Which statement best contrasts competitive and noncompetitive inhibitors?

• A. Competitive inhibitors bind at the active site; noncompetitive inhibitors bind away from the active site, causing conformational change.
• B. Both inhibitors bind only at the active site.
• C. Competitive inhibitors increase Vmax; noncompetitive inhibitors decrease Km.
• D. Noncompetitive inhibitors compete with substrate for the active site.

Answer: (A)

Inhibition types hinge on where the molecule binds and how that affects the enzyme’s shape and activity. A competitive inhibitor sits right in the active site, blocking substrate binding by occupying the spot the substrate would use. A noncompetitive inhibitor, on the other hand, binds to a different site on the enzyme and causes a conformational change that lowers the enzyme’s catalytic efficiency, regardless of how much substrate is present.

This makes the statement correct: it captures the essential distinction—competitive inhibition targets the active site, while noncompetitive inhibition acts at an allosteric site to alter the enzyme’s activity. In terms of enzyme kinetics, competitive inhibition can be overcome by increasing substrate concentration, and it does not change the maximum rate (Vmax) of the reaction, though the apparent affinity (Km) for the substrate increases. Noncompetitive inhibition cannot be overcome by more substrate, and it lowers Vmax while Km remains essentially unchanged. The other statements blur these differences, misplacing where noncompetitive inhibitors bind or mis-stating the kinetic effects.