What characterizes uncompetitive inhibition in enzymatic reactions?

Uncompetitive inhibition is characterized by the inhibitor binding only to the enzyme-substrate complex, not to the free enzyme. This leads to a unique effect on the kinetic parameters of the reaction. Specifically, it decreases both the Michaelis constant (Km) and the maximum velocity (Vmax) of the reaction.

When an uncompetitive inhibitor binds to the enzyme-substrate complex, it effectively prevents the complex from converting into product. As a result, the concentration of substrate needed to reach half of Vmax is reduced, thereby lowering Km. Since some enzyme-substrate complexes are sequestered by the inhibitor, the overall Vmax of the reaction is also decreased because fewer complexes are available to be converted into product at maximum substrate levels.

This distinguishes uncompetitive inhibition from other types of inhibition, such as competitive inhibition, which affects Km without changing Vmax, or non-competitive inhibition, which can lower Vmax without altering Km. Thus, the correct characterization of uncompetitive inhibition reflects its distinct impact on both Km and Vmax.