Spinach, carrots, pumpkins, sweet potatoes (yellow) are example of vegetables rich in beta-carotene and other provitamin A carotenoids.
Acyclic carotenes, such as lycopene (the main carotenoid in the human diet), and xanthophylls, except those mentioned above (beta-cryptoxanthin, alpha-cryptoxanthin, and beta-carotene-5,6-epoxide), cannot be converted to vitamin A.

References

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

Johnson E.J. The role of carotenoids in human health. Nutr Clin Care 2002;5(2):56-65. doi:10.1046/j.1523-5408.2002.00004.x

Olson, J.A. 1999. Carotenoids. p. 525-541. In: Shils M.E., Olson J.A., Shike M., Ross A.C. “Modern nutrition in health and disease” 9th ed., by Lippincott, Williams & Wilkins, 1999

Ross A.B., Thuy Vuong L., Ruckle J., Synal H.A., Schulze-König T., Wertz K., Rümbeli R., Liberman R.G., Skipper P.L., Tannenbaum S.R., Bourgeois A., Guy P.A., Enslen M., Nielsen I.L.F., Kochhar S., Richelle M., Fay L.B., and Williamson G. Lycopene bioavailability and metabolism in humans: an accelerator mass spectrometry study. Am J Clin Nutr 2011;93:1263-1273. doi:10.3945/ajcn.110.008375

Role of carotenoids in plants and foods

Through the course of evolution, carotenoids, thank to their unique physical and chemical properties, have proven to be highly versatile molecules, being able to perform many functions in many different organisms, like plants.

Carotenoids in photosynthesis

Carotenoids, in the early stages of the emergence of single-celled photosynthetic organisms, are probably been used for light harvesting at wavelengths different from those covered by chlorophyll. Therefore carotenoids, acting as light absorbing accessory pigments, have allowed to expand the range of solar radiation absorbed and so utilized for photosynthesis, energy that is then transferred to chlorophyll itself.
The major carotenoids involved in light harvesting, that accumulate in green plant tissues, are beta-carotene, lutein, neoxanthin, and violaxanthin, that absorb light energy in the 400- to 500-nm range.
Moreover, they protect chlorophyll from photooxidation (in humans, they may contribute to the protection of photo-oxidative damage caused by UV rays, thus acting as a endogenous photo-protective agents).

Carotenoids and autumn leaf color

Leaf color of deciduous plants in different seasons, green, yellow, orange or red, is due to the presence in them of natural pigments.
In spring and summer, the predominant pigment present in the leaf is chlorophyll, and therefore the color is green.
Carotenoids and PlantsDuring the fall, the color changes from green to yellow, orange or red, depending on the type of plant: this is a consequence of the change, both qualitative and quantitative, in the pigment content.
In fact, as a result of the decrease of the temperature and daylight hours, the production of chlorophyll is interrupted and that already present is demolished into colorless metabolites. In this way the predominant pigments become carotenoids (yellow-orange), molecules much more stable than the chlorophyll, which remain in the leaf coloring it (it do not seem to be synthesized de novo), and anthocyanins (red-purple), which, unlike carotenoids, are not present during the growing season, but are synthesized in autumn, just before leaf fall. Therefore, it can be concluded that the red-purple color assumed from the leaves of certain plants is not a side effect of leaf senescence but results from anthocyanins de-novo synthesis.
Depending on the prevalence of carotenoids or anthocyanins, leaf color changes from green to yellow/orange, as in Ginkgo biloba (yellow), or red-purple as in some maples.

And plants with non green leaves?
Their color is not due to the absence of chlorophyll but the presence of very high amounts of other pigments, typically carotenoids and anthocyanins, that “cover” the chlorophyll, determining the color of the leaf.

Some functions of apocarotenoids in plants and foods

These oxygenated carotenoids, containing fewer than 40 carbon atoms, have many functions in plants and animals and are also important for the aroma and flavor of foods.
Some of their main functions include the following.

  • Apocarotenoids have significant roles in the response signals involved in the development and in the response to the environment (for example abscisic acid).
  • They can act as visual or volatile signals to attract pollinators.
  • They are important in the defense mechanisms of plants.
  • They have a role in regulating plant architecture.
  • An apocarotenal, trans-beta-apo-8′-carotenal, found in citrus fruits and spinach, with a low provitamin A activity, is used in pharmaceuticals and cosmetics, and is also a food additive (E160e) legalized by the European Commission for human consumption.
  • Apocarotenoids make an important contribution to the nutritional quality and flavor of many types of foods such as fruits, wine and tea. Two natural apocarotenoids, crocetin and bixina, have economic importance as they are used as pigments and aroma in foods.
  • Finally, a broad range of apocarotenals derive from oxidative reactions that occur in food processing; these molecules are intermediates in the formation of smaller molecules, important for the color and flavor of the food.

References

Archetti, M., Döring T.F., Hagen S.B., Hughes N.M., Leather S.R., Lee D.W., Lev-Yadun S., Manetas Y., Ougham H.J. Unravelling the evolution of autumn colours: an interdisciplinary approach. Trends Ecol Evol 2009;24(3):166-73. doi:10.1016/j.tree.2008.10.006

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

Maltodextrin, fructose and endurance sports

Carbohydrate ingestion can improve endurance capacity and performance.
The ingestion of different types of carbohydrates, which use different intestinal transporters, can:

  • increase total carbohydrate absorption;
  • increase exogenous carbohydrate oxidation;
  • and therefore improve performance.

Glucose and fructose

When a mixture of glucose and fructose is ingested (in the analyzed literature, respectively 1.2 and 0.6 g/min, ratio 2:1, for total carbohydrate intake rate to 1.8 g/min), there is less competition for intestinal absorption compared with the ingestion of an iso-energetic amount of glucose or fructose,  two different intestinal transporters being involved. Furthermore, fructose absorption is stimulated by the presence of glucose.

This can:

  • contribute to a faster rate of monosaccharide absorption;
  • increase the availability of exogenous carbohydrates in the bloodstream;
  • cause the higher exogenous carbohydrate oxidation rates in fructose plus glucose combination compared to high glucose intake alone.

The combined ingestion of glucose and fructose allows to obtain exogenous carbohydrate oxidation rate around 1,26 g/min, therefore, higher than the rate reported with glucose alone (1g/min), also in high concentration.
The observed difference (+0,26 g/min) can be fully attributed to the oxidation of ingested fructose.

Sucrose and glucose

The ingestion of sucrose and glucose, in the same conditions of the ingestion of glucose and fructose (therefore, respectively 1.2 and 0.6 g/min, ratio 2:1, for total carbohydrate intake rate to 1.8 g/min), gives similar results.

Glucose, sucrose and fructose

Very high oxidation rates are found with a mixture of glucose, sucrose, and fructose (in the analyzed literature, respectively 1.2, 0.6 and 0.6 g/min, ratio 2:1:1, for total carbohydrate intake rate to 2.4 g/min; however, note the higher amounts of ingested carbohydrates).

Maltodextrin and fructose

High oxidation rates are also observed with combinations of maltodextrin and fructose, in the same conditions of the ingestion of glucose plus fructose (therefore, respectively 1.2 and 0.6 g/min, ratio 2:1, for total carbohydrate intake rate to 1.8 g/min).

Such high oxidation rates can be achieved with carbohydrates ingested in a beverage, in a gel or in a low-fat, low protein, low-fiber energy bar.

The best combination of carbohydrates ingested during exercise seems to be the mixture of maltodextrin and fructose in a 2:1 ratio, in a 5% solution, and in a dose around 80-90 g/h.

Maltodextrin and Fructose: Oxidation of Ingested Carbohydrates
Fig. 1 – Oxidation of Ingested Carbohydrates

Why?

  • This mixture has the best ratio between amount of ingested carbohydrates and their oxidation rate and it means that smaller amounts of carbohydrates remain in the stomach or gut reducing the risk of gastrointestinal complication/discomfort during prolonged exercise (see brackets grafa in the figure).
  • A solution containing a combination of multiple transportable carbohydrates and a carbohydrate content not exceeding 5% optimizes gastric emptying rate and improves fluid delivery.

Example of a 5% carbohydrate solution containing around 80-90 g of maltodextrin and fructose in a 2:1 rate; ingestion time around 1 h.

  • 1.5 L solution: 80 g of carbohydrates, including around 55 g of maltodextrin and around 25 of fructose.
  • 1.8 L solution: 90 g of carbohydrates, including 60 g of maltodextrin and 30 of fructose.

Conclusion

During prolonged exercise, when high exogenous carbohydrate oxidation rates are needed, the ingestion of multiple transportable carbohydrates is preferred above that of large amounts of a single carbohydrate.
The best mixture seems to be maltodextrin and , in a 2:1 ratio, in a 5% concentration solution, and at ingestion rate of around 80-90 g/h.

References

Burke L.M., Hawley J.A., Wong S.H.S., & Jeukendrup A. Carbohydrates for training and competition. J Sport Sci 2011;29:Sup1,S17-S27. doi:10.1080/02640414.2011.585473

Jentjens R.L.P.G., Moseley L., Waring R.H., Harding L.K., and Jeukendrup A.E. Oxidation of combined ingestion of glucose and fructose during exercise. J Appl Physiol 2004:96;1277-1284. doi:10.1152/japplphysiol.00974.2003

Jentjens R.L.P.G., Venables M.C., and Jeukendrup A.E. Oxidation of exogenous glucose, sucrose, and maltose during prolonged cycling exercise. J Appl Physiol 2004:96;1285-1291. doi:10.1152/japplphysiol.01023.2003

Jeukendrup A.E. Carbohydrate feeding during exercise. Eur J Sport Sci 2008:2;77-86. doi:10.1080/17461390801918971

Jeukendrup A.E. Nutrition for endurance sports: marathon, triathlon, and road cycling. J Sport Sci 2011:29;sup1, S91-S99. doi:10.1080/02640414.2011.610348

Prolonged exercise and carbohydrates

During prolonged exercise (>90 min), like marathon, Ironman, cross-country skiing, road cycling or open water swimming, the effects of supplementary carbohydrates on performance are mainly metabolic rather than central and include:

  • the provision of an additional muscle fuel source when glycogen stores become depleted;
  • muscle glycogen sparing;
  • the prevention of low blood glucose concentrations.

