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Metabolism, Proteins

Multifunctional enzymes

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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;303-8. 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):17857-71

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 [Google eBook]

Metabolism, Proteins

Multienzyme complexes

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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:

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:

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+.

Multienzyme Complexes
Fig. 1 – The Five Reactions Catalyzed by the PDC

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.
Keto Acids
Fig. 2 – Keto Acids

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

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

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):268-79. 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):17857-71

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):193-7. 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):14802-07. doi:10.1073/pnas.011597698

Proteins

Gluten: definition, structure, properties, wheat, cereal list

7 Comments

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.
Gluten
Fig. 1 – 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
Fig. 2 – 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

Carbohydrates, Proteins

Digestion of starch and alpha-amylase

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alpha-amylase
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.
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 polysaccharides 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

Arienti G. “Le basi molecolari della nutrizione”. Seconda edizione. Piccin, 2003

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

Cozzani I. and Dainese E. “Biochimica degli alimenti e della nutrizione”. Piccin Editore, 2006

Giampietro M. “L’alimentazione per l’esercizio fisico e lo sport”. Il Pensiero Scientifico Editore, 2005

Mahan LK, Escott-Stump S.: “Krause’s foods, nutrition, and diet therapy” 10th ed. 2000

Mariani Costantini A., Cannella C., Tomassi G. “Fondamenti di nutrizione umana”. 1th ed. Il Pensiero Scientifico Editore, 1999

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

Shils M.E., Olson J.A., Shike M., Ross A.C. “Modern nutrition in health and disease” 9th ed., by Lippincott, Williams & Wilkins, 1999

Stipanuk M.H.. “Biochemical and physiological aspects of human nutrition” W.B. Saunders Company-An imprint of Elsevier Science, 2000

Nutrition and sport, Proteins

Daily protein requirements for athletes

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Proteins Requirements
Fig. 1 – Food High in Proteins

It is now accepted by athletes, coaches and athletic trainers that proper diet is one of the cornerstones for achieving better athletic performance. Despite this widely spread assumption, many, even at the highest levels, still believe that an high protein intake is fundamental in the athlete’s diet. This opinion is not new and is deeply rooted in the imaginary of many people almost as if, eating meat, even of big and strong animals, we were able to gain their strength and vitality too.
The function of proteins as energy-supplier for working muscle was hypothesized for the first time by von Liebig in ‘800 and it is because of his studies if, even today, animal proteins, and therefore meats, are often believed having great importance in the energy balance in the athlete’s diet, despite nearly two centuries in which biochemistry and sports medicine have made enormous progress.
Really, by the end of ‘800 von Pettenkofer and Voit and, at the beginning of ‘900, Christensen and Hansen retrenched their importance for energy purposes, also for the muscle engaged in sport performance, instead bringing out the prominent role played by carbohydrates and lipids.
Of course we shouldn’t think that proteins are not useful for the athlete or sedentary people. The question we need to answer is how many proteins a competitive athlete, engaged in intense and daily workouts, often two daily sessions (for 3-6 hours), 7/7, for more than 10 months a year, needs per day. We can immediately say that, compared to the general population, and with the exception of some sports, (see below) the recommended amount of protein is greater.

Metabolic fate of proteins at rest and during exercise

In a healthy adult subject engaged in a non-competitive physical activity, the daily protein requirements is about 0.85 g/kg desirable body weight, as shown by WHO.
Proteins turnover in healthy adults, about 3-4 g/kg body weight/day (or 210-280 g for a 70 kg adult), is slower for the muscle than the other tissues and decreasing with age, and is related to the amount of amino acids in the diet and protein catabolism.
At rest the anabolic process, especially of synthesis, uses about 75% amino acids while the remaining 25% undergoes oxidative process, that will lead to CO2 and urea release (for the removal of ammonia).
During physical activity, as result of the decreased availability of sugars, i.e. muscle glycogen and blood glucose used for energy purposes, as well as the intervention of cortisol, the percentage of amino acids destinated to anabolic processes is reduced while it increases that of amino acids diverted to catabolic processes, that is, it occurs an increase in the destruction of tissue proteins.
At the end of physical activity, for about two hours, anabolic processes remain low whereupon it occurs their sharp increase that brings them to values higher than basal ones, so, training induces an increase in protein synthesis even in the absence of an increase in proteins intake.

What determines the daily protein requirements?

There are many factors to be taken into account in the calculation of the daily protein requirements.

