What happens to Vmax when a noncompetitive inhibitor is present?

When a noncompetitive inhibitor is present, the maximum rate of reaction, known as Vmax, decreases. Noncompetitive inhibitors bind to an enzyme at a site other than the active site, which alters the enzyme's activity regardless of whether the substrate is bound. This binding reduces the overall number of functional enzyme molecules available to catalyze the reaction, hence lowering Vmax.

Unlike competitive inhibitors, which primarily affect the apparent affinity of the enzyme for the substrate but do not change Vmax, noncompetitive inhibitors lower the Vmax directly as they inhibit the enzyme's activity at all levels of substrate concentration. This means that even if you increase the substrate concentration, the maximum possible velocity of the reaction cannot be reached as long as the inhibitor is present. This fundamental effect on Vmax is a characteristic distinguishing feature of noncompetitive inhibition in enzyme kinetics.