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. There 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.

See also agonist.