In the presence of a competitive inhibitor, how does the affinity for the substrate change?

In the presence of a competitive inhibitor, the affinity of the enzyme for the substrate effectively decreases, but this is nuanced by the fact that the binding of the substrate is still possible. Competitive inhibitors bind to the active site of the enzyme, competing directly with the substrate for binding. This means that for a fixed concentration of enzyme and substrate, the likelihood of the substrate binding is reduced because some of the active sites are occupied by the inhibitor.

However, it's critical to note that this decrease in affinity is not permanent; increasing the concentration of substrate can outcompete the inhibitor for binding to the active site. Therefore, while the apparent affinity (as indicated by the Michaelis-Menten constant, Km) increases in the presence of a competitive inhibitor, the overall effect is that it appears as if the enzyme has a lower affinity for the substrate under these conditions.

This scenario highlights the classic concept of competition where the inhibitor can be overcome by high substrate concentrations, but as long as the inhibitor is present, the effective affinity is reduced because the substrate is less likely to bind when the inhibitor occupies the active site.