How many carbohydrates should an athlete take?

The optimal amount of ingested carbohydrate is that which results in the maximal rate of exogenous carbohydrate oxidation without causing gastrointestinal discomfort”. (Jeukendrup A.E., 2008, see References).

Prolonged exercise: which carbohydrates should an athlete take?

Until 2004 it was believed that carbohydrates ingested during exercise (also prolonged exercise) could be oxidized at a rate no higher than 1 g/min, that is, 60 g/h, independent of the type of carbohydrate.
Exogenous carbohydrate oxidation is limited by their intestinal absorption and the ingestion of more than around 60 g/min of a single type of carbohydrate will not increase carbohydrate oxidation rate but it is likely to be associated with gastrointestinal discomfort (see later).
Why?
At intestinal level, the passage of glucose (and galactose) is mediated by a sodium dependent transporter called SGLT1. This transporter becomes saturated at a carbohydrate intake about 60 g/h and this (and/or glucose disposal by the liver that regulates its transport into the bloodstream) limits the oxidation rate to 1g/min or 60 g/h. For this reason, also when glucose is ingested at very high rate (>60 g/h), exogenous carbohydrate oxidation rates higher 1.0-1.1 g/min are not observed.

Role of nutrition in prolonged exercise

The rate of oxidation of ingested maltose, sucrose, maltodextrins and glucose polymer is fairly similar to that of ingested glucose.

Fructose uses a different sodium independent transporter called GLUT5. Compared with glucose, fructose has, like galactose, a lower oxidation rate, probably due to its lower rate of intestinal absorption and the need to be converted into glucose in the liver, again like galactose, before it can be oxidized.
However, if the athlete ingests different types of carbohydrates, which use different intestinal transporters, exogenous carbohydrate oxidation rate can increase significantly.
It seems that the best mixture is maltodextrins and fructose.

Note: the high rates of carbohydrate ingestion may be associated with delayed gastric emptying and fluid absorption; this can be minimized by ingesting combinations of multiple transportable carbohydrates that enhance fluid delivery compared with a single carbohydrate. This also causes relatively little gastrointestinal distress.

Conclusion

The ingestion of different types of carbohydrates that use different intestinal transporters can:

  • increase total carbohydrate absorption;
  • increase exogenous carbohydrate oxidation;
  • improve performance.

References

Burke L.M., Hawley J.A., Wong S.H.S., & Jeukendrup A. Carbohydrates for training and competition. J Sport Sci 2011;29:Sup1,S17-S27. doi:10.1080/02640414.2011.585473

Jentjens R.L.P.G., Moseley L., Waring R.H., Harding L.K., and Jeukendrup A.E. Oxidation of combined ingestion of glucose and fructose during exercise. J Appl Physiol 2004:96;1277-1284. doi:10.1152/japplphysiol.00974.2003

Jentjens R.L.P.G., Venables M.C., and Jeukendrup A.E. Oxidation of exogenous glucose, sucrose, and maltose during prolonged cycling exercise. J Appl Physiol 2004:96;1285-1291. doi:10.1152/japplphysiol.01023.2003

Jeukendrup A.E. Carbohydrate feeding during exercise. Eur J Sport Sci 2008:2;77-86. doi:10.1080/17461390801918971

Jeukendrup A.E. Nutrition for endurance sports: marathon, triathlon, and road cycling. J Sport Sci 2011:29;sup1, S91-S99. doi:10.1080/02640414.2011.610348

Carbohydrate ingestion during short duration high intensity exercise

High Intensity: During-Exercise Nutrition
Fig. 1- During-Exercise Nutrition

Carbohydrate ingestion during intermittent high intensity or prolonged (>90 min) sub-maximal exercise can:

  • increase exercise capacity;
  • improve exercise performance;
  • postpone fatigue.

The intake of very small amounts of carbohydrates or carbohydrate mouth rinsing (for example with a 6% maltodextrin solution) may improve exercise performance by 2-3% when the exercise is of relatively short duration (<1 h) and high intensity (>75% VO2max), that is, an exercise not limited by the availability of muscle glycogen stores, given adequate diet.
The underlying mechanisms for the ergogenic effect of carbohydrates during this type of activity are not metabolic but may reside in the central nervous system: it seems that carbohydrates are detected in the oral cavity by unidentified receptors, promoting an enhanced sense of well-being and improving pacing.
These effects are independent of taste or sweet and non-sweet of carbohydrates but are specific to carbohydrates.

It should be noted that performance effects with drink ingestion are similar to the mouth rinse; therefore athletes, when they don’t complain of gastrointestinal distress when ingesting too much fluid, may have an advantage taking the drink (in endurance sports, dehydration and carbohydrate depletion are the most likely contributors to fatigue).

Conclusion
It seems that during exercise of relatively short duration (<1 h) and high intensity (>75% VO2max) it is not necessary to ingest large amounts of carbohydrates: a carbohydrate mouth rinsing or the intake of very small amounts of carbohydrates may be sufficient to obtain a performance benefit.

References

Burke L.M., Hawley J.A., Wong S.H.S., & Jeukendrup A. Carbohydrates for training and competition. J Sport Sci 2011;29:Sup1,S17-S27. doi:10.1080/02640414.2011.585473

Jentjens R.L.P.G., Moseley L., Waring R.H., Harding L.K., and Jeukendrup A.E. Oxidation of combined ingestion of glucose and fructose during exercise. J Appl Physiol 2004:96;1277-1284. doi:10.1152/japplphysiol.00974.2003

Jentjens R.L.P.G., Venables M.C., and Jeukendrup A.E. Oxidation of exogenous glucose, sucrose, and maltose during prolonged cycling exercise. J Appl Physiol 2004:96;1285-1291. doi:10.1152/japplphysiol.01023.2003

Jeukendrup A.E. Carbohydrate feeding during exercise. Eur J Sport Sci 2008:2;77-86. doi:10.1080/17461390801918971

Carotenoids: definition, structure and classification

Carotenoids
Fig. 1 – Carrots

Carotenoids are soluble-fat pigments found throughout nature.
Carotenoids were discovered during the 19th century.
In 1831 Wachen proposed the term “carotene” for a pigment crystallized from carrot roots.
Berzelius called the more polar yellow pigments extracted from autumn leaves “xanthophylls” (originally phylloxanthins), from Greek words xanthos, meaning yellow, and phyllon, meaning leaf.
Tswett separated many pigments and called them “carotenoids.”
They occur in the chromoplasts of plants and some other photosynthetic organisms such as algae and in some types of fungi and bacteria; they are also produced by some invertebrates (Aphids).
There are more than 750 different carotenoids ranging in color from red (such as lycopene), to orange (such as alpha-carotene, beta-carotene, and gamma-carotene) or yellow (such as lutein, alfa-cryptoxanthin or violaxanthin); more than 100 have been found in fruits and vegetables.
In some green plants and in their parts, generally the darker the green color, the higher the carotenoid content: for example, carotenoid content in pale green cabbage is less than 1% of that in dark green one.
Fruit carotenoids are very different, and those present in ripe fruits may be different from those present in unripe fruits.
They also occur extensively in microorganisms and animals.
In plants, microorganism and animals carotenoids have diverse and important functions and actions.

CONTENTS

Chemical structure of carotenoids

Carotenoids are a class of hydrocarbon compounds consisting of 40 carbon atoms (tetraterpenes), with a structure characterized by an extensive conjugated double-bond system that determines the color (it serves as a light-absorbing chromophore): as the number of conjugated double-bond increases, color changes from pale yellow, to orange, to red.
In nature, they exist primarily in the more stable all-trans isomeric configuration, even though small amounts of cis isomers do occur too (they can be produced from all-trans forms also during processing).
Traditionally, carotenoids have been given trivial names derived from the biological source from which they are extracted. However, a semisystematic scheme exists: it allows carotenoids to be named in a way that describes and defines their structure.

Classification

Depending on the presence or absence of oxygen in the molecule, they can be divided into:

  • xanthophylls, which contain oxygen, such as:

Antheraxanthin
Astaxanthin (red)
Auroxanthin
Bixin, E160b
Canthaxanthin (red), E161g
Capsanthin, E160c
Capsorubin, E160c
beta-Carotene-5,6-epoxide
alfa-Cryptoxanthin (yellow)
beta-Cryptoxanthin (orange)
Crocetin
Lutein (yellow), E161b
Lutein-5,6-epoxide or taraxanthin
Luteoxanthin
Lycophyll
Lycoxanthin
Neoxanthin
Rubixanthin
Tunaxanthin
Violaxanthin (yellow)
Zeaxanthin (yellow-orange)
Zeinoxanthin

  • carotenes, which lack oxygen, as such:

alfa-Carotene (orange)
beta-Carotene (orange), E160a
delta-Carotene
gamma-Carotene (orange)
Lycopene (red), E160d
Neurosporene
Phytoene (colorless)
Phytofluene
alfa-Zeacarotene
beta-Zeacarotene
zeta-Carotene

Depending on chemical structure they can be divided into:

  • acyclic carotenes: formed by a linear carbon chain such as:

zeta-Carotene
Phytoene (colorless)
Lycopene (red), E160d
Neurosporene
Phytofluene

  • cyclic carotenes: containing one or two cyclic structures such as:

alfa-Carotene (orange)
beta-Carotene (orange), E160a
gamma-Carotene (orange)
delta-Carotene
alfa-Zeacarotene
beta-Zeacarotene

  • hydroxycarotenoids (or carotenols): containing at least an hydroxyl group (xanthophylls) such as:

alfa-Cryptoxanthin (yellow)
beta-Cryptoxanthin (orange)
Lutein (yellow), E161b
Lycofill
Lycoxanthin
Rubixanthin
Zeaxanthin (yellow-orange)
Zeinoxanthin

  • epoxycarotenoids: containing at least an epoxic group (xanthophylls) such as:

Antheraxanthin
Auroxanthin
beta-Carotene-5,6-epoxide
Lutein-5,6-epoxide
Luteoxanthin
Neoxanthin
Violaxanthin (yellow)

  • uncommon or species-specific carotenoids such as:

Bixin, E160b
Capsanthin, E160c
Capsorubin, E160c
Crocetin

Note: Although green leaves contain unesterified hydroxycarotenoids, most carotenols in ripe fruits are esterified with fatty acids, a class od lipids. However, those of some fruits, particularly those that remain green when ripe (example kiwi fruit) undergo no or limited esterification.

