Pharmacology Definitions

An agonist is any molecule, large or small, that is able to act as a ligand and bind to a receptor by one of a variety of mechanisms, such as covalent linkage, ionic interaction, hydrogen bonding, Van der Waals forces, hydrophobic interactions or the zipper mechanism, which results in the receptor being taken from its resting state to an activated state which consequently results in a response from the cell. If the agonist can give a maximum response it is called a full agonist, whereas a partial agonist is not able to achieve the maximum response even when all the receptors are occupied. This is due to differences in efficacy. Agonists have both affinity for the receptor and efficacy with respect to a response.