Antagonist

A compound (usually synthetic) that inhibits the action of its natural counterpart by binding to a receptor or protein. There are three classes of antagonist, chemical, physiological and pharmacological.

Pharmacological antagonists can be further characterised as:

• Competitive which binds to the binding site of a receptor - via the induced fit method - and can be overcome by increasing the concentration of an agonist (competitor). Example: Atropine, Tropicamide, Hyoscine (all these bind to muscarinic receptors)
• Irreversible competitive which forms a covalent bond with the receptor.
• Non-competitive antagonists which blocks the signal transduction event of the receptor. The term non-competitive antagonists are used to describe two different situations: one where the antagonist binds to the allosteric site of the receptor itself and the other is when the antagonist binds to the active site of the receptor.

A pharmacalogical example of an antagonist is Tamoxifen, a drug which acts on estrogen receptor and is often used in the treatment of breast cancer.

Chemical antagonism occurs when the drug in question causes an effect by preventing the formation of a chemical compound.

See also agonist.