In biochemistry and pharmacology, an allosteric modulator (allo- from the Greek meaning "other") is a substance which indirectly influences (modulates) the effects of an agonist or inverse agonist at a target protein, for example a receptor. Allosteric modulators bind to a site distinct from that of the orthosteric agonist binding site. Usually they induce a conformational change within the protein structure. A positive allosteric modulator (PAM), which is also called an allosteric enhancer, induces an amplification of the agonists effect. A negative modulator (NAM) reduces the effects of the orthosteric ligand, but is inactive in the absence of the orthosteric ligand. Substances that occupy the allosteric binding site and are functionally neutral are called silent allosteric modulators (SAMs). Classic benzodiazepines are well-known PAMs.
The modulatory activity can be of first-order or of second-order (or both). Second-order modulators alter the modulatory activity of first-order modulators. (−)‐Epigallocatechin‐3‐gallate is one such example of a second-order modulator at GABAA receptors.
From modulators the allosteric agonists are to be distinguished. They are defined as ligands able to directly activate a receptor by binding to a receptor site distinct from the primary (orthosteric) site, the allosteric site. They compete for occupancy of the allosteric receptor, and can bind in the absence of an orthosteric ligand. This is distinct from an allosteric modulator in that it directly produces an effect, where as modulators induce an indirect effect, thus have no direct effect of their own. Ago-allosteric modulators are both allosteric agonists and allosteric modulators.
An ago-allosteric modulator acts as an agonist and an enhancer for endogenous agonists in increasing agonist potency (dose range over which a response is produced) and providing "superagonism". Superagonism results when the efficacy is greater than 100 percent. Ago-allosteric modulators can be neutral, negative, or positive. Neutral ago-allosteric modulators increase efficacy, but have no affect on potency. A negative ago-allosteric modulator has a negative effect on the potency but a positive effect on the efficacy of an agonist. A positive ago-allosteric modulator increases both efficacy and potency.
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