In noncompetitive inhibition, what happens if more substrate is added?

• A. Rate increases
• B. Rate unchanged
• C. Rate not increased
• D. Rate decreases

Answer: (C)

In noncompetitive inhibition, the inhibitor binds to the enzyme at a site other than the active site and can bind whether or not the substrate is bound. This lowers the maximum rate of catalysis (Vmax) because some enzyme molecules become unable to turnover product, but it doesn’t change the enzyme’s affinity for the substrate (Km stays the same).

If more substrate is added, more enzyme–substrate complexes can form, so the rate does increase with substrate concentration. However, because part of the enzyme population is inhibited, the rate can never reach the original, uninhibited maximum. It approaches a reduced maximum instead.