All of the following are true regarding glycolytic intermediates and their fates EXCEPT:

Question

Accepted Answer

A. 3-Phosphoglycerate is the first ATP-generating step in glycolysis via substrate-level phosphorylation. ## Glycolytic Intermediates: Fates and Checkpoints

### Correct Statements (Options 0, 1, 3)

**High-Yield:** Glucose-6-phosphate is a critical branch point. It can enter glycolysis OR be shunted to the pentose phosphate pathway (oxidative phase) to generate NADPH for reductive biosynthesis (fatty acids, cholesterol, nucleotides) and antioxidant defense (glutathione reduction).

**Key Point:** Aldolase cleaves fructose-1,6-bisphosphate into dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P) in equimolar amounts (1:1). DHAP is then isomerized to G3P, so both triose phosphates proceed through the lower glycolytic pathway.

**Clinical Pearl:** Pyruvate is a metabolic hub. It can be:
- **Transaminated** to alanine (glucose-alanine cycle during fasting)
- **Carboxylated** to oxaloacetate (gluconeogenesis, TCA cycle entry)
- **Oxidatively decarboxylated** to acetyl-CoA (fed state, lipogenesis)

### The Incorrect Statement (Option 2)

**Warning:** Option 2 contains a critical error in the **sequence of ATP generation**. The first ATP-generating step in glycolysis is NOT at 3-phosphoglycerate.

**Mnemonic:** **GAPDH Goes first** — **G**lyceraldehyde-**3**-**P**hosphate **D**ehydrogenase catalyzes the FIRST substrate-level phosphorylation, producing 1,3-bisphosphoglycerate, which then transfers its acyl phosphate to ADP via phosphoglycerate kinase to form ATP.

### Timeline of ATP Generation in Glycolysis

| Step | Enzyme | Substrate | Product | ATP Generated? |
|------|--------|-----------|---------|----------------|
| 6 | Glyceraldehyde-3-phosphate dehydrogenase | G3P | 1,3-bisphosphoglycerate | **FIRST ATP** (coupled) |
| 7 | Phosphoglycerate kinase | 1,3-bisphosphoglycerate | 3-phosphoglycerate | ATP ✓ |
| 10 | Pyruvate kinase | Phosphoenolpyruvate | Pyruvate | ATP ✓ |

**Key Point:** 3-Phosphoglycerate is the PRODUCT of the first ATP-generating step, not the site where ATP is generated. The high-energy acyl phosphate bond is in 1,3-bisphosphoglycerate (the immediate predecessor of 3-phosphoglycerate).

[cite:Lehninger Principles of Biochemistry 8e Ch 14; Stryer Biochemistry 9e Ch 16]

Answer

B. Fructose-1,6-bisphosphate is cleaved by aldolase into dihydroxyacetone phosphate and glyceraldehyde-3-phosphate in a 1:1 molar ratio

Answer

C. Glucose-6-phosphate can be diverted to the pentose phosphate pathway to generate NADPH for biosynthesis

Answer

D. Pyruvate can be transaminated to alanine, carboxylated to oxaloacetate, or oxidatively decarboxylated to acetyl-CoA

All of the following are true regarding glycolytic intermediates and their fates EXCEPT:

Explanation

Glycolytic Intermediates: Fates and Checkpoints

Correct Statements (Options 0, 1, 3)

The Incorrect Statement (Option 2)

Timeline of ATP Generation in Glycolysis

Practice similar questions

Join our NEET PG community

Step	Enzyme	Substrate	Product	ATP Generated?
6	Glyceraldehyde-3-phosphate dehydrogenase	G3P	1,3-bisphosphoglycerate	FIRST ATP (coupled)
7	Phosphoglycerate kinase	1,3-bisphosphoglycerate	3-phosphoglycerate	ATP ✓
10	Pyruvate kinase	Phosphoenolpyruvate	Pyruvate	ATP ✓