Apocarotenoids

Apocarotenoids are a class of carotenoids containing less than 40 carbon atoms, very widespread in nature and with extremely different structures.
They derive from 40 carbon atom carotenoids by oxidative cleavage that can occurs through non-specific mechanisms, such as photo-oxidation, or through the action of specific enzymes (these enzymatic activities, identified in plants, animals and microorganisms, are collectively referred to as carotenoid cleavage dioxygenases).
Some of the most well-known

  • vitamin A
  • abscisic acid
  • bixin, E160b
  • crocetin
  • trans-β-apo-8′-carotenal, E160e

References

Boileau A.C., Merchen N.R., Wasson K., Atkinson C.A. and Erdman Jr J.W. cis-Lycopene is more bioavailable than trans-lycopene in vitro and in vivo in lymph-cannulated ferrets. J Nutr 1999;129:1176-1181. doi:10.1093/jn/129.6.1176

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

Engelmann N.J., Clinton S.K., and Erdman Jr J.W. Nutritional aspects of phytoene and phytofluene,carotenoid precursors to lycopene. Adv Nutr 2011:2;51-61. doi:10.3945/​an.110.000075

Olempska-Beer Z. Lycopene (synthetic): chemical and technical assessment (CTA). Office of Food Additive Safety, Center for Food Safety and Applied Nutrition. U.S. Food and Drug Administration. College Park, Maryland, USA.

Periago M.J., Bravo S., García-Alonso F.J., and Rincón F. Detection of key factors affecting lycopene in vitro accessibility. J Agr Food Chem 2013;61(16):3859-3867. doi:10.1021/jf3052994

Ross A.B., Thuy Vuong L., Ruckle J., Synal H.A., Schulze-König T., Wertz K., Rümbeli R., Liberman R.G., Skipper P.L., Tannenbaum S.R., Bourgeois A., Guy P.A., Enslen M., Nielsen I.L.F., Kochhar S., Richelle M., Fay L.B., and Williamson G. Lycopene bioavailability and metabolism in humans: an accelerator mass spectrometry study. Am J Clin Nutr 2011;93:1263-1273. doi:10.3945/ajcn.110.008375

Wang X-D. Lycopene metabolism and its biological significance. Am J Clin Nutr 2012:96;1214S-1222S. doi:10.3945/​ajcn.111.032359

Hydration before endurance sports

Pre-hydration
Fig. 1 – Pre-hydration

In endurance sports, like Ironman, open water swimming, road cycling, marathon, or cross-country skiing, the most likely contributors to fatigue are dehydration and carbohydrate (especially liver and muscle glycogen) depletion.

Pre-hydration

Due to sweat loss needed to dissipate the heat generated during exercise, dehydration can compromise exercise performance.
It is important to start exercising in a euhydrated state, with normal plasma electrolyte levels, and attempt to maintain this state during any activity.
When an adequate amount of beverages with meals are consumed and a protracted recovery period (8-12 hours) has elapsed since the last exercise, the athlete should be euhydrated.
However, if s/he has not had adequate time or fluids/electrolytes volume to re-establish euhydration, a pre-hydration program may be useful to correct any previously incurred fluid-electrolyte deficit prior to initiating the next exercise.

Pre-hydration program

If during exercise the nutritional target is to reduce sweat loss to less than 2–3% of body weight, prior to exercise the athlete should drink beverages at least 4 hours before the start of the activity, for example, about 5-7 mL/kg body weight.
But if the urine is still dark (highly concentrated) and/or is minimal, s/he should slowly drink more beverages, for example, another 3-5 mL/kg body weight, about 2 hours before the start of activity so that urine output normalizes before starting the event.

It is advisable to consume small amounts of sodium-containing foods or salted snacks and/or beverages with sodium that help to stimulate thirst and retain the consumed fluids.
Moreover, palatability of the ingested beverages is important to promote fluid consumption before, during, and after exercise. Fluid palatability is influenced by several factors, such as:

  • temperature, often between 15 and 21 °C;
  • sodium content;
  • flavoring.

And hyper-hydration?

Hyper-hydration, especially in the heat, could improve thermoregulation and exercise performance, therefore, it might be useful for those who lose body water at high rates, as during exercise in hot conditions or who have difficulty drinking sufficient amounts of fluid during exercise.
However there are several risks:

  • fluids that expand the intra- and extra-cellular spaces (e.g. glycerol solutions plus water) greatly increase the risk of having to void during exercise;
  • hyper-hydration may dilute and lower plasma sodium which increases the risk of dilutional hyponatraemia, if during exercise, fluids are replaced aggressively.

Finally, it must be noted that plasma expanders or hyper-hydrating agents are banned by the World Anti-Doping Agency (WADA).

Conclusion
“Pre-hydrating with beverages, if needed, should be initiated at least several hours before the exercise task to enable fluid absorption and allow urine output to return toward normal levels. Consuming beverages with sodium and/or salted snacks or small meals with beverages can help stimulate thirst and retain needed fluids” (Sawka et al., 2007, see References).

References

Jeukendrup A.E. Nutrition for endurance sports: marathon, triathlon, and road cycling. J Sport Sci 2011:29;sup1, S91-S99. doi:10.1080/02640414.2011.610348

Sawka M.N., Burke L.M., Eichner E.R., Maughan, R.J., Montain S.J., Stachenfeld N.S. American College of Sports Medicine position stand: exercise and fluid replacement. Med Sci Sport Exercise 2007;39:377-390. doi:10.1249/mss.0b013e31802ca597

Shirreffs S., Sawka M.N. Fluid and electrolyte needs for training, competition and recovery. J Sport Sci 2011;29:sup1, S39-S46. doi:10.1080/02640414.2011.614269

Hypoglycemia and carbohydrate ingestion 60 min before exercise

Hypoglycemia: Fatigue
Fig. 1 – Fatigue

From several studies it appears that the risk of developing hypoglycemia (blood glucose < 3.5 mmol /l or < 63 mg/l) is highly individual: some athletes are very prone to develop it and others are much more resistant.

Strategies to limit hypoglycemia in susceptible subjects

A strategy to minimize glycemic and insulinemic responses during exercise is to delay carbohydrate ingestion just prior to exercise: in the last 5-15 min before exercise or during warm-up (even though followed by a short break).
Why?

  • Warm-up and then exercise increase catecholamine concentrations blunting insulin response.
  • Moreover, it has been shown that ingestion of carbohydrate-containing beverages during a warm-up (even if followed by a short break) does not lead to rebound hypoglycemia, independent of the amount of carbohydrates, but instead increases glycemia. When carbohydrates are ingested within 10 min before the onset of the exercise, exercise will start before the increase of insulin concentration.

Therefore, this timing strategy would provide carbohydrates minimizing the risk of a possible reactive hypoglycaemia.
In addition, it is possible to choose low glycemic index carbohydrates that lead to more stable glycemic and insulinemic responses during subsequent exercise.

Example: a 5-6% carbohydrate solution, often maltodextrin (i.e. 50-60 g maltodextrin in 1000 ml) or maltodextrin plus fructose (e.g. respectively 33 g plus 17 g in 1000 ml).

An intriguing observation is the lack of a clear relation between hypoglycaemia and its symptoms (likely related to a reduced delivery of glucose to the brain). In fact, symptoms are often reported in the absence of true hypoglycemia and hypoglycemia is not always associated with symptoms. Though the cause of the symptoms is still unknown, it is clearly not related to a glycemic threshold.

Conclusion
Some athletes develop symptoms similar to those of hypoglycemia, even though they aren’t always linked to actual low glycemia. To minimize these symptoms, for these subjects an individual approach is advisable. It may include:

  • carbohydrate ingestion just before the onset of exercise or during warm-up;
  • choose low-to-moderate GI carbohydrates that result in more stable glycemic and insulinemic responses;
  • or avoid carbohydrates 90 min before the onset of exercise.

References

Jeukendrup A.E. Carbohydrate feeding during exercise. Eur J Sport Sci 2008:2;77-86. doi:10.1080/17461390801918971

Jeukendrup A.E., C. Killer S.C. The myths surrounding pre-exercise carbohydrate feeding. Ann Nutr Metab 2010;57(suppl 2):18-25. doi:10.1159/000322698

Moseley L., Lancaster G.I, Jeukendrup A.E. Effects of timing of pre-exercise ingestion of carbohydrate on subsequent metabolism and cycling performance. Eur J Appl Physiol 2003;88:453-458. doi:10.1007/s00421-002-0728-8

Carbohydrate loading before competition

Carbohydrate loading is a good strategy to increase fuel stores in muscles before the start of the competition.

What does the muscle “eat” during endurance exercise?

Muscle “eats” carbohydrates, in the form of glycogen, stored in the muscles and liver, carbohydrates ingested during the exercise or just before that, and fat.
During endurance exercise, the most likely contributors to fatigue are dehydration and carbohydrate depletion, especially of muscle and liver glycogen.
To prevent the “crisis” due to the depletion of muscle and liver carbohydrates, it is essential having high glycogen stores before the start of the activity.

What does affect glycogen stores?

