inhibition

Meaning: An inhibitor lowers enzyme activity. Reversible inhibitors bind noncovalently; irreversible ones usually bind covalently and permanently. Competitive inhibitors bind the active site and compete with substrate. Noncompetitive inhibitors bind an allosteric site on enzyme and enzyme-substrate complex equally, so less working enzyme is available. Uncompetitive inhibitors bind only the enzyme-substrate complex. Mixed inhibitors bind both forms with unequal affinity.

Equation: v = Vmax[S] / (Km(1 + [I]/Ki) + [S]) for competitive inhibition. v is initial rate (M/s), Vmax is maximum rate (M/s), [S] is substrate concentration (M), Km is the Michaelis constant (M), [I] is inhibitor concentration (M), Ki is the inhibitor dissociation constant (M).

On the MCAT: Passages give Michaelis-Menten or Lineweaver-Burk plots (1/v versus 1/[S]) with and without inhibitor. Competitive lines share the y-intercept (same Vmax) but have a different x-intercept. Pure noncompetitive lines share the x-intercept (same Km) and have a higher y-intercept. Uncompetitive lines are parallel. Feedback inhibition, where a pathway's end product inhibits an early enzyme, also appears.

Common confusion: Students say a competitive inhibitor lowers Vmax because it slows the reaction. Enough substrate outcompetes it, so Vmax is unchanged and only apparent Km rises. Extra substrate does not rescue noncompetitive inhibition.