Question of the day-Enzyme catalyzing Carbon-Carbon bond cleavage in Glycolysis

Which of the following enzymes cleaves a carbon-carbon bond in glycolysis?

A. Phospho triose isomerase

B. Aldolase

C. Phospho glycerate kinase

D. Enolase

E. Pyruvate Kinase

The correct answer is B- Aldolase.

The Aldolase cleaves fructose-1,6-bisphosphate (the product of 2nd glycolytic reaction catalyzed by Phospho fructo kinase -1) between the C-3 and C-4 carbons to yield two triose phosphates. The products are dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (figure).

Reactions of glycolysis

Figure- Reactions of glycolysis.

Phospho triose isomerase

Of the two products of the Aldolase reaction, only glyceraldehyde-3-phosphate goes directly into the second phase of glycolysis. The other triose phosphate, dihydroxyacetone phosphate, must be converted to glyceraldehyde-3-phosphate by the enzyme Phospho triose Isomerase also called Triose phosphate Isomerase (Figure-reaction-5). This reaction thus permits both products of the aldolase reaction to continue in the glycolytic pathway, and in essence makes the C-1, C-2, and C-3 carbons of the starting glucose molecule equivalent to the C-6, C-5, and C-4 carbons, respectively.  The triose phosphate Isomerase reaction completes the first phase of glycolysis, each glucose molecule that passes through being converted to two molecules of glyceraldehyde-3-phosphate.

Phospho glycerate kinase

The enzyme phosphoglycerate kinase (reaction-7) transfers a phosphoryl group from 1,3-bisphosphoglycerate to ADP to form an ATP. Because each glucose molecule sends two molecules of glyceraldehyde-3-phosphate into the second phase of glycolysis and because two ATPs were consumed per glucose in the first-phase reactions, the phosphoglycerate kinase reaction “pays off” the ATP debt created by the priming reactions. ADP has been phosphorylated to form ATP at the expense of a substrate, this is an example of substrate-level phosphorylation.The other kind of phosphorylation, oxidative phosphorylation, is driven energetically by the transport of electrons from appropriate coenzymes and substrates to oxygen.


Enolase catalyzes the formation of phosphoenolpyruvate from 2-phosphoglycerate (Figure-reaction-9). The reaction in essence involves a dehydration—the removal of a water molecule—to form the enol structure of PEP. The enzyme is strongly inhibited by fluoride ion in the presence of phosphate. Inhibition arises from the formation of fluorophosphate (FPO32-), which forms a complex with Mg2+ at the active site of the enzyme. A mixture of sodium fluoride and potassium oxalate is used while collecting the blood sample for glucose estimation. Sodium fluoride prevents glucose loss from the sample by preventing glycolysis.

Pyruvate Kinase

The second ATP-synthesizing reaction of glycolysis is catalyzed by pyruvate kinase, which brings the pathway at last to its pyruvate branch point. Pyruvate kinase mediates the transfer of a phosphoryl group from phosphoenolpyruvate to ADP to make ATP and pyruvate (Figure- reaction-10). For each glucose molecule in the glycolysis pathway, two ATPs are made at the pyruvate kinase stage (because two triose molecules were produced per glucose in the aldolase reaction).

Thus the most appropriate answer for C-C bond cleavage is Aldolase

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