How does a noncompetitive inhibitor decrease the rate of an enzyme-catalyzed reaction?

• A. It increases the enzyme's affinity for the substrate, thereby decreasing \(K_m\).
• B. It competes with the substrate for binding at the active site, increasing the apparent \(K_m\) but not affecting \(V_{max}\).
• C. It binds to an allosteric site on the enzyme, reducing the maximum velocity (\(V_{max}\)) without affecting the substrate's binding affinity (\(K_m\)).
• D. It binds irreversibly to the enzyme's active site, permanently inactivating the enzyme.

Answer: (C)

Verified step by step guidance

1. Understand the concept of enzyme inhibition: Noncompetitive inhibitors bind to an allosteric site on the enzyme, which is a site other than the active site. This binding alters the enzyme's structure or function, reducing its catalytic efficiency.
2. Recognize the key distinction between noncompetitive inhibition and other types of inhibition: Unlike competitive inhibitors, noncompetitive inhibitors do not compete with the substrate for the active site. Therefore, the substrate's binding affinity (\(K_m\)) remains unchanged.
3. Analyze the effect on reaction kinetics: Noncompetitive inhibitors decrease the maximum velocity (\(V_{max}\)) of the enzyme-catalyzed reaction because they reduce the number of functional enzyme molecules available for catalysis. This occurs regardless of substrate concentration.
4. Understand why \(K_m\) is unaffected: Since the inhibitor does not interfere with substrate binding at the active site, the enzyme's affinity for the substrate remains constant, and \(K_m\) is not altered.
5. Summarize the mechanism: Noncompetitive inhibitors bind to an allosteric site, causing a conformational change in the enzyme that reduces its catalytic activity. This results in a lower \(V_{max}\) while leaving \(K_m\) unchanged.