Enzyme inhibition can be ______, ______, or ______ inhibition.

• A. Competitive
• B. Noncompetitive
• C. Uncompetitive
• D. Mixed

Answer: (C)

Enzyme inhibition is categorized by where the inhibitor binds relative to the substrate. The main reversible types you’ll see taught are competitive (binds free enzyme at the active site), noncompetitive (binds either free enzyme or ES with no preference in many cases, leaving Km unchanged), and uncompetitive (binds only to the enzyme–substrate complex). The statement can be completed with any of these classic types, and the trio often emphasized in teaching is competitive, noncompetitive, and uncompetitive (with mixed inhibition recognized as a separate, fourth type in some curricula).

Focusing on uncompetitive inhibition: the inhibitor binds only to the enzyme–substrate complex, forming a complex that cannot proceed to product. This sequesters ES and reduces the overall catalytic turnover, so Vmax decreases. Because the inhibitor stabilizes ES, the apparent affinity for substrate increases, causing Km to decrease as well. In kinetic plots, this yields parallel lines because both Vmax and Km are reduced by the same factor.