How do competitive and noncompetitive inhibitors affect Km and Vmax?

• A. Competitive inhibitors increase apparent Km (decrease affinity) with no change in Vmax; noncompetitive inhibitors decrease Vmax with no change in Km.
• B. Competitive inhibitors increase apparent Km (decrease affinity) with no change in Vmax; noncompetitive inhibitors decrease Vmax with no change in Km.
• C. Competitive inhibitors decrease Km and increase Vmax; noncompetitive inhibitors increase Km with no change in Vmax.
• D. Competitive inhibitors have no effect on Km or Vmax; noncompetitive inhibitors decrease Km.

Answer: (B)

This question focuses on how inhibitors alter Km and Vmax in enzyme kinetics. A competitive inhibitor competes with the substrate for the active site, so you need more substrate to reach half-max velocity. That makes the apparent Km higher, reflecting a lower apparent affinity, but Vmax remains the same because with enough substrate you can outcompete the inhibitor and achieve the same maximum rate. A noncompetitive inhibitor binds to an allosteric site and reduces the enzyme’s catalytic efficiency without blocking substrate binding, so the maximum rate decreases (fewer functional enzymes or slower turnover), while Km stays the same since substrate affinity isn’t directly affected. So the standard pattern is: competitive inhibitors raise Km with no change in Vmax, and noncompetitive inhibitors lower Vmax with no change in Km. This matches why the correct answer is that first effect on Km and Vmax, and the second on Vmax with Km unchanged.