Antagonist

An antagonist is a chemical that binds to a receptor of a cell, but does not trigger a response by that cell. In pharmacology, antagonists have affinity but no efficacy for their receptors, and binding will disrupt the interaction and inhibit the function of an agonist or inverse agonist at receptors.

Antagonists mediate their effects by binding to the active site or to allosteric sites on receptors, or they may interact at unique binding sites not normally involved in the biological regulation of the receptor's activity. Antagonist activity may be reversible or irreversible depending on the longevity of the antagonist–receptor complex, which, in turn, depends on the nature of antagonist receptor binding. The majority of drug antagonists achieve their potency by competing with endogenous ligands or substrates at structurally defined binding sites on receptors.

Types of Antagonist

Antagonists

There are several types of antagonist:

• A competitive antagonist reversibly binds to receptors at the same binding site as the endogenous ligand or agonist, but without activating the receptor. Agonists and antagonists "compete" for the same binding site on the receptor. Once bound, an antagonist will block agonist binding.
• A non-competitive antagonist binds to either the active site of the receptor or binds to an allosteric site. Unlike competitive antagonists, which affect the amount of agonist necessary to achieve a maximal response but do not affect the magnitude of that maximal response, non-competitive antagonists reduce the magnitude of the maximum response that can be attained by any amount of agonist. This property earns them the name "non-competitive" because their effects cannot be negated, no matter how much agonist is present.

Partial Agonists

Although they are agonists, partial agonists can act as a competitive antagonist in the presence of a full agonist, as it competes with the full agonist for receptor occupancy. A partial agonist has an efficacy to the receptor that is lower than that of a full agonist, so the overall response is lowered.

Inverse Agonists

An inverse agonist is an agent that binds to the same receptor as an agonist but induces a pharmacological response opposite to that agonist. Although it acts upon the receptor as an agonist, the effects are opposite to what an agonist would produce.

See also

• Agonist
• Reuptake Inhibitor
• Releasing Agent

References

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