The following points highlight the eight main glycolytic reactions. The reactions are: 1. Phosphohexose Isomerase 2. Phosphofructokinase 3. Aldolase or Fructose-Bisphosphate Aldolase 4. Glyceraldehyde-3-℗ Dehydrogenase 5. 3-Phosphoglycerate Kinase 6. Phosphoglycerate Mutase 7. Enolase 8. Pyruvate Kinase.
Reaction # 1. Phosphohexose Isomerase:
This enzyme isomerizes glucose-6-℗ (resulting, via glucose-1-℗, from phos-phorolysis of the reserve glycogen or from the phosphorylation of glucose having entered the cell) to fructose-6-℗ (see fig. 4-24).
This reaction is catalyzed by phosphoglucose isomerase (or glucose-6-phosphate isomerase), and there exists a very similar enzyme, phosphomannose isomerase (or mannose-6-phosphate isomerase), which transforms mannose-6-℗ (an epimer of glucose-6-℗, which differs from it only by the configuration of the hydroxyl on carbon 2: see fig. 4-5) also into fructose-6-℗.
Reaction # 2. Phosphofructokinase:
Fructose-6-℗, formed by isomerization of glucose-6-℗, or by phosphorylation of fructose (resulting from the hydrolysis of diet sucrose), undergoes a second phosphorylation and is converted into fructose-1, 6-bisphosphate (see fig. 4-25). As in the case of the reaction catalyzed by glucokinase or hexokinase, this is a transphosphorylation taking place at the cost of ATP.
The break of a phosphoanhydride bond is accompanied by the formation of a phosphoester linkage, which results in a considerable decrease in free energy (ΔG0 # – 4 kcal/mole), so that the reaction is also not reversible (here also it is a phosphatase which will permit the reconversion from 1, 6 bis-℗ to fructose 6-℗). We will see in the following that the activity of phosphofructokinase is subjected to an allosteric type of control which enables the regulation of glycolysis.
Reaction # 3. Aldolase or Fructose-Bisphosphate Aldolase:
This enzyme owes its name to the fact that it catalyzes an aldol condensation (the addition of an activated methylene group on the double bond of a carbonyl) or the reverse reaction (which is the case here in glycolysis).
Fructose, 1, 6-bis-℗ is thus split into 2 molecules of triosephosphates, glyceraldehyde-3-℗ and dihydroxyacetone-℗, which are isomers and interconvertible (like the hexose-phosphates; see above the first reaction of glycolysis) under the action of a triosephosphate-isomerase (see fig. 4-26).
The equilibrium of this isomerizaticn reaction is very largely in favour of dihydroxyacetone-℗ (96.5%), but gradually the equilibrium moves towards the formation of glyceraldehyde- 3-℗ which alone is implied in the following glycolytic reactions.
Thus, dihydroxyacetone-℗ is transformed into glyceraldehyde-3-℗, and while drawing the balance of glycolysis, it must be remembered that 2 molecules of the latter are in fact formed from each molecule of fructose 1, 6 bis-℗, i.e. from each molecule of glucose.
Reaction # 4. Glyceraldehyde-3-℗ Dehydrogenase:
As indicated by the name of this enzyme, this step will comprise an oxidation, the first in this sequence of glycolytic reactions; it will affect the aldehyde group of glyceraldehyde-3-℗ and one will finally obtain a mixed anhydride between the carbonyl thus formed and phosphoric acid.
Apparently, it seems that the following 3 reactions take place (illustrated in figure 4-27):
a) Reaction of the aldehyde group with a sulphydryl group of the enzyme (which must be in the reduced form to exert its catalytic activity) and formation of a thio-hemiacetal.
b) Oxidation (dehydrogenation) of this thio-hemiacetal to thioester by NAD+ linked to the enzyme.
c) Phosphorolysis of this thio-ester with formation of the mixed anhydride (1, 3-di-℗-glyceric acid) which has a “high potential energy” bond.
We will see in the following how, depending on the physiological conditions (aerobiosis or anaerobiosis), reoxidation of NADH can take place.
Reaction # 5. 3-Phosphoglycerate Kinase:
The mixed anhydride (or acyl-phosphate) formed will transfer its phosphate group to ADP to form ATP, thanks to the 3-phosphoglycerate kinase (whose name suggests the reverse reaction). This is a phosphorylation of ADP to ATP coupled with an oxidation, the mechanism (see fig. 3-8).
Reaction # 6. Phosphoglycerate Mutase:
This enzyme catalyzes the migration of the phosphate group and thus permits the conversion of 3-phosphoglyceric acid into 2-phosphoglyceric acid (see fig. 4-29).
Reaction # 7. Enolase:
It catalyzes the dehydration of 2-phosphoglyceric acid which is transformed into phosphoenolpyruvic acid, a compound of “high potential energy” (ΔG0 of hydrolysis ≠ -12.8 kcal/mole).
Reaction # 8. Pyruvate Kinase:
The phosphoenolpyruvic acid can transfer a phosphate group to ADP to form ATP and thus give pyruvic acid (it is probable that the enol form appears first and then undergoes a rearrangement to give the keto tautomeric form). The latter three reactions are illustrated in figure 4-29.
Glycolytic reactions can take place in aerobiosis or anaerobiosis; but the NAD+ content of cells is very low, so that glycolysis may be blocked at the stage of the oxidation of glyceraldehyde-3-℗, a step which requires NAD+ (see fig. 4-27), if there is no process enabling the reoxidation of NADH to NAD+.
We have already seen (see fig. 3-6) how this reoxidation takes place in aerobiosis, thanks to the electron carrier system and it should be remembered that for each molecule of NADH oxidized, 3 molecules of ATP are formed. But we must now examine what happens in anaerobiosis.
