Allosteric modulator

In pharmacology and biochemistry, allosteric modulators are a group of substances that bind to a receptor to change that receptor's response to stimuli. Some of them, like benzodiazepines or alcohol, function as psychoactive drugs. The site that an allosteric modulator binds to (i.e., an allosteric site) is not the same one to which an endogenous agonist of the receptor would bind (i.e., an orthosteric site). Modulators and agonists can both be called receptor ligands.

Allosteric modulators can be 1 of 3 types either: positive, negative or neutral. Positive types increase the response of the receptor by increasing the probability that an agonist will bind to a receptor (i.e. affinity), increasing its ability to activate the receptor (i.e. efficacy), or both. Negative types decrease the agonist affinity and/or efficacy. Neutral types don't affect agonist activity but can stop other modulators from binding to an allosteric site. Some modulators also work as allosteric agonists and yield an agonistic effect by themselves.

The term "allosteric" derives from the Greek language. Allos means "other", and stereos, "solid" or "shape". This can be translated to "other shape", which indicates the conformational changes within receptors caused by the modulators through which the modulators affect the receptor function.

Allosteric modulators can alter the affinity and efficacy of other substances acting on a receptor. A modulator may also increase affinity and lower efficacy or vice versa. Affinity is the ability of a substance to bind to a receptor. Efficacy is the ability of a substance to activate a receptor, given as a percentage of the ability of the substance to activate the receptor as compared to the receptor's endogenous agonist. If efficacy is zero, the substance is considered an antagonist.

The site to which endogenous agonists bind to is named the orthosteric site. Modulators don't bind to this site. They bind to any other suitable sites, which are named allosteric sites. Upon binding, modulators generally change the three-dimensional structure (i.e. conformation) of the receptor. This will often cause the orthosteric site to also change, which can alter the effect of an agonist binding. Allosteric modulators can also stabilize one of the normal configurations of a receptor.

In practice, modulation can be complicated. A modulator may function as a partial agonist, meaning it doesn't need the agonist it modulates to yield agonistic effects. Also, modulation may not affect the affinities or efficacies of different agonists equally. If a group of different agonists that should have the same action bind to the same receptor, the agonists might not be modulated the same by some modulators.

A modulator can have 3 effects within a receptor. One is its capability or incapability to activate a receptor (2 possibilities). The other two are agonist affinity and efficacy. They may be increased, lowered or left unaffected (3 and 3 possibilities). This yields 17 possible modulator combinations. There are 18 (=2*3*3) if neutral modulator type is also included.

For all practical considerations, these combinations can be generalized only to 5 classes and 1 neutral: