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

Explanation

## The Committed Step of Glycolysis **Key Point:** Phosphofructokinase-1 (PFK-1) catalyzes the phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate and is the rate-limiting enzyme of glycolysis. ### Why PFK-1 is the Committed Step 1. **Irreversibility**: The reaction is highly exergonic (ΔG°' = −14.2 kJ/mol) and essentially irreversible under physiological conditions. 2. **Unique to glycolysis**: Unlike glucose-6-phosphate (which can enter other pathways like glycogen synthesis or the pentose phosphate pathway), fructose-1,6-bisphosphate is committed exclusively to glycolysis. 3. **No other pathway**: Fructose-1,6-bisphosphate cannot be diverted to gluconeogenesis or other major metabolic routes. ### Allosteric Regulation of PFK-1 | Regulator | Effect | Mechanism | |-----------|--------|----------| | AMP, ADP | Activators | Signal low energy state; increase enzyme activity | | ATP | Inhibitor | Signal high energy state; decrease enzyme activity | | Citrate | Inhibitor | Signal abundant biosynthetic precursors | | F-2,6-BP | Potent activator | Hormonal signal (insulin); most important regulator | | H^+^ | Inhibitor | Low pH decreases activity | **High-Yield:** PFK-1 is often called the "valve" of glycolysis because its regulation determines the rate of glucose breakdown. In exams, when asked about the rate-limiting step or allosteric regulation in glycolysis, PFK-1 is the answer. **Mnemonic:** **PFK = Primary Flux Keeper** — it controls the entry of glucose into the glycolytic pathway after the initial trapping step. ### Comparison with Other Glycolytic Enzymes | Enzyme | Role | Reversibility | Regulation | |--------|------|---------------|----------| | Hexokinase | Glucose trapping | Reversible | Product inhibition by G6P | | PFK-1 | Committed step | Irreversible | Allosteric (ATP, AMP, F-2,6-BP, citrate) | | Pyruvate kinase | Final ATP generation | Irreversible | Allosteric (F-1,6-BP, ATP, alanine) | | Aldolase | Cleavage | Reversible | No allosteric regulation | **Clinical Pearl:** Deficiency of PFK-1 causes glycogen storage disease type VII (Tarui disease), characterized by muscle weakness, hemolytic anemia, and exercise intolerance due to inability to generate ATP from glucose.