Definition
A molecule that binds a receptor and activates it, producing a biological response.
An agonist binds a receptor and stabilises its active state, triggering downstream signalling. Full agonists produce the maximal response the system can generate, while partial agonists produce a submaximal response even when every receptor is occupied, and can reduce the effect of a full agonist competing for the same site. Agonists are characterised by potency, often expressed as EC50, and by efficacy, the maximal response. Some agonists preferentially activate one signalling pathway over another at the same receptor, a property called biased agonism that is under active study for GPCRs. Many research peptides, including GLP-1 analogues, GHRH analogues and ghrelin-receptor ligands, are described by the receptors they activate.