What is a competitive inhibitor in enzymology?

• A. A molecule that speeds up an enzyme's activity.
• B. A molecule that degrades enzymes.
• C. A molecule that binds to an enzyme's active site, preventing substrate binding.
• D. A molecule that binds to an enzyme's product site enhancing product release.

Answer: (C)

A competitive inhibitor is a molecule that resembles the substrate enough to fit into the enzyme’s active site. By occupying that site, it blocks substrate binding and prevents catalysis until the inhibitor leaves. This can be overcome by increasing the substrate concentration, since the substrate can outcompete the inhibitor for the active site. In kinetics terms, competitive inhibition raises the apparent Km (lower apparent affinity) but does not change Vmax when substrate becomes plentiful. The idea fits best because it describes direct competition for the active site, rather than affecting the enzyme itself, degrading it, or interacting with product release.