Which statement best describes uncompetitive inhibition?

Which statement best describes uncompetitive inhibition?

• A. Binds to the active site; Km increases; Vmax unchanged.
• B. Binds to an allosteric site; Vmax decreases; Km unchanged.
• C. Binds only to the enzyme–substrate complex; lowers both Km and Vmax proportionally.
• D. Binds to the active site; Vmax decreases; Km increases.

Answer: (C)

Uncompetitive inhibition occurs when the inhibitor binds only to the enzyme–substrate complex, not to the free enzyme. By attaching to ES, it prevents the catalytic step, so fewer product molecules are formed and Vmax decreases. At the same time, binding to ES pulls the equilibrium toward more ES formation, which makes the apparent affinity for substrate seem higher, so Km decreases. The two parameters drop by the same factor, so their ratio stays the same and plots of 1/v versus 1/[S] appear parallel. This contrasts with competitive inhibition, which binds the active site and raises Km without changing Vmax; noncompetitive inhibition, which lowers Vmax without changing Km; and mixed inhibition, which alters both but not proportionally.

Uncompetitive inhibition occurs when the inhibitor binds only to the enzyme–substrate complex, not to the free enzyme. By attaching to ES, it prevents the catalytic step, so fewer product molecules are formed and Vmax decreases. At the same time, binding to ES pulls the equilibrium toward more ES formation, which makes the apparent affinity for substrate seem higher, so Km decreases. The two parameters drop by the same factor, so their ratio stays the same and plots of 1/v versus 1/[S] appear parallel. This contrasts with competitive inhibition, which binds the active site and raises Km without changing Vmax; noncompetitive inhibition, which lowers Vmax without changing Km; and mixed inhibition, which alters both but not proportionally.