  • The diet in the days before the competition.
  • The level of training (well-trained athletes synthesize more glycogen and have potentially higher stores, because they have more efficient enzymes).
  • The activity in the day of the competition and the days before (if muscle doesn’t work it doesn’t lose glycogen). Therefore, it is better to do light trainings in the days before the competition, not to deplete glycogen stores, and to take care of nutrition.

The “Swedish origin” of carbohydrate loading

Very high muscle glycogen levels (the so-called glycogen supercompensation) can improve performance, i.e. time to complete a predetermined distance, by 2-3% in the events lasting more than 90 minutes, compared with low to normal glycogen, while benefits seem to be little or absent when the duration of the event is less than 90 min.
Well-trained athletes can achieve glycogen supercompensation without the depletion phase prior to carbohydrate loading, the old technique discovered by two Swedish researchers, Saltin and Hermansen, in 1960s.
The researchers discovered that muscle glycogen concentration could be doubled in the six days before the competition following this diet:

  • three days of low carb menu (a nutritional plan very poor in carbohydrates, i.e. without pasta, rice, bread, potatoes, legumes, fruits etc.);
  • three days of high carbohydrate diet, the so-called carbohydrate loading (a nutritional plan very rich in carbohydrates).

This diet causes a lot of problems: the first three days are very hard and there may be symptoms similar to depression due to low glucose delivery to brain, and the benefits are few.
Moreover, with the current training techniques, the type and amount of work done, we can indeed obtain high levels of glycogen: above 2.5 g/kg of body weight.

The “corrent” carbohydrate loading

If we compete on Sunday, a possible training/nutritional plan to obtain supercompensation of glycogen stores can be the following:

  • Wednesday, namely four days before the competition, moderate training and then dinner without carbohydrates;
  • from Thursday on, namely the three days before the competition, hyperglucidic diet and light trainings.
Example of nutritional plan for carbohydrate loading and glycogen supercompensation
Carbohydrate Loading: 2500 kcal Diet

The amount of dietary carbohydrates needed to recover glycogen stores or to promote glycogen loading depends on the duration and intensity of the training programme, and they span from 5 to 12 g/kg of body weight/d, depending on the athlete and his activity. With higher carbohydrate intake you can achieve higher glycogen stores but this does not always results in better performance; moreover, it should be noted that glycogen storage is associated with weight gain due to water retention (approximately 3 g per gram of glycogen), and this may not be desirable in some sports.

References

Burke L.M., Hawley J.A., Wong S.H.S., & Jeukendrup A. Carbohydrates for training and competition. J Sport Sci 2011;29:Sup1,S17-S27. doi:10.1080/02640414.2011.585473

Hargreaves M., Hawley J.A., & Jeukendrup A.E. Pre-exercise carbohydrate and fat ingestion: effects on metabolism and performance. J Sport Sci 2004;22:31-38. doi10.1080/0264041031000140536

Jeukendrup A.E., C. Killer S.C. The myths surrounding pre-exercise carbohydrate feeding. Ann Nutr Metab 2010;57(suppl 2):18-25. doi:10.1159/000322698

Moseley L., Lancaster G.I, Jeukendrup A.E. Effects of timing of pre-exercise ingestion of carbohydrate on subsequent metabolism and cycling performance. Eur J Appl Physiol 2003;88:453-458. doi:10.1007/s00421-002-0728-8

Alkaline diet and health benefits

The acid-ash hypothesis posits that protein and grain foods, with a low potassium intake, produce a diet acid load, net acid excretion, increased urine calcium, and release of calcium from the skeleton, leading to osteoporosis.” (Fenton et al., 2009, see References).
Is it true?
Calcium, present in bones in form of carbonates and phosphates, represents a large reservoir of base in the body. In response to an acid load such as the high protein diets these salts are released into the circulation to bring about pH homeostasis. This calcium is lost in the urine and it has been estimated that the quantity lost with the such diet over time could be as high as almost 480 g over 20 years or almost half the skeletal mass of calcium!
Even these losses of calcium may be buffered by ingestion of foods that are alkali rich as fruit and vegetables, and on-line information promotes an alkaline diet for bone health as well as a number of books, a recent meta-analysis has shown that the causal association between osteoporotic bone disease and dietary acid load is not supported by evidence and there is no evidence that the alkaline diet is protective of bone health (but it is protective against the risk for kidney stones).

Note: it is possible that fruit and vegetables are beneficial to bone health through mechanisms other than via the acid-ash hypothesis.

And protein?
Excess dietary protein with high acid renal load may decrease bone density, if not buffered by ingestion of foods that are alkali rich, that is fruit and vegetables. However, an adequate protein intake is needed for the maintenance of bone integrity. Therefore, increasing the amount of fruit and vegetables may be necessary rather than reducing protein too much.
Therefore it is advisable to consume a normo-proteic diet rich in fruits and vegetables and poor in sodium, that is, a Mediterranean Diet-like eating patterns, eating foods with a negative acid load together with foods with a positive acid load. Example: pasta plus vegetables or meats plus vegetables and fruits (see figure below).

Alkaline Diet: Food and Acid Load
Food and Acid Load

Alkaline diet and muscle mass


As we age, there is a loss of muscle mass, which predispose to falls and fractures. A diet rich in potassium, obtained from fruits and vegetables, as well as a reduced acid load, results in preservation of muscle mass in older men and women.

Alkaline diet and growth hormone

In children, severe forms of metabolic acidosis are associated with low levels of growth hormone with resultant short stature; its correction with potassium or bicarbonate citrate increases growth hormone significantly and improves growth. In postmenopausal women, the use of enough potassium bicarbonate in the diet to neutralize the daily net acid load resulted in a significant increase in growth hormone and resultant osteocalcin.
Improving growth hormone levels may reduce cardiovascular risk factors, improve quality of life, body composition, and even memory and cognition.

Conclusion

Alkaline diet may result in a number of health benefits.

  • Increased fruits and vegetables would improve the K/Na ratio and may benefit bone health, reduce muscle wasting, as well as mitigate other chronic diseases such as hypertension and strokes.
  • The increase in growth hormone may improve many outcomes from cardiovascular health to memory and cognition.
  • The increase in intracellular magnesium is another added benefit of the alkaline diet (e.g. magnesium, required to activate vitamin D, would result in numerous added benefits in the vitamin D systems).

It should be noted that one of the first considerations in an alkaline diet, which includes more fruits and vegetables, is to know what type of soil they were grown in since this may significantly influence the mineral content and therefore their buffering capacity.

References

Fenton T.R., Lyon A.W., Eliasziw M., Tough S.C., Hanley D.A. Meta-analysis of the effect of the acid-ash hypothesis of osteoporosis on calcium balance. J Bone Miner Res 2009;24(11):1835-1840. doi:10.1359/jbmr.090515

Fenton T.R., Lyon A.W., Eliasziw M., Tough S.C., Hanley D.A. Phosphate decreases urine calcium and increases calcium balance: a meta-analysis of the osteoporosis acid-ash diet hypothesis. Nutr J 2009;8:41. doi:10.1186/1475-2891-8-41

Fenton T.R., Tough S.C., Lyon A.W., Eliasziw M., Hanley D.A. “Causal assessment of dietary acid load and bone disease: a systematic review and meta-analysis applying Hill’s epidemiologic criteria for causality.” Nutr J 2011;10:41. doi:10.1186/1475-2891-10-41

Schwalfenberg G.K. The alkaline diet: is there evidence that an alkaline pH diet benefits health? J Environ Public Health 2012; Article ID 727630. doi:10.1155/2012/727630

Metabolic acidosis and human diet

Life depends on appropriate pH levels around and in living organisms and cells.
We requires a tightly controlled pH level in our serum of about 7.4 (a slightly alkaline range of 7.35 to 7.45) to avoid metabolic acidosis and survive. As a comparison, in the past 100 years the pH of the ocean has dropped from 8.2 to 8.1 because of increasing carbon dioxide (CO2) deposition with a negative impact on life in the ocean (it may lead to the collapse of the coral reefs).

Metabolic Acidosis: The pH Scale
Fig. 1 – The pH Scale

Even the mineral content of the food we eat (minerals are used as buffers to maintain pH within the aforementioned range) is considerabled influence by the pH of the soil in which plants are grown. The ideal pH of soil for the best overall availability of essential nutrients is between 6 and 7: an acidic soil below pH of 6 may have reduced magnesium and calcium, and soil above pH 7 may result in chemically unavailable zinc, iron, copper and manganese.

Metabolic acidosis and agricultural and industrial revolutions

In the human diet, there has been considerable change from the hunter gather civilization to the present in the pH and net acid load. With the agricultural revolution (last 10,000 years) and even more recently with industrialization (last 200 years) it has been seen:

  • an increase in sodium compared to potassium (the ratio potassium/sodium has reversed from 10 to 1 to a ratio of 1 to 3 in the modern diet) and in chloride compared to bicarbonate in the diet,;
  • a poor intake of magnesium and fiber;
  • a large intake of simple carbohydrates and saturated fats.

This results in a diet that may induce metabolic acidosis which is mismatched to the genetically determined nutritional requirements.
Moreover, with aging, there is a gradual loss of renal acid-base regulatory function and a resultant increase in diet-induced metabolic acidosis.
Finally, a high protein low-carbohydrate diet with its increased acid load results in very little change in blood chemistry, and pH, but results in many changes in urinary chemistry: urinary calcium, undissociated uric acid, and phosphate are increased, while urinary magnesium, urinary citrate and pH are decreased.
All this increases the risk for kidney stones.

pH as a protective barrier

The human body has an amazing ability to maintain a steady pH in the blood with the main compensatory mechanisms being renal and respiratory.
The pH in the body vary considerably from one area to another. The highest acidity is found in the stomach (pH of 1.35 to 3.5) and it aids in digestion and protects against opportunistic microbial organisms. The skin is quite acidic (pH 4-6.5) and this provides an acid mantle as a protective barrier to the environment against microbial overgrowth (this is also seen in the vagina where a pH of less than 4.7 protects against microbial overgrowth).
The urine have a variable pH from acid to alkaline depending on the need for balancing the internal environment.

