Glycolysis: steps, enzymes, 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.^[5]

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, glycolysis 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.^[2][5]

Contents

• The discovery
• Why is it so important?
• Sequence of reactions
  • Reaction 1
    • The importance of glucose phosphorylation
    • Other possible fates of glucose 6-phosphate
  • Reaction 2
  • Reaction 3
  • Reaction 4
  • Reaction 5
  • Reaction 6
  • Reaction 7
    • What is substrate-level phosphorylation?
  • Reaction 8
    • Synthesis of 2,3-bisphosphoglycerate and the Rapoport-Luebering pathway
  • Reaction 9
  • Reaction 10
• Fate of NADH and pyruvate
  • Lactic acid fermentation
  • Alcoholic fermentation
  • Fate of pyruvate and NADH under aerobic conditions
    • Anabolic fates of pyruvate
• Glycolysis and ATP production
  • ATP production under anaerobic conditions
  • ATP production under aerobic conditions
• Feeder pathways
  • Glycogen and starch
  • Fructose
    • Fructose and hexokinase
    • Sorbitol
  • Galactose
  • Mannose
• Regulation of glycolysis
  • Hexokinase
    • Comparison of the kinetic properties of hexokinase isoenzymes
    • Regulation of the activity of hexokinases I-III
    • Regulation of the activity of hepatic glucokinase
  • Regulation of phosphofructokinase 1 activity
  • Regulation of pyruvate kinase activity
• References

The discovery

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.^[2][5]
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 glycolysis.
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 it 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.^[7]
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 a 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.

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 glycogen synthesis, and the synthesys of fatty acids, triglycerides, nucleotides, some amino acids, and 2,3-bisphosphoglycerate (2,3-BPG).

Sequence of reactions

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.^[2][5]
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.^[2][5]

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 glycogenolysis.

Reaction 1

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 MgATP^2-, 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 P_i 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 glucose has some functions.^[2]

• 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. 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.

• It can be metabolized by the pentose phosphate pathway, that provides cells with:

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

In the second step of the glycolysis, 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

These molecules are an example of functional group isomerism.
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 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

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 (PP_i) 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 + PP_i → Fructose 1,6-bisphosphate + P_i

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

In the fourth step of the glycolysis, 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 K_m 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

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 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 percent of the triose 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 K_cat/K_M 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 K_eq 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 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 source of glycerol 3-phosphate in adipose tissue and small intestine.

Reaction 6

In the sixth step of the glycolysis, 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⁺ + P_i ⇄ 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

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

ΔG°’ is of -18.5 kJ/mol (-4.4 kcal/mol), namely, it is an exergonic reaction, whereas 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 P_i.
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 + P_i + 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

In the eighth step of the glycolysis, 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. 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.
In red blood cells this reaction is catalyzed by the bisphosphoglycerate mutase,^[6] 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.

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

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 P_i, 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

In the final step of the glycolysis, 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.

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 P_i, 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.

Fate of NADH and pyruvate

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.^[2] 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.^[5]

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 P_i + 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 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 P_i + 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.
Pyruvate, in the mitochondrial matrix, is first converted to acetyl-CoA in the reactions catalyzed by the pyruvate dehydrogenase complex, one of the mitochondrial multienzyme complexes. In the reaction, an oxidative decarboxylation, pyruvate loses a carbon atom as CO₂, and the remaining two carbon units iare 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.

Glycolysis and ATP production

In the glycolytic pathway the glucose molecule is degraded to two molecules of pyruvate.
During 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.^[5]

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 P_i, 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 P_i → 2 ATP + 2 H₂O

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

Glucose + 2 NAD⁺ + 2 ADP + 2 P_i → 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 percent.
Notice that the overall equation of glycolysis can also be derived by considering all the reagents, ATP, NAD⁺, ADP, and P_i and all the products.

Glucose + 2 ATP + 2 NAD⁺ + 4 ADP + 2 P_i → 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.

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 percent, [(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 percent of such free energy, [(61/183.6) x 100] is stored within ATP, whereas it is 41.8 percent 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 P_i is much higher than that required under standard conditions, namely, approximately 50 percent of the energy released is stored within ATP.

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

Other carbohydrates besides glucose, both simple and complex, can be catabolized via glycolysis, after enzymatic conversion to one of the glycolytic intermediates.^[2][7]
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.

At the intestinal level, starch and disaccharides encounter the enzymes responsible for carbohydrate digestion. Then, the absorption of monosaccharides released or derived from diet occur. 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 catalyze 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. The enzyme is also part of the polyol pathway, for the conversion of glucose 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.
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⁺

The sugar 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</p

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. 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.^[7] A such fine regulation could not be achieved if a single enzyme operates in both directions.
The control of the glycolysis involves essentially the reactions catalyzed by hexokinase, PFK-1, and pyruvate kinase, whose activity is regulated through:

• allosteric modifications, that occur on a timescale of milliseconds and are instantly reversible;
• covalent modifications, that is, phosphorylations and dephosphorylations, that occur on a timescale of seconds;
• changes in enzyme concentrations, resulting from changes in the rate of their synthesis and/or degradation, that occur on a timescale 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.^[2][5]
Hexokinase I is the predominant isozyme in the brain, whereas in skeletal muscle hexokinase I and II are present, accounting for 70-75 percent and 25-30 percent of the isozymes, respectively.
Hexokinase IV, also known as glucokinase (EC 2.7.1.2), is mainly present in hepatocytes and beta 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.^[1]

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 K_m 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.^[7]
Conversely, glucokinase has a high K_m 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 K_m 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 V_max.

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 the regulation of blood glucose levels.^[1] Why?

• As previously said, glucokinase has a K_m for glucose of about 10 mM, much higher than the K_m 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 K_m 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 K_m 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 K_m 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 beta 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.^[4] The mechanism of inhibition by GKRP occurs via the anchorage of glucokinase inside the nucleus, where it is separated from the other glycolytic enzymes.

  

  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 glycolysis.
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, P_i and fructose 2,6-bisphosphate are allosteric activators.^[8][9]

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 P_i 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 K_m 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 percent 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, K_eq, of the reaction is:

K_eq = [ATP][AMP]/[ADP]²= 0.44

Under normal conditions, ADP and AMP concentrations are about 10 percent and often less than 1 percent 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 isn’t 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.
• Another metabolite involved in the control of the flow of carbon through glycolysis and gluconeogenesis is xylulose 5-phosphate,^[3] 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.^[5][7]
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.

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 K_m 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

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