Agonists, Antagonists & Allosteric Modulators

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Cellular receptors are essential proteins situated on or within the surface of cells, responsible for receiving and processing signals. These receptors bind with specific molecules, known as ligands, which are dispatched by other cells to trigger various biological responses. These ligands, termed agonists, can be of endogenous (originating from within the body) or exogenous (originating from outside the body) nature. Agonists bind to receptors and instigate a response. They exist on a spectrum of activity: full agonists deliver a response equal to that of the endogenous agonist, partial agonists offer weaker responses, while super agonists exhibit even greater efficacy than endogenous or full agonists. Another category, inverse agonists, uniquely decrease the basal receptor activity, setting them apart from antagonists.

In contrast to agonists, antagonists work by binding to receptors, but instead of activating them, they block and inhibit their function, preventing agonists from binding. Depending on their mechanism, antagonists can be classified as either competitive (binding directly to the receptor's binding site) or noncompetitive (binding to a different, allosteric site). An example of a noncompetitive antagonist is ketamine, which targets the NMDA-glutamate receptor. The binding of both agonists and antagonists to their receptors can be either temporary (reversible) or permanent (irreversible). Allosteric modulators operate by attaching to sites distinct from the primary receptor binding location, consequently altering the receptor's binding site conformation. This can either bolster or diminish receptor binding. Benzodiazepines serve as an example of allosteric modulators, amplifying the frequency of opening of GABA-A receptor ion channels when stimulated by GABA"

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What is the difference between an agonist and an antagonist?

An agonist refers to a molecule that activates a receptor to elicit a biological response, whereas an antagonist is a molecule that prevents the activation of the receptor by blocking the binding of agonists. Agonists act as a trigger, cueing the receptor for action, while antagonists act more like locks, blocking the receptor from being activated.

What is the definiton of an allosteric modulators in pharmacology?

'Allosteric modulators are substances that bind to a receptor at a site different from the primary active site. Instead of directly activating or inhibiting the receptor, they alter its sensitivity or response to the primary agent (agonist or antagonist). For example, benzodiazepines are allosteric modulators that enhance the response of GABA-A receptors when stimulated by GABA.

What distinguishes 'full', 'partial', and 'inverse' agonists from each other?