Metabolici Acidosis: pH of Selected Fluids, Organs, and Membranes
Fig. 2 – pH of Selected Fluids, Organs, and Membranes

References

Fenton T.R., Lyon A.W., Eliasziw M., Tough S.C., Hanley D.A. Meta-analysis of the effect of the acid-ash hypothesis of osteoporosis on calcium balance. J Bone Miner Res 2009;24(11):1835-1840. doi:10.1359/jbmr.090515

Fenton T.R., Lyon A.W., Eliasziw M., Tough S.C., Hanley D.A. Phosphate decreases urine calcium and increases calcium balance: a meta-analysis of the osteoporosis acid-ash diet hypothesis. Nutr J 2009;8:41. doi:10.1186/1475-2891-8-41

Fenton T.R., Tough S.C., Lyon A.W., Eliasziw M., Hanley D.A. Causal assessment of dietary acid load and bone disease: a systematic review and meta-analysis applying Hill’s epidemiologic criteria for causality. Nutr J 2011;10:41. doi:10.1186/1475-2891-10-41

Schwalfenberg G.K. The alkaline diet: is there evidence that an alkaline pH diet benefits health? J Environ Public Health 2012; Article ID 727630. doi:10.1155/2012/727630

Endurance sports and nutrition

In the last years endurance sports, defined in the PASSCLAIM document of the European Commission as those lasting 30 min or more, are increasing in popularity and competitions as half marathons, marathons, even ultramarathons, half Ironmans, or Ironman competitions attract more and more people.
Open water swimming, an endurance sportThey are competitions which can last hours, or days in the more extreme case of ultramarathons.
Athletes at all levels should take care of training and nutrition to optimize performance and to avoid potential health threats.
In endurance sports the most likely contributors to fatigue are dehydration and carbohydrate depletion (especially liver and muscle glycogen).

Dehydration and endurance sports

Dehydration is due to sweat losses needed to dissipate the heat that is generated during exercise. To prevent the onset of fatigue from this cause, the nutritional target is to reduce sweat losses to less than 2–3% of body weight; it is equally important to avoid drinking in excess of sweating rate, especially low sodium drinks, to prevent hyponatraemia (low serum sodium levels).

Glycogen depletion

Muscle glycogen and blood glucose are the most important substrates from which muscle obtains the energy needed for contraction.
Fatigue during prolonged exercise is often associated with reduced blood glucose levels and muscle glycogen depletion; therefore, it is essential starting exercise/competition with high pre-exercise muscle and liver glycogen concentrations, the last one for the maintaining of normal blood glucose levels.

Other problems which reduce performance and can be an health threat of the athlete, especially in long-distance races, are gastrointestinal problems, hyperthermia and hyponatraemia.
Hyponatraemia has occasionally been reported, especially among slower competitors with very high intakes of low sodium drinks.
Gastrointestinal problems occur frequently, especially in long-distance races; both genetic predisposition and the intake of highly concentrated carbohydrate solutions, hyperosmotic drinks, as well as the intake of fibre, fats, and proteins seem to be important in their occurrence.

References

Burke L.M., Hawley J.A., Wong S.H.S., & Jeukendrup A. Carbohydrates for training and competition. J Sport Sci 2011;29:Sup1,S17-S27. doi:10.1080/02640414.2011.585473

Saris W.H., Antoine J.M., Brouns F., Fogelholm M., Gleeson M., Hespel P., Jeukendrup A.E., Maughan R.J., Pannemans D., Stich V. PASSCLAIM – Physical performance and fitness. Eur J Nutr. 2003;42(Suppl 1):i50-i95. doi:10.1007/s00394-003-1104-0

Jeukendrup A.E. Carbohydrate feeding during exercise. Eur J Sport Sci 2008:2;77-86. doi:10.1080/17461390801918971

Jeukendrup A.E. Nutrition for endurance sports: marathon, triathlon, and road cycling. J Sport Sci 2011:29;sup1, S91-S99. doi:10.1080/02640414.2011.610348

Sawka M.N., Burke L.M., Eichner E.R., Maughan, R.J., Montain S.J., Stachenfeld N.S. American College of Sports Medicine position stand: exercise and fluid replacement. Med Sci Sport Exercise 2007;39:377-390. doi:10.1249/mss.0b013e31802ca597

Shirreffs S., Sawka M.N. Fluid and electrolyte needs for training, competition and recovery. J Sport Sci 2011;29:sup1, S39-S46. doi:10.1080/02640414.2011.614269

Fruits and vegetables in season

Numerous studies showed that seasonality plays a key role in optimizing the antioxidant properties of fruits and vegetables. For example, a recent Chinese study have investigated the influence of growing season (summer vs winter) on the synthesis and accumulation of phenolic compounds and antioxidant properties in five grape cultivars. The study showed that both phenolic compounds and antioxidant properties in the skin and seed of winter berries were significantly higher than those of summer berries for all of the cultivars investigated. Finally, to choose seasonal vegetables and fruits also ensures considerable saving of money.

List of vegetables and fruits in season

Fruits and Vegetables: Fruits in Season
Fig. 1 – Fruits in Season
Fruits and Vegetables: Vegetables in Season
Fig. 2 – Vegetables in Season

References

Xu C., Zhang Y., Zhu L., Huang Y., and Jiang Lu J. Influence of growing season on phenolic compounds and antioxidant properties of grape berries from vines Grown in Subtropical Climate. J Agric Food Chem 2011:59(4);1078-1086. doi:10.1021/jf104157z

Carbohydrate mouth rinse and endurance exercise performance

The importance of carbohydrates as an energy source for exercise is well known: one of the first study to hypothesize and recognize their importance was the study of Krogh and Lindhardt at the beginning of the 20th century (1920); later, in the mid ‘60’s, Bergstrom and Hultman discovered the crucial role of muscle glycogen on endurance capacity.
Nowdays, the ergogenic effects of carbohydrate supplementation on endurance performance are well known; they are mediated by mechanisms such as:

  • a sparing effect on liver glycogen;
  • the maintenance of glycemia and rates of carbohydrate oxidation;
  • the stimulation of glycogen synthesis during low-intensity exercise ;
  • a possible stimulatory effect on the central nervous system.

However, their supplementation, immediately before and during exercise, has an improving effect also during exercise (running or cycling) of a shorter and more intense nature: >75% VO2max (maximal oxygen consumption) and ≤1 hour, during which euglycaemia is rarely challenged and adequate muscle glycogen store remains at the cessation of the exercise.

Hypothesis for carbohydrate mouth rinse

In the absence of a clear metabolic explanation it was speculated that ingesting carbohydrate solutions may have a ‘non-metabolic’ or ‘central effect’ on endurance performance. To explore this hypothesis many studies have investigated the performance responses of subjects when carbohydrate solutions (about 6% carbohydrate, often maltodextrins) are mouth rinsed during exercise, expectorating the solution before ingestion.
By functional magnetic resonance imaging and transcranial stimulation it was shown that carbohydrates in the mouth stimulate reward centers in the brain and increases corticomotor excitability, through oropharyngeal receptors which signal their presence to the brain.
Probably salivary amylase releases very few glucose units from maltodextrins which is probably what is needed in order to activate the purported carbohydrate receptors in the oropharynx (no glucose transporters in the oropharynx are known).
However, the performance response appears to be dependent upon the pre-exercise nutritional status of the subject: most part of the studies showing an improving effect on performance was conducted in a fasted states (3- to 15-h fasting).
Only one study has shown improvements of endurance capacity; in both fed and fasted states by carbohydrate mouth rinse, but in non-athletic subjects.

References

Beelen M., Berghuis J., Bonaparte B., Ballak S.B., Jeukendrup A.E., van Loon J. Carbohydrate mouth rinsing in the fed state: lack of enhancement of time-trial performance. Int J Sport Nutr Exerc Metab 2009;19(4):400-409. doi:10.1123/ijsnem.19.4.400

Bergstrom J., Hultman E. A study of glycogen metabolism during exercise in man. Scand J Clin Invest 1967;19:218-228. doi:10.3109/00365516709090629

Bergstrom J., Hultman E. Muscle glycogen synthesis after exercise: an enhancing factor localized in muscle cells in man. Nature 1966;210:309-310. doi:10.1038/210309a0

de Salles Painelli V.S., Nicastro H., Lancha A. H.. Carbohydrate mouth rinse: does it improve endurance exercise performance? Nutrition Journal 2010;9:33. doi:10.1186/1475-2891-9-33

Fares E.J., Kayser B. Carbohydrate mouth rinse effects on exercise capacity in pre- and postprandial States. J Nutr Metab 2011, Article ID 385962. doi:10.1155/2011/385962

Krogh A., Lindhard J. The relative value of fat and carbohydrate as sources of muscular energy. Biochem J 1920;14:290-363. doi:10.1042/bj0140290

Rollo I. Williams C. Effect of mouth-rinsing carbohydrate solutions on endurance performance. Sports Med. 2011;41(6):449-461. doi:10.2165/11588730-000000000-00000

Blood glucose levels: the role of the liver and glucose-6-phosphatase

One of the main functions of the liver is to participate in the maintaining of blood glucose levels within well defined range (in the healthy state before meals 60-100 mg/dL or 3.33-5.56 mmol/L). To do it the liver releases glucose into the bloodstream in:

  • fasting state;
  • between meals;
  • during physical activity.

