Which type of inhibition keeps Km constant while decreasing Vmax?

Non-competitive inhibition is characterized by its effect on enzyme kinetics, particularly concerning Vmax and Km. In this mode of inhibition, an inhibitor binds to an enzyme regardless of whether the substrate is present, leading to a reduction in the maximum rate of reaction (Vmax) without affecting the affinity of the enzyme for its substrate, which is represented by the Michaelis constant (Km).

When a non-competitive inhibitor is present, the binding of this inhibitor does not interfere with the binding of substrate to the enzyme, so the affinity (Km) remains unchanged. However, since the inhibitor reduces the overall number of available enzyme molecules for catalysis (even if the substrate is present), the maximum achievable rate of reaction (Vmax) is decreased.

This distinction is critical; the fact that Km stays constant indicates that the effectiveness of the enzyme at a given substrate concentration is not altered by non-competitive inhibition. Thus, even at high substrate concentrations, the presence of non-competitive inhibitors will not allow the enzyme to reach its maximum catalytic activity, thereby lowering Vmax but retaining Km.