Antagonist

An antagonist is a ligand that binds to a protein and prevents stimulation of the protein by an agonist. Usually, binding of an antagonist is competitive and reversible, i.e. binding occurs to the same site at which agonists bind. While antagonist is bound, agonist can not bind and activate the receptor; the agonist must wait until the antagonist dissociates from the protein.

The effects of a competitive antagonist can be overcome by increasing the concentration of agonist. The reason antagonists have a therapeutic effect when administered to patients is because, in the body, receptors are being continuously (or regularly) exposed to endogenous agonist, so the presence of antagonist reduces the cells’ response to the endogenous agonist through competition.

If the antagonist binds reversibly to an allosteric site, there is no direct competition for binding, and effects of the antagonist can not be overcome by increasing the agonist concentration.

A few antagonists can bind irreversibly to the agonist binding site. Several enzyme inhibitors bind in this way, to the substrate binding site on the enzyme. Inhibition (the term used to specify antagonism of an enzyme) can not be overcome by increasing concentrations of an agonist or substrate.