Blood glucose levels and hepatic glucose-6-phosphatase

In the liver, glycogen is the storage form of glucose which is released from the molecule not as such, but in the phosphorylated form i.e. with charge, the glucose 1-phosphate (this process is called glycogenolysis). The phosphorylated molecule can’t freely diffuse from the cell, but in the liver it is present the enzyme glucose-6-phosphatase that hydrolyzes glucose 6-phosphate, produced from glucose 1-phosphate in the reaction catalyzed by phosphoglucomutase, to glucose (an irreversible dephosphorylation).

glycogen(n glucose residues) + Pi → glucose 1-phosphate + glycogen(n-1 glucose residues)

glucose 1-phosphate ↔ glucose 6-phosphate

glucose 6-phosphate + H2O → glucose + Pi

Role of the liver and glucose 6-phosphatase in regulating blood glucose levelsThen, glucose can diffuse from the hepatocyte, via a transporter into the plasma membrane called GLUT2, into the bloodstream to be delivered to extra-hepatic cells, in primis neurons and red blood cells for which it is the main, and for red blood cells the only energy source (neurons, with the exception of those in some brain areas that can use only glucose as energy source, can derive energy from another source, the ketone bodies, which becomes predominant during periods of prolonged fasting).

Note: the liver obtains most of the energy required from the oxidation of fatty acids , a class of lipids, and not from glucose.
Glucose-6-phosphatase is present also in the kidney and gut but not in the muscle and brain; therefore in these tissues glucose-6-phosphate can’t be released from the cell.
Glucose-6-phosphatase plays an important role also in gluconeogenesis.

Glucose-6-phosphatase is present into the membrane of endoplasmic reticulum and the hydrolysis of glucose-6-phosphate occurs into its lumen (therefore this reaction is separated from the process of glycolysis). The presence of a specific transporter, the glucose-6-phosphate translocase, is required to transport the phosphorylated molecule from citosol into the lumen of endoplasmic reticulum. Although a glucose transporter is present into the membrane of endoplasmic reticulum, most of the released glucose is not transported back into the cytosol of the cell but is secreted into the bloodstream. Finally, an ion transporter transports back into the cytosol the inorganic phosphate released into the endoplasmic reticulum.

References

Nelson D.L., M. M. Cox M.M. Lehninger. Principles of biochemistry. 6th Edition. W.H. Freeman and Company, 2012

Roach P.J., Depaoli-Roach A.A., Hurley T.D and Tagliabracci V.C. Glycogen and its metabolism: some new developments and old themes. Biochem J 2012;441:763-787. doi:10.1042/BJ20111416

Stipanuk M.H., Caudill M.A. Biochemical, physiological, and molecular aspects of human nutrition. 3rd Edition. Elsevier health sciences, 2012

What is glycogen and why do we need it?

Glycogen is a branched polymer made up of D-glucose units, the most abundant monosaccharide in nature.
Due to the branched structure, glycogen is a compact and soluble macromolecule, has a low osmotic pressure and allows rapid release of the stored glucose when needed. As it is synthesized without a template, unlike proteins and nucleic acids it exists as a population of molecules with different chemical structures and sizes.
Glycogen molecule forms assemblages with proteins that are essential for its metabolism, such as glycogenin (EC 2.4.1.186), glycogen synthase (EC 2.4.1.11), glycogen phosphorylase (EC 2.4.1.1), debranching enzyme (EC 2.4.1.25 and EC 3.2.1.33), and those that regulate its anchoring to cytoskeleton and membranes. Such polysaccharide-proteins are called beta granules and are found in the cytosol of bacteria, Archea, Fungi and animal cells.
Phosphate groups are covalently linked to the polysaccharide, and, as well as associated proteins, seem to be involved in the regulation of its metabolism.
Glycogen is stored in cell in times of nutritional plenty and serves as storage form of glucose and therefore of energy. In animals, it is found in practically all cells and, in mammals, it is most abundant in the liver and skeletal muscle. In the liver, several beta granules arrange in a broccoli-like fashion to form the so-called alpha granules. Glycogen is also found in lysosomes.
In humans, it represents less than 1% of the body’s energy stores, and is essential in maintaining blood glucose homeostasis, too.
It is absent in plants, where starch is the storage form of glucose. Therefore, the polymerization of glucose represents a universal mechanism for energy storage.
From a human nutrition point of view, glycogen has little significance as after an animal has been killed it is mostly broken down to glucose and then to lactic acid. It should be noted that the acidity consequently to lactic acid production gradually improves the texture and keeping qualities of the meat. The only dietary sources are oysters and other shellfish that are eaten virtually alive; they contain about 5% glycogen.

CONTENTS

The discovery of glycogen

Glycogen was discovered in 1857 by the French physiologist Claude Bernard, considered the founder of experimental medicine. In the second half of the last century, studies on glycogen metabolism led to several significant discoveries, such as:

  • the reversible phosphorylation of proteins;
  • protein kinases and protein phosphatases;
  • the effect of insulin on the activity of intracellular enzymes.

In turn, this led to the award of four Nobel Prizes, three for physiology or medicine, to Carl Ferdinand Cori and Gerty Theresa Cori, née Radnitz in 1947, Earl Sutherland Jr. in 1971, and Edwin Krebs and Edmond Fischer in 1992, and one for chemistry, to Louis Leloir in 1970.

Chemical and molecular structure of glycogen

Individual glycogen molecule is a branched polymer of D-glucose in the pyranose form, namely, a stiff six-membered heterocyclic ring, five carbons and one oxygen, with chair conformation.
The central priming protein glycogenin and phosphate groups are covalently bound to the polysaccharide  chain.
Most of the glucose units are linked by α-1,4 glycosidic bonds, where each unit is linked to the next by a bond between C-1 of one unit and the hydroxyl group on the C-4 of the next unit, with an oxygen atom acting as a bridge between the two carbon atoms.
The branch points are introduced by α-1,6 glycosidic bonds, that occur approximately every 8-12 residues, again with an oxygen atom acting as a bridge between the two carbon atoms, in this case, C-1 and C-6, and with an average chain length of about 13 residues in mammals. Because each branch ends in a non-reducing residue, there are n+1 non-reducing ends in the molecule, where n is number of chains, but only one reducing end to which glycogenin is linked.
Note: in disaccharides, oligosaccharides, and polysaccharide the non-reducing end is the end that lacks a free anomeric carbon atom.

Chemical and molecular structure of glycogen

Having the same types of bonds, the primary structure of glycogen resembles that of amylopectin, which, with amylose, is one of the two polymers of D-glucose units composing starch. However, compared to amylopectin where branches occur every 25-30 glucose units, glycogen is more branched, and the branches are smaller.
Unlike proteins and nucleic acids, polysaccharides are synthesized without a template, resulting from the addition of monosaccharides or oligosaccharides to the growing structure. In addition, because branches occur without a precise localization, molecules with the same mass will not necessarily have the same structure. Hence, for each type of molecules there are different chemical structures. Moreover,  glycogen isolated from different biological sources exists as a population of molecules of different sizes. Therefore, the best way to describe its chemistry is to define the distribution of the molecular masses, and the average frequency with which branches occurs and their average length.
Finally, it should be emphasized that glycogen is not a static entity but constantly vary over the course of its existence.

As glucose is a chiral molecule, it exists as a pair of enantiomers, indicated according to the Fischer convention as D-glucose, the most widespread in nature and the monomeric unit of glycogen and starch, and L-glucose.

Who does stabilize 3D structure?

The folding into three-dimensional structures of macromolecules such as proteins, nucleic acids and polysaccharides is governed by the same principles: the monomeric units, namely, amino acids, nucleotides, and monosaccharides, with their more-or-less rigid structure, are joined by covalent bonds to form one dimensional polymers that spontaneously fold into three-dimensional structures stabilized by noncovalent interactions such as:

  • hydrogen bonds;
  • van der Waals interactions;
  • hydrophobic interactions;
  • ionic interactions, when charged subunits are present.

These interactions can occur within macromolecules or between macromolecules, as in supramolecular complexes such as cellulose or multienzyme complexes.
As the pyranose ring of glucose is a rigid structure, the three-dimensional conformation of the oligosaccharides and polysaccharides results from rotation about both C−O bonds of the glycosidic bond, with the bond angles labeled φ and ψ. However, there is no free rotation about each C−O bonds due to the steric interference by substituents. Hence, some conformations will be more stable than others. For amylose and glycogen, the most stable 3D structure is a tightly coiled helix stabilized by interchain hydrogen bonds.

What are the advantages of the branched structure?

The highly branched structure of glycogen offer several advantages.

  • The non-reducing ends present on the outermost tier can act as a substrate for glycogen phosphorylase. Therefore, many glycogen phosphorylases can work simultaneously allowing a rapid mobilization of stored glucose as glucose 1-phosphate.
  • The highly branched structure allows stored glucose to exert a much lower osmotic pressure than it would exert if it were in its monomeric form. For example, hepatocytes store an amount of glucose that, in free form, would have a concentration of ~0.4 M, against a glycogen concentration of ~0.01 mM. Therefore, if glucose were in free form, the resulting osmolarity would be so elevated to cause an osmotic entry of water that would lead to cell lysis. Moreover, as extracellular  concentration of glucose is ~5 mM, glucose uptake into a cell with a glucose concentration of 0.4 M would be particularly energetically expensive.
  • Branches allow the formation of compact granules.
  • If branches were absent or at least few, a very large number of long linear polymers would have to have to be present to have a number of non-reducing ends comparable to those present in glycogen and to store a comparable amount of glucose. This could cause cell damage. Evidence in favor of this hypothesis comes from a rare genetic disease, Anderson’s disease or amylopectinosis or glycogen storage disease type 4, due to mutations in the gene for branching enzyme. These mutations lead to a deficiency of the enzyme activity and accumulation in different tissues of abnormally branched glycogen that resembles amylopectin.
  • Branches allow glycogen to remain soluble, unlike starch.

Whelan’s model

Due to the action of glycogenin, and then of glycogen synthase and branching enzyme (EC 2.4.1.18), glycogen molecule grows exponentially in concentric tiers around the glycogenin core. According to Whelan’s model of glycogen structure, two types of glucose chain can be categorized:

  • A-chains, unbranched and present only on the surface;
  • B-chains, internal and, on average, with two branching points.

