Receptor Agonist
Definition
A molecule that binds a receptor and switches it on, producing the kind of response the endogenous signal produces. Most peptide drugs are agonists at a receptor their parent hormone already occupies.
In more detail
An agonist binds a receptor and activates it, producing the response the receptor's natural ligand would. The term is relative to a system: the same molecule can be a full agonist at one receptor subtype and a partial agonist or an antagonist at another, which is why a compound is described as an agonist at a named target rather than in the abstract.
Two properties are involved and they vary independently. Affinity is how readily the molecule binds; efficacy is how much response follows once it has. A compound can bind tightly and activate weakly, which is what partial agonism describes, and that combination can reduce the response to the natural ligand by occupying the site.
What this means for research sourcing
Mechanistic terms like this describe how research compounds behave in vitro. See the mechanism overviews for compounds Volta stocks.
Related Terms in Pharmacology
Receptor Antagonist
A molecule that occupies a receptor without activating it, so the natural ligand cannot bind. The effect is to suppress a pathway rather than drive it.
Partial Agonist
An agonist that cannot produce the full response however much of it is present, because it activates the receptor less completely than the natural ligand does. Against a full agonist it can behave as a brake.
Dual Agonist
One molecule that activates two receptors. GLP-2 TZ is a GLP-1 and GIP agonist; survodutide combines GLP-1 with glucagon-receptor activity. The pairing is a design decision about which pathways to engage at once.
Triple Agonist
One molecule that activates three receptors. GLP-3 RT acts at the GLP-1, GIP and glucagon receptors, and it is the compound that put the class into clinical development.
Binding Affinity
How tightly a molecule holds its receptor, reported as a dissociation constant where a lower number means tighter binding. Affinity alone does not predict the size of a response: that also depends on how well the bound molecule activates the receptor.
EC50
The concentration producing half of a compound's maximum effect in a given assay. It is a potency measure and is only comparable between compounds tested in the same system, which is why cross-paper EC50 comparisons mislead so often.
Potency
How little of a compound is needed to produce a given effect. A more potent molecule works at a lower concentration, which says nothing about whether its maximum achievable effect is larger.
Receptor Selectivity
How far a compound acts on its intended receptor rather than on related ones. Selectivity is the difference between a GHRP that raises growth hormone alone and one that also moves cortisol and prolactin.
