Final answer:
A noncompetitive inhibitor decreases the maximal reaction rate (Vmax) of an enzyme-catalyzed process without affecting the substrate affinity (KM), because it binds at a separate site from the active site and induces a conformational change.
Step-by-step explanation:
The effect of a noncompetitive inhibitor on an enzyme-mediated reaction is to decrease the maximal reaction rate (Vmax), while the KM (Michaelis-Menten constant) remains unchanged. This occurs because a noncompetitive inhibitor binds to the enzyme at a site other than the active site, which induces a conformational change that affects the active site, even when the substrate is present.
As a result, adding more substrate does not overcome the inhibition, unlike with competitive inhibitors where increasing substrate can alleviate the inhibitory effects. Therefore, for a noncompetitive inhibitor, even if the substrate concentration increases, the enzyme activity can only reach a lower than normal maximum reaction rate.