It has been calculated that the maximum size would be of 12 tiers, for a diameter of ~42 nm, a total number of ~55,000 glucose units, and a molecular mass of ~107 kDa.
Moreover, considering that each tier has a thickness of 3.8 nm, assuming glycogen molecule to be a sphere, from the third to the twelfth tier:

  • the diameter increases by 5.4 times;
  • the volume, which grows according to the cube of its radius, increases 156 times;
  • carbohydrate content increases 45.6 fold;
  • the number of A-chains in each outermost tier increase exponentially, and is equal to 2n-1, where n corresponds to the number of the tier.

Distribution of glucose residues in a glycogen molecule

Considering skeletal muscle, the analysis of the size of glycogen molecules by electron microscopy showed the presence of few full-size particles, and an average diameter of ~25 nm, corresponding to seven tiers.
An important feature of the Whelan’s model  is that the outermost tier would contain, in the form of A-chains, ~50% of all of glucose molecules. This does not mean that these molecules are all accessible to glycogen phosphorylase because the enzyme stalls four residues from the branch point. The intervention of the debranching enzyme, whose activity is slower than glycogen phosphorylase activity, removes the branch and allows glycogenolysis to proceed.

Why is 13th tier not possible? The 13th tier seems to be not possible because of the steric hindrance due to high density of glucose units on the molecule surface. Such an high density of glucose residues would lead to insufficient space for the interaction between the catalytic region of glycogen metabolism enzymes, and then of glycogen synthase, too, and the growing chains.
Furthermore, through mathematical analyzes, it has been suggested that values concerning branch length, ~13 residues in mammals, average branching frequencies per tier, 2, and the maximum number of tiers, 12, are optimal for mobilization of the maximum amount of glucose molecules in the shortest possible time.

Glicogenin

The structure of the glycogen molecule includes the protein glycogenin, which is covalently bonded to the polysaccharide chain. Glycogenin initiates the synthesis of glycogen by autoglycosylation, catalyzing the addition of 7-11 glucose units to a specific tyrosine residue. This primer chain then acts as substrate for  glycogen synthase. In addition, binding to actin filaments, glycogenin anchors the oligosaccharide primer chain to the cytoskeleton.

Phosphate groups

In addition to glycogenin, glycogen molecule covalently binds phosphate groups.
For many years they were considered a contaminant and their amounts were inversely correlated with purity of the sample. Only in the early 1980s they were recognized as an integral part of the polysaccharide, where they seems to be linked to C-2 e C-3 as monoester, probably as a result of a side reaction during the activity of glycogen synthase.
Many studies have suggested that their presence plays a role in regulating glycogen metabolism, similarly to what happens for starch metabolism in plants. Evidence supporting this hypothesis are the identification of laforin, a glycogen phosphatases, and that its mutation is a key factor in Lafora disease, a form of epilepsy characterized, among other things, by an excessive phosphorylation of glycogen.
But how would they act? Several hypotheses have been proposed, and two are reported below.

  • It has been suggested that phosphate group, which are hydrophilic, could expose hydrophobic regions, reducing glycogen solubility. The dephosphorylation by laforin, allowing glycogen molecule to remain soluble, would facilitate branch formation.
  • Another hypothesis suggests that the degree of phosphorylation would be related to the age of the molecule, acting as a kind quality control. The increase in phosphorylation, which causes a reduction in the solubility of the polysaccharide, would be considered as a metabolic marker that directs it towards the lysosomal degradation, a process called glycophagy, rather than towards glycogenolysis.

Beta granules

Individual glycogen molecules are too small to be detected by light microscopy. Conversely, electron microscopy allowed to identify three types of structures: beta granules, gamma particles, and alpha granules.
Beta granules are made up of the polysaccharide, glycogenin and gamma-particles, which are protein-rich particles of ~3 nm in diameter. Beta granules have a molecular mass of ~106-107 kDa, a diameter of ~20-30 nm, with a rosette-like appearance.
They are considered a rapid energy source.
Under physiological conditions, proteins account for 66-80% of their weight. These proteins also bind to each other, to cytoskeleton or to membranes, and are all involved in the metabolism of the polysaccharide. Some of these are:

  • glycogen synthase, the debranching enzyme, and glycogen phosphorylase;
  • several regulatory proteins, such as:

laforin and phosphoprotein phosphatase 1 or PP1 (EC 3.1.3.17);

phosphorylase kinase (EC 2.7.11.19) and AMPK (EC 2.7.11.31);

the membrane anchoring protein STDB1; note that phosphorylase kinase binds the membranes, too;

malin or E3-ubiquitin ligase (EC 2.3.2.27), which binds to glycogen via laforin, and TRIM7.

Unlike the pyruvate dehydrogenase complex or ribosomes, the stoichiometry and the composition of the beta granules is not constant, but rather dynamic, as proteins associate or dissociate from the granule depending on cellular conditions. In addition, differences are observed not only between different cell types but also within the same cell type, for example in skeletal muscle cells depending on different subcellular localizations.

Alpha granules

In the liver, beta granules are organized to form structures called alpha granules.
They are made up of several beta granules, are ~108 kDa in molecular weight, up to ~300 nm in diameter, with a broccoli-like appearance.
Alpha granules are considered a slower energy source than beta granules.
To date, the mechanism underlying their formation is not yet clear, although it seems that beta granules are linked through a protein skeleton rich in disulfide bonds.

Where is glycogen found in humans?

In humans, glycogen occurs in all cells, although the main stores are found in the liver and skeletal muscle, where, depending on nutritional status, it can represent up to 10% of the liver mass and 2% of muscle mass. Therefore, skeletal muscle has a limited capacity to store glycogen than liver; however, as its mass is greater than that of the liver, the muscle content of glycogen is about double than that of the liver. For example, in a non-fasting 70 kg adult male there are ~100 g of glycogen in the liver and ~250 g in the muscle. Athletes can reach higher values, as in best male marathon runners, whose stores in liver and muscle are equal to ~475 g, corresponding to ~1,900 kcal.
The amount of glycogen stored are much lower than those of fats because fats are a form of energy storage much more efficient. Why?

  • They can be stored in anhydrous form, whereas the amount of water bound to glycogen is equal to 2-3 times its weight.
  • As the stored fats are insoluble in water they are osmotically inert;
  • The oxidation of one gram of glycogen yield about 4 kcal, whereas one gram of fat about 9 kcal, then about double the energy.

Why is glycogen important to humans?

Fats, proteins and glycogen are energy stores that the body uses when needed.
In animals, fats are second only to proteins as reserve of energy, although proteins are an energy store of last resort, such as during a prolonged fast.
In a healthy adult subject, fat mass accounts for about 21% of the body weight in man and 26% in woman. In an adult male of 70 kg body weight, fat mass is sufficient for about 2 months of the body’s energy expenditures. Conversely, glycogen stores are sufficient for about one day of the body’s energy expenditures. Nevertheless, glycogen is accumulated. Why?

  • Unlike glucose, fatty acids, a class of lipids cannot be metabolized anaerobically and therefore cannot be used to produce energy by skeletal muscle during anaerobic exercises. Moreover, muscle cannot oxidize fatty acids as quickly as it does with glucose stored in glycogen.
  • Animals cannot convert fatty acids into glucose, so they cannot be used to maintain glycemic homeostasis. Although glucose released from muscle glycogen remains within the cell, glucose released from hepatic glycogen, and to a lesser extent from renal glycogen, thanks to the enzyme glucose 6-phosphatase, enter the systemic circulation contributing to the regulation of blood glucose levels.
  • Glycogen has a specialized role in fetal lung type II pulmonary cells, or type II pneumocytes, the lung cells  susceptible to SARS-CoV-2. At about 26 weeks of gestation they start to accumulate glycogen that serves as major substrate for the synthesis of the lipids of pulmonary surfactant, of which dipalmitoylphosphatidylcholine is the major component.
  • The brain contains a small amount of glycogen, too, primarily in astrocytes. It accumulates during sleep and is mobilized upon waking, therefore suggesting its functional role in the conscious brain. These glycogen stores also provide a moderate degree of protection against hypoglycemia.

Glycogen and muscle work

Carbohydrates, namely glucose, and fatty acids are the main energy sources for muscle during exercise, and their relative contribution varies depending on the intensity and duration of exercise, as summarized below:

  • <30% VO2max: mainly fatty acids;
  • 40-60% VO2max: fatty acids and carbohydrates;
  • 75% VO2max: mainly carbohydrates;
  • >80% VO2max: almost exclusively carbohydrates.

Therefore, the contribution of glycogen to the energy needed to support muscle work increases with increasing exercise intensity, whereas that of fatty acids decreases. Furthermore, when no carbohydrates are ingested, performance is determined by glycogen stores in skeletal muscle and liver, whose relative consumption is different: as the intensity increases, muscle glycogen consumption increases whereas liver glycogen consumption remains more or less constant.

Note: the relative contribution of fatty acids and glycogen as energy sources also varies according to the athlete’s level of training.

