A reduction in the rate of a catalyzed reaction by substances called inhibitors.

Inhibitors may work by poisoning catalysts for the reaction or by removing free radicals in a chain reaction. Enzyme inhibition affects biochemical reactions, in which the catalysts are enzymes.

Competitive inhibition occurs when the inhibitor molecules resemble the substrate molecules and bind to the active site of the enzyme, so preventing normal enzymatic activity.

Competitive inhibition can be reversed by increasing the concentration of the substrate.

In noncompetitive inhibition the inhibitor binds to a part of the enzyme or enzyme-substrate complex other than the active site, known as an allosteric site. This deforms the active site so that the enzyme cannot catalyze the reaction.

Noncompetitive inhibition cannot be reversed by increasing the concentration of the substrate.