Noncompetitive inhibitors are defined as which of the following?

• A. Binds to the active site blocking substrate.
• B. Binds to the enzyme away from the active site altering its shape so the active site no longer functions.
• C. Competes with substrate for active site.
• D. Serves as a cofactor to increase activity.

Answer: (B)

Noncompetitive inhibitors bind to an allosteric site on the enzyme, away from the active site, and this binding changes the enzyme’s shape so the active site no longer catalyzes the reaction effectively, even when the substrate is present. Because they don’t block substrate binding directly, the substrate can still bind, but the enzyme’s activity is reduced, so the maximum rate (Vmax) decreases while Km is typically unchanged in pure noncompetitive inhibition. This description matches the mechanism given. The other ideas describe blocking the active site or competing for it, or refer to a molecule that enhances activity rather than inhibits.