During noncompetitive inhibition, what happens to Km and Vmax?

• A. Km remains the same; Vmax decreases
• B. Km increases; Vmax remains the same
• C. Km decreases; Vmax increases
• D. Km increases; Vmax decreases

Answer: (A)

Noncompetitive inhibition changes how much enzyme is capable of turning substrate into product, not how tightly the substrate binds. The inhibitor binds to a site other than the active site and can attach to either the free enzyme or the enzyme–substrate complex, reducing the number of active catalytic sites. Because substrate binding to the active site isn’t affected, the affinity for substrate remains the same, so Km stays the same. But the overall catalytic capacity drops because some enzyme molecules are inhibited, so the maximum rate Vmax decreases. If Km were to change, that would imply altered substrate affinity (as seen in competitive or mixed inhibition), and if both Km and Vmax changed, that would point to a different inhibition type.