Phosphofructokinase-1 (PFK-1) is allosterically regulated by multiple effectors. A newly synthesized competitive inhibitor mimics ATP binding at the allosteric site rather than the substrate site. Which statement about this inhibitor's effect on PFK-1 kinetics is MOST accurate?
- A It decreases Vmax without altering Km for fructose-6-phosphate
- B It shifts the sigmoidal kinetics to hyperbolic by reducing cooperativity ✓
- C It increases Km for fructose-6-phosphate without decreasing Vmax
- D It decreases substrate affinity at the active site and reduces maximal velocity
Explanation
PFK-1 is a homotetramer exhibiting sigmoidal (cooperative) kinetics with its substrate fructose-6-phosphate because it is allosterically regulated. High ATP binds the allosteric site and reduces cooperativity, converting sigmoidal to more hyperbolic kinetics, effectively shifting the S0.5 rightward without a simple Km change. An inhibitor mimicking ATP at the allosteric site would similarly reduce cooperativity (Hill coefficient decreases), flattening the sigmoid. This is distinct from competitive inhibition at the active site (which shifts Km) or non-competitive inhibition (which decreases Vmax). Understanding cooperative enzyme regulation is clinically relevant since citrate and ATP are physiological inhibitors of PFK-1.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
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