A competitive inhibitor of an enzyme will
A. alter the Vmax of the reaction
B. bind to the same site as the substrate
C. decrease the apparent Km for the substrate
D. decrease the turnover number
E. form an irreversible complex with the enzyme
The correct answer is B. Substances that reduce the activity of an enzyme are called inhibitors. Reversible inhibitors bind to an enzyme but rapidly dissociate from it [in contrast to irreversible inhibitors (choice E), which bind tightly and dissociate very slowly from the enzyme]. There are several types of reversible inhibitors:
Competitive inhibitors usually resemble the substrate and compete with it for binding at the active site (choice B). Thus, increasing the concentration of substrate will decrease the percent inhibition of the enzyme. The Vmax is unchanged, but the Km is increased.
A noncompetitive inhibitor binds with equal affinity to both enzyme and enzyme-substrate complex. This binding leads to a distortion of the substrate binding site, so new substrate cannot bind and/or the product cannot be released. In this kind of inhibition, the Vmax is decreased (choice A), but the Km is not altered. Adding more substrate will not reverse this type of inhibition. This is the equivalent of decreasing the turnover number (choice D).
An uncompetitive inhibitor does not bind to free enzyme, but binds to the enzyme-substrate complex at a site other than the catalytic site. Once bound by the inhibitor, the enzyme is trapped in the enzyme-substrate complex state until the inhibitor dissociates. In this kind of inhibition, the slope of the reaction (which is the ratio Km/Vmax) remains the same, but both Vmax (choice A) and Km (choice C) are reduced.