competitive inhibition

competitive inhibition, in biochemistry, phenomenon in which a substrate molecule is prevented from binding to the active site of an enzyme by a molecule that is very similar in structure to the substrate. Thus, the inhibitor molecule and the substrate that the enzyme acts on “compete” for the same binding site. The degree of competitive inhibition is proportional to the amount of inhibitor present, and, because the inhibitor binds reversibly to the enzyme, its effects can be overcome through an increase in the concentration of the competing substrate.

Competitive inhibition has various effects on cells and living systems, all of which stem from a common mechanism—namely, decreased enzymatic activity that occurs via a reduction in catalytic efficiency, or the rate at which an enzyme converts a given substrate into a product. In living organisms, the products of enzymatic reactions are involved in numerous cell signaling and regulatory pathways, including those that initiate cellular activities, such as cell division and growth, and that regulate the function of tissues and organ systems. Therefore, a reduction in products arising from competitive inhibition can significantly alter cell and system function.

There are many examples of molecules that act as competitive inhibitors. For instance, the antimetabolite methotrexate, which is used to slow cancer growth, is similar in structure to the vitamin folic acid and competes with folic acid for binding sites on the enzyme dihydrofolate reductase. The enzyme-inhibitor complex that forms when methotrexate is bound to dihydrofolate reductase blocks the production of nucleic acids that are necessary for DNA synthesis. Thus, methotrexate is effective against cancer, because it disrupts DNA synthesis and inhibits cell division. Another example is penicillin, which is a competitive inhibitor that blocks the active site of an enzyme that certain types of bacteria use to construct their cell walls.