Which enzyme catalyzes the first committed step of glycolysis and is the primary site of allosteric regulation?

Question

Accepted Answer

A. Phosphofructokinase-1. ## Glycolysis Regulation: The Rate-Limiting Step

**Key Point:** Phosphofructokinase-1 (PFK-1) catalyzes the phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate and is the primary rate-limiting enzyme of glycolysis.

### Why PFK-1 is the Committed Step

While hexokinase catalyzes the first step (glucose → glucose-6-phosphate), glucose-6-phosphate is a branch point that can enter glycolysis, glycogen synthesis, or the pentose phosphate pathway. PFK-1 catalyzes an irreversible reaction that commits the carbon skeleton specifically to glycolysis.

### Allosteric Regulation of PFK-1

| Regulator | Effect | Mechanism |
| --- | --- | --- |
| ATP | Inhibition | Signals energy sufficiency |
| AMP / ADP | Activation | Signals energy depletion |
| Citrate | Inhibition | Signals adequate biosynthetic precursors |
| Fructose-2,6-bisphosphate | Activation | Most potent activator; fed state signal |
| H⁺ | Inhibition | Acidosis suppresses glycolysis |

**High-Yield:** PFK-1 is the most heavily regulated enzyme in glycolysis and is the textbook example of allosteric control in biochemistry exams.

**Mnemonic:** **"PFK is the COMMITTED step"** — it is the first irreversible step unique to glycolysis, not shared with other pathways.

Answer

B. Pyruvate kinase

Answer

C. Aldolase

Answer

D. Hexokinase

Which enzyme catalyzes the first committed step of glycolysis and is the primary site of allosteric regulation?

Explanation

Glycolysis Regulation: The Rate-Limiting Step

Why PFK-1 is the Committed Step

Allosteric Regulation of PFK-1

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Regulator	Effect	Mechanism
ATP	Inhibition	Signals energy sufficiency
AMP / ADP	Activation	Signals energy depletion
Citrate	Inhibition	Signals adequate biosynthetic precursors
Fructose-2,6-bisphosphate	Activation	Most potent activator; fed state signal
H⁺	Inhibition	Acidosis suppresses glycolysis