Competitive inhibition

Competitive inhibition occurs when an inhibitor competes with the normal substrate for the same binding site on an enzyme, usually the active site. The inhibitor often looks like the substrate, so it can slip into the site, but the enzyme cannot convert it into product. While the inhibitor is bound, that enzyme molecule is blocked. Because the binding is usually reversible, substrate and inhibitor take turns at the site in proportion to their concentrations and how tightly each one binds.

Its hallmark is that a high substrate concentration overcomes it. With enough substrate, the inhibitor is crowded out and the enzyme still reaches its normal maximum rate, Vmax. What changes is the apparent Michaelis constant, Km: more substrate is needed to reach half of Vmax, so Km rises. On a Lineweaver-Burk plot, the lines with and without inhibitor cross the y-axis at the same point. This differs from noncompetitive inhibition, in which the inhibitor binds elsewhere on the enzyme and lowers Vmax.

Many drugs work this way. Methotrexate, used in cancer treatment, competes with folate for the enzyme dihydrofolate reductase.