An enzyme's Km for substrate A is 0.5 mM. A competitive inhibitor raises the apparent Km to 2 mM without altering Vmax. A non-competitive inhibitor reduces Vmax by 50% without altering Km. If you add both inhibitors simultaneously, what happens to Vmax and apparent Km?

• A Vmax reduced to 25%; apparent Km raised to 4 mM — inhibitors synergize on both parameters
• B Vmax reduced to 50%; apparent Km raised to 2 mM — competitive and non-competitive effects are independent and additive on their respective parameters ✓
• C Vmax is unchanged; only Km is affected because competitive inhibitor dominates
• D Both Km and Vmax are halved because both inhibitors reduce enzyme efficiency equally

Explanation

Competitive inhibitors increase apparent Km (alpha × Km, where alpha = 1 + [I]/Ki) but do not affect Vmax (excess substrate can overcome inhibition). Non-competitive (pure) inhibitors reduce Vmax (alpha' × Vmax reduction) but do not affect true Km (inhibitor binds equally to E and ES). When both are present simultaneously, their effects on their respective parameters are independent: the competitive inhibitor still affects only apparent Km, and the non-competitive inhibitor still affects only Vmax. This reflects that they bind different sites and have different mechanisms.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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