What blocks enzymes activity by binding to the site of an enzyme?

Competitive inhibitors reduce enzyme activity by binding (in competition with the enzyme's substrate) to the active site. These inhibitors may be reversible or irreversible. With reversible inhibitors, which may release the enzyme, concentrations much higher than the concentration of the substrate would be required to completely block enzyme activity, and even then one or two reactions may take place over long periods of time. With irreversible inhibitors, which permanently attach to the enzyme, enzyme activity could be completely blocked when the amount of inhibitor matches the amount of enzyme. Competitive inhibition reduces the enzymes ability to bind substrate (so it lowers the K_M) but does not alter the maximum rate (very high substrate concentrations would out compete for enzyme binding).

Other types of inhibitors work in other ways. Non-competitive inhibitors bind to the enzyme on a site other than the active site. These too may be reversible or irreversible. Binding does not compete with substrate, so concentrations to completely block enzyme activity do not have to be as high as reversible competitive inhibitors. Non-competitive inhibition reduces the apparent maximum rate for the enzyme.

Uncompetitive inhibitors bind only when the substrate is also bound to the enzyme (they bind to the enzyme-substrate complex). Both the maximum rate and substrate binding affinities appear lower.