Which is the most common site of regulation in glycolysis?

Question

Accepted Answer

B. Phosphofructokinase-1 (PFK-1) catalysing the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate. ## Primary Site of Glycolytic Regulation

**Key Point:** Phosphofructokinase-1 (PFK-1) is the rate-limiting enzyme and the most common site of regulation in glycolysis. This is the committed step of the pathway.

### Why PFK-1 is the Primary Control Point

1. **Committed step**: Unlike hexokinase (which produces G6P that can enter multiple pathways), PFK-1 catalyzes the first irreversible, pathway-specific reaction.
2. **Allosteric regulation**: PFK-1 is inhibited by ATP and citrate (signals of energy abundance) and activated by AMP and ADP (signals of energy depletion).
3. **Thermodynamically unfavorable**: The reaction has a large negative ΔG under cellular conditions, making it essentially irreversible and ideal for regulation.

### Regulation of Other Glycolytic Enzymes

| Enzyme | Type of Regulation | Significance |
|--------|-------------------|---------------|
| Hexokinase | Product inhibition by G6P | Secondary control; G6P can enter other pathways |
| PFK-1 | **Allosteric + covalent** | **PRIMARY CONTROL** |
| Pyruvate kinase | Allosteric (inhibited by ATP, alanine) | Tertiary control; feedback regulation |
| Aldolase | No significant regulation | Equilibrium enzyme; not rate-limiting |

**High-Yield:** PFK-1 is often called the "committed" enzyme because it catalyzes the first irreversible step unique to glycolysis. This is the most frequently tested regulatory concept in glycolysis.

**Mnemonic:** **PFK = Primary Flux Keeper** — it controls the rate at which glucose enters the main glycolytic pathway.

**Clinical Pearl:** In metabolic disorders affecting glycolysis (e.g., glycogen storage diseases), understanding PFK-1 regulation is crucial for predicting metabolic consequences.

Answer

A. Aldolase catalysing the cleavage of fructose-1,6-bisphosphate into DHAP and G3P

Answer

C. Hexokinase catalysing the conversion of glucose to glucose-6-phosphate

Answer

D. Pyruvate kinase catalysing the conversion of phosphoenolpyruvate to pyruvate

Which is the most common site of regulation in glycolysis?

Explanation

Primary Site of Glycolytic Regulation

Why PFK-1 is the Primary Control Point

Regulation of Other Glycolytic Enzymes

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Enzyme	Type of Regulation	Significance
Hexokinase	Product inhibition by G6P	Secondary control; G6P can enter other pathways
PFK-1	Allosteric + covalent	PRIMARY CONTROL
Pyruvate kinase	Allosteric (inhibited by ATP, alanine)	Tertiary control; feedback regulation
Aldolase	No significant regulation	Equilibrium enzyme; not rate-limiting