24. During glycolysis, fructose-1,6-diphosphate undergoes __________ to produce two C3 – fragments.
1. Retroaldol reaction
2. Oxidation
3. Isomerisation
4. Hydrolysis

What Happens to Fructose-1,6-Bisphosphate During Glycolysis?

Glycolysis is a central metabolic pathway that breaks down glucose to produce energy. One of its key steps involves the cleavage of fructose-1,6-bisphosphate (F-1,6-BP) into two three-carbon molecules. This reaction is both chemically interesting and biologically critical. Let’s dive into the nature of this transformation.

The Step in Focus: Cleavage of Fructose-1,6-Bisphosphate

During glycolysis, the enzyme aldolase acts on fructose-1,6-bisphosphate, a six-carbon sugar with phosphate groups at carbons 1 and 6. This molecule is split into two three-carbon (C3) fragments:

• Glyceraldehyde-3-phosphate (G3P)

• Dihydroxyacetone phosphate (DHAP)

What Type of Reaction Is It?

This cleavage is classified as a retroaldol reaction, which is the reverse of an aldol condensation. In organic chemistry, a retroaldol reaction involves breaking a carbon-carbon bond between an α- and a β-carbon, typically facilitated by an electron-withdrawing group such as a carbonyl or phosphate.

In this case:

• Aldolase catalyzes the retroaldol cleavage of the carbon-carbon bond between carbons 3 and 4 of fructose-1,6-bisphosphate.

• This produces the two triose phosphates needed for the continuation of glycolysis.

Analysis of Options:

1. Retroaldol reaction – ✅ Correct. This is the actual chemical mechanism catalyzed by aldolase.

2. Oxidation – ❌ Incorrect. Oxidation occurs later, when G3P is converted into 1,3-bisphosphoglycerate.

3. Isomerisation – ❌ Incorrect. Isomerisation happens when DHAP is converted into G3P by triose phosphate isomerase.

4. Hydrolysis – ❌ Incorrect. No water molecule is involved in cleaving the bond here.

✅ Correct Answer:

(1) Retroaldol reaction

Conclusion:

The cleavage of fructose-1,6-bisphosphate into two C3 fragments during glycolysis is a retroaldol reaction. This reaction is catalyzed by the enzyme aldolase, enabling the metabolic pathway to proceed efficiently toward ATP generation. Understanding the type of reaction involved helps clarify the biochemical logic and mechanism of this crucial step.