Allosteric enzymes differ from Michaelis-Menten enzymes in having sigmoidal kinetics. The Hill coefficient (n) for a perfectly cooperative allosteric enzyme equals the:
- A Ratio Vmax/Km, representing catalytic efficiency
- B Number of subunits in the enzyme regardless of cooperativity
- C Number of cooperative binding sites; n > 1 indicates positive cooperativity, n = 1 indicates no cooperativity, n < 1 indicates negative cooperativity ✓
- D Turnover number (kcat) of the enzyme
Correct answer: C. Number of cooperative binding sites; n > 1 indicates positive cooperativity, n = 1 indicates no cooperativity, n < 1 indicates negative cooperativity
Explanation
The Hill equation v = Vmax × [S]^n / (K0.5^n + [S]^n) describes sigmoidal kinetics. The Hill coefficient n reflects the degree of cooperativity: n = 1 is Michaelis-Menten (hyperbolic, no cooperativity); n > 1 is positive cooperativity (substrate binding enhances further binding, e.g., haemoglobin n ≈ 2.8, ATCase); n < 1 is negative cooperativity (rare). In practice, n is a phenomenological parameter and equals the number of cooperative sites only if cooperativity is perfect. K0.5 is the substrate concentration at half-Vmax (analogous to Km).
Reference: Harper's Illustrated Biochemistry, 32nd ed.
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