References

Alberts B., Johnson A., Lewis J., Morgan D., Raff M., Roberts K., Walter P. Molecular Biology of the Cell. 6th Edition. Garland Science, Taylor & Francis Group, 2015

Beelen M., Burke L.M., Gibala M.J., van Loon J.C. Nutritional strategies to promote postexercise recovery. Int J Sport Nutr Exerc Metab 2010:20(6);515-32. doi:10.1123/ijsnem.20.6.515

Berg J.M., Tymoczko J.L., and Stryer L. Biochemistry. 5th Edition. W. H. Freeman and Company, 2002

Deng B., Sullivan M.A., Chen C., Li J., Powell P.O., Hu Z., Gilbert R.G. Molecular structure of human-liver glycogen. PLoS ONE 2016;11(3):e0150540. doi:10.1371/journal.pone.0150540

Fontana J.D. The presence of phosphate in glycogen. FEBS Lett 1980;1:109(1):85-92. doi:10.1016/0014-5793(80)81317-2

Garrett R.H., Grisham C.M. Biochemistry. 4th Edition. Brooks/Cole, Cengage Learning, 2010

Gentry M.S., Guinovart J.J., Minassian B.A., Roach P.J., and Serratosa J.M. Lafora disease offers a unique window into neuronal glycogen metabolism. J Biol Chem 2018;293(19):7117-25. doi:10.1074/jbc.R117.803064

Gunja-Smith Z., Marshall J.J., Mercier C., Smith E.E. and Whelan W.J. A revision of the Meyer-Bernfeld model of glycogen and amylopectin. FEBS Lett 1970:12(2);101-104. doi:10.1016/0014-5793(70)80573-7

Melendez R., Melendez-Hevia E., and Cascante M. How did glycogen structure evolve to satisfy the requirement for rapid mobilization of glucose? A problem of physical constraints in structure building. J Mol Evol 1997;45(4):446-55. doi:10.1007/PL00006249

Moran L.A., Horton H.R., Scrimgeour K.G., Perry M.D. Principles of Biochemistry. 5th Edition. Pearson, 2012

Nelson D.L., M. M. Cox M.M. Lehninger. Principles of biochemistry. 6th Edition. W.H. Freeman and Company, 2012

Prats C., Graham T.E., and Shearer J. The dynamic life of the glycogen granule. J Biol Chem 2018;293(19):7089-98. doi:10.1074/jbc.R117.802843

Roach P.J., Depaoli-Roach A.A., Hurley T.D and Tagliabracci V.C. Glycogen and its metabolism: some new developments and old themes. Biochem J 2012;441:763-87. doi:10.1042/BJ20111416

Rosenthal M.D., Glew R.H. Medical Biochemistry – Human Metabolism in Health and Disease. John Wiley J. & Sons, Inc., 2009

Shearer J. and Graham T.E. Novel aspects of skeletal muscle glycogen and its regulation during rest and exercise. Exerc Sport Sci Rev 2004;32(3):120-6. doi:10.1097/00003677-200407000-00008

Stipanuk M.H., Caudill M.A. Biochemical, physiological, and molecular aspects of human nutrition. 3rd Edition. Elsevier health sciences, 2013 [Google eBooks]

Voet D. and Voet J.D. Biochemistry. 4th Edition. John Wiley J. & Sons, Inc. 2011

Whelan W.J. Enzymic explorations of the structures of starch and glycogen. Biochem J 1971;122(5):609-622. doi:10.1042/bj1220609

How to reduce body fat

The international scientific literature is unanimous in setting the lower limit for the daily caloric intake to 1200 kcal for women and 1500 kcal for men (adults).
To make negative the daily caloric intake, and therefore lose body weight, but expecially lose body fat, evaluation of actual caloric needs of the subject will be alongside:

  • the correct distribution of meals during the day;
  • an increased physical activity, by which the negative balance can be achieved without major sacrifices during meals.
Body Fat
Daily Caloric Balance

This will make weight loss easier and protect from subsequent weight gains and yo-yo effect.
Ultimately, there must be a change in lifestyle.

CONTENTS

Body fat and “miracle diets”

So, the best strategy for losing body fat is not a drastic reduction in caloric intake, nor follow constrictive or “strange” diets, such as hcg diet plan, sacred heart diet, paleo diet, Master Cleanse diet (the diet that Beyonce did), etc., that require to eliminate or greatly reduce the intake of certain macronutrients, mostly carbohydrates.
Such conducts can be:

  • very stressful from psychological point of view;
  • not passable for long periods;
  • hazardous to health because of inevitable nutrient deficiencies.

Finally, they do not ensure that all the weight lost is only or almost only body fat and are often followed by substantial increases in body weight and/or by yo-yo effect.
Why?

Body fat and reduction of energy intake

An excessive reduction of energy intake means eating very little and this determines the risk, high, not to take adequate amounts of the various essential nutrients, that is, what we can’t synthesize, such as vitamins, certain amino acids, some fatty acids, a class of lipids, and minerals, including e.g. calcium, essential for bone metabolism at every stage of life, or iron, used in many body functions as the transport of oxygen to the tissues. This results in a depression of metabolism and hence a reduction in energy expenditure.
Whether the reduction in energy intake is excessive, or even there are periods of fasting, it adds insult to injury because a proportion of free fatty mass will be lost. How?

Reduction in energy intake and role of carbohydrates

Glucose is the only energy source for red blood cells and some brain areas, while other brain areas can also derive energy from ketone bodies, which are a product of fatty acid metabolism.
At rest, brain extracts 10% of the glucose from the bloodstream, a significant amount, about 75 mg/min., considering that its weight is about 1.5 kg. To maintain a constant glycemia, and thus ensure a constant supply of glucose to tissues, we needs to take carbohydrates or alternatively amino acids, both easily obtained from foods.
In the case of a low or absent dietary intake of carbohydrates, whereas after about 18 hours liver glycogen, which releases glucose into circulation, depletes, body synthesizes de novo glucose from certain amino acids through a process called gluconeogenesis (actually this metabolic pathway is active even after a normal meal but increases its importance in fasting).
But what’s the main source of amino acids in the body when their dietary intake is low or absent? Endogenous proteins, and there is a hierarchy in their use that is before we consume the less important and only after the most important ones. For the first digestive enzymes, pepsin, chymotrypsin, elastase, carboxypeptidase and aminopeptidase (around 35-40 g) will be used; successively liver and pancreas slow down their synthesis activities for export proteins and unused amino acids are directed to gluconeogenesis. It’s clear that these are quite modest reserves of amino acids and it is the muscle that will undertake to provide the required amounts of amino acids that is proteolysis of muscle proteins begins.
Note: Anyway, there is no absolute sequentiality in the degradation of several proteins, there is instead a plot in which, proceeding, some ways lose their importance and others will buy. So, to maintain constant glycemia the protein component of muscle is reduced, including skeletal muscle that is a tissue that represents a fairly good portion of the value of the basal metabolism and that, with exercise, can significantly increase its energy consumption: thus essential for weight loss and subsequent maintenance. It is as if the engine capacity was reduced.
One thing which we don’t think about is that heart is a muscle that may be subject to the same processes seen for skeletal muscle.
Ultimately make glucose from proteins, also food-borne, is like heat up the fire-place burning the furniture of the eighteenth century, amino acids, having available firewood, dietary carbohydrates.
Therefore, an adequate intake of carbohydrates with diet prevents excessive loss of proteins, namely, there is a saving effect of proteins played by carbohydrates.
Mammals, and therefore humans, can’t synthesize glucose from fats.

What goes in when carbohydrates goes out?

The elimination or substantial reduction in carbohydrate intake in the diet results in an increased intake of proteins, lipids, including cholesterol, because it will increase the intake of animal products, one of the main defects in hyperproteic diet.
In the body there are no amino acids reserves, thus they are metabolized and, as a byproduct of their use, ammonia is formed and it’ll be eliminated as toxic. For this reason high-protein diets imply an extra work for liver and kidneys and also for this they are not without potential health risks.
An increased fat intake often results into an increased intake of saturated fatty acids, trans fatty acids, and cholesterol, with all the consequences this may have on cardiovascular health.
What has been said so far should not induce to take large amounts of carbohydrates; this class of macronutrients should represent 55-60% of daily calories, fats 25-30% (primarily extra-virgin olive oil) and the remainder proteins: thus a composition in macronutrient that refers to prudent diet or Mediterranean Diet.

Body fat and the entry in a phase of famine/disease

A excessive reduction in caloric intake is registered by our defense mechanisms as an “entry” in a phase of famine/disease.
The abundance of food is a feature of our time, at least in industrialized countries, while our body evolved over hundreds of thousands of years during which there was no current abundance: so it’s been programmed to try to overcome with minimal damage periods of famine. If caloric intake is drastically reduced it mimics a famine: what body does is to lower consumption, lower the basal metabolism, that is, consumes less and therefore also not eating much we will not get great results. It is as if a machine is lowered the displacement, it’ll consume less (our body burns less body fat).

Yo-yo effect

Yo-yo effect or weight cycling, namely, repeated phases of loss and weight gain, appears related to excess weight and accumulation of fat in the abdomen.
Several studies suggest a link in women with:

  • increased blood pressure;
  • hypercholesterolemia;
  • gallbladder disease;
  • significant increase in binge eating disorder;
  • a sense of depression with regard to weight.

Lastly, yo-yo effect is related to a greater easy to gain weight than those who are not subject to it. In this regard there should be emphasized that the weight cycling occurs over years, during which, aging, the rate of metabolism inevitably tends to decrease: this could make more difficult the subsequent losses.

References

Cereda E., Malavazos A.E., Caccialanza R., Rondanelli M., Fatati G. and Barichella M. Weight cycling is associated with body weight excess and abdominal fat accumulation: a cross-sectional study. Clin Nutr 2011;30(6):718-723. doi:10.1016/j.clnu.2011.06.009

Montani J-P., Viecelli A.K., Prévot A. & Dulloo A.G. Weight cycling during growth and beyond as a risk factor for later cardiovascular diseases: the ‘repeated overshoot’ theory. Int J Obes (Lond) 2006;30:S58-S66. doi:10.1038/sj.ijo.0803520

Ravussin E., Lillioja S., Knowler W.C., Christin L., Freymond D., Abbott W.G.H., Boyce V., Howard B.V., and Bogardus C. Reduced rate of energy expenditure as a risk factor for body-weight gain. N Engl J Med 1988;318:467-472. doi:10.1056/NEJM198802253180802

Sachiko T. St. Jeor S.T. St., Howard B.V., Prewitt T.E., Bovee V., Bazzarre T., Eckel T.H., for the AHA Nutrition Committee. Dietary Protein and Weight Reduction. A Statement for Healthcare Professionals From the Nutrition Committee of the Council on Nutrition, Physical Activity, and Metabolism of the American Heart Association. Circulation 2001;104:1869-1874. doi:10.1161/hc4001.096152