In noncompetitive inhibition, where does the inhibitor bind?

• A. Active site
• B. Allosteric site
• C. Substrate binding site
• D. Cofactor site

Answer: (B)

Noncompetitive inhibition relies on the inhibitor binding somewhere other than the active site. The binding occurs at an allosteric site, a separate regulatory region on the enzyme. This interaction induces a conformational change that lowers the enzyme’s catalytic efficiency, so the reaction rate drops even if the substrate can still bind. Because substrate binding isn’t blocked, Km (the substrate affinity) stays the same, but Vmax decreases since fewer enzymes are effectively catalyzing the reaction. This is what distinguishes noncompetitive inhibition from competitive inhibition, where the inhibitor blocks the active site and affects substrate binding.