  • The age of the subject (if, for example, he/she is in the age of development).
  • Gender: female athletes may require higher levels as their energy intake is lower.
  • An adequate carbohydrate intake reduces their consumption.
    During physical activity, glucogenic amino acids may be used as energy source directly in the muscle, after their conversion to glucose in the liver through gluconeogenesis.
    An adequate carbohydrate intake before and during prolonged exercise lowers the use of body proteins.
  • The amount of carbohydrates stored in muscles and liver (glycogen) (see above).
  • The energy intake of the diet.
    A reduced energy intake increases protein requirements; conversely, the higher energy intake, the lower the amount of protein required to achieve nitrogen balance; usually there is a nitrogen retention of 1-2 mg per kcal introduced.
    If the athlete is engaged in very hard competition/workouts, or if he requires an increase in muscle masses (e.g. strength sports) nitrogen balance must be positive; a negative balance indicates a loss of muscle mass.
    The nitrogen balance is calculated as difference between the nitrogen taken with proteins (equal to: g. proteins/6.25) and the lost one (equal to: urinary urea in 24 hours, in g., x0.56]; in formula:

Nb (nitrogen balance) = (g. protein/6.25) – [urinary urea in 24 hours, in g., x0.56)]

  • The type of competition/workouts that the athlete is doing, either resistance or endurance, as well as the duration and intensity of the exercise itself.
    Resistance training leads to an increase in protein turnover in muscle, stimulating protein synthesis to a greater extent than protein degradation; both processes are influenced by the recovery between a training and the next one as well as by the degree of training (more training less loss).
    In the resistance and endurance performances the optimal protein requirements in younger people as for those who train less time are estimated at 1.3 to 1.5 g protein/kg body weight, while in adult athletes who train more time is slightly lower, about 1-1.2 g/Kg of body weight.
    Why?
    In subjects engaged in a hard physical activity, proteins are used not only for plastic purposes, which are incremented, but also for energy purposes being able to satisfy in some cases up to 10-15% of the total energy demand.
    Indeed, intense aerobic performances, longer than 60 minutes, obtain about 3-5% of the consumed energy by the oxidation of protein substrates; if we add to this the proteins required for the repair of damaged tissue protein structures, it results a daily protein demand about 1.2 to 1.4 g/kg body weight.
    If the effort is intense and longer than 90 minutes (as it may occur in road cycling, running, swimming, or cross-country skiing), also in relation to the amount of available glycogen in muscle and liver (see above), the amount of proteins used for energy purposes can get to satisfy, in the latter stages of a prolonged endurance exercise, 15% of the energy needs of the athlete.
  • The physical condition.
  • When needed, the desired weight.
    Athletes attempting to lose weight or maintain a low weight may need more proteins.

From the above, protein requirements don’t exceed 1.5 g/kg body weight, also for an adult athlete engaged in intense and protracted workouts, while if you consider the amount of protein used for energy purposes, you do not go over 15% of the daily energy needs.
So, it’s clear that diets which supply higher amounts (sometimes much higher) of proteins aren’t of any use, stimulate the loss of calcium in bones and overload of work liver and kidney. Moreover, excess proteins don’t accumulate but are used to fat synthesis.

How to meet the increased protein requirements of athletes

Protein Requirements
Fig. 2 – Road Cycling

A diet that provides 12 to 15% of its calories from protein will be quite sufficient to satisfy the needs of almost all of the athletes, also those engaged in exhausting workouts.
In fact, with the exception of some sports whose energy expenditure is low, close to that of sedentary subject (for example: shooting, or artistic and rhythmic gymnastics), athletes need a high amount of calories and, for some sports such as road cycling, swimming or cross-country skiing, it may be double/triple than that of a sedentary subject.
The increase in food intake is accompanied by a parallel increase in protein intake, because only a few foods such as honey, maltodextrin, fructose, sugar and vegetable oils are protein-free, or nearly protein-free.

Calculation of protein requirements of athletes

If you consider an energy demand of 3500 kcal/die, with a protein intake equal to 15% of total daily calories, you have:

3500 x 0.15 = 525 Kcal

As 1 gram of protein contains 4 calories, you obtain:

525/4 = 131 g of proteins

Dividing the number found by the highest protein requirements seen above (1.5 g/kg body weight/day), you obtain:

131/1.5 = 87 kg

that is, the energy needs of a 87 kg athlete engaged in intense workouts are satisfied.
Repeating the same calculations for a caloric intake of 5000 , you obtain 187 g of protein; dividing it by 1.5 the result is 125 kg, that is, the energy needs of a 125 kg athlete are satisfied.
These protein intakes can be met by a Mediterranean-type diet, without protein or amino acids supplements.

References

Giampietro M. L’alimentazione per l’esercizio fisico e lo sport. Il Pensiero Scientifico Editore. Prima edizione 2005

Protein and amino acid requirements in human nutrition. Report of a joint FAO/WHO/UNU expert consultation. 2002 (WHO technical report series ; no. 935).

Stipanuk M.H., Caudill M.A. Biochemical, physiological, and molecular aspects of human nutrition. 3rd Edition. Elsevier health sciences, 2013 [Google eBooks]