Which type of enzyme inhibition is demonstrated by substrate inhibition, where high substrate concentrations decrease enzyme activity?

• A. Substrate inhibition
• B. Competitive inhibition
• C. Noncompetitive inhibition
• D. Uncompetitive inhibition

Answer: (A)

High substrate concentrations can slow enzyme activity when the substrate binds to a second, inhibitory site on the enzyme, forming a nonproductive complex such as E-S-S. As more substrate floods in, more enzyme molecules become trapped in these nonproductive forms, so the overall rate drops despite more substrate being available. This phenomenon is referred to as substrate inhibition, and it contrasts with other classic inhibition types: competitive inhibition involves a molecule that competes with the substrate for the active site and can be overcome by high substrate; noncompetitive inhibition involves an inhibitor binding to a site other than the active site to reduce Vmax regardless of substrate; uncompetitive inhibition involves an inhibitor binding only to the enzyme-substrate complex, lowering both Km and Vmax. In substrate inhibition, the key factor is that the substrate itself acts as the inhibitory species at high concentrations, decreasing catalytic productivity as substrate increases.