inhibitor

Meaning: An enzyme inhibitor binds an enzyme and lowers its reaction rate. Reversible inhibitors bind noncovalently; irreversible ones bind covalently and permanently remove active enzyme. A competitive inhibitor resembles the substrate and occupies the active site. A noncompetitive inhibitor binds an allosteric site on E and ES equally well. An uncompetitive inhibitor binds only the enzyme-substrate complex. A mixed inhibitor binds both E and ES with different affinity.

Equation: Km,app = Km(1 + [I]/Ki) for competitive inhibition, where Km is the substrate concentration at half Vmax (M), [I] is the inhibitor concentration (M), and Ki is the inhibitor's dissociation constant (M). Vmax is unchanged.

On the MCAT: Passages show Michaelis-Menten or Lineweaver-Burk plots with and without inhibitor. Competitive: lines meet on the y-axis (same Vmax, larger Km). Noncompetitive: lines meet on the x-axis (lower Vmax, same Km). Uncompetitive: parallel lines (both Vmax and Km fall). Feedback inhibition, where a pathway's end product inhibits an early enzyme, also appears.

Common confusion: Adding more substrate overcomes only competitive inhibition, because substrate and inhibitor compete for the same site. Noncompetitive inhibition lowers Vmax at any substrate concentration, so extra substrate cannot rescue it.