How does the presence of a non-competitive inhibitor affect substrate saturation?

The presence of a non-competitive inhibitor does not affect the maximum rate of reaction achievable by the enzyme, which is determined by the enzyme's ability to bind to substrate. In the presence of a non-competitive inhibitor, the inhibitor can bind to the enzyme regardless of whether the substrate is bound or not. This means that even if the substrate concentration increases, the maximum reaction rate (Vmax) is lowered because the inhibitor effectively reduces the quantity of active enzyme available to catalyze the reaction.

However, the binding of the substrate to the enzyme remains unchanged, meaning that the enzyme still binds substrates at the same affinity (Km remains constant). Consequently, substrate saturation—the point at which increasing the substrate concentration does not lead to an increase in reaction rate—remains unchanged because it depends on the enzyme's ability to bind the substrate rather than the presence of the non-competitive inhibitor. Thus, the relationship between substrate concentration and enzyme activity is affected in terms of maximum reaction rate but not in the context of substrate saturation. Hence, the state of substrate saturation is unaffected when a non-competitive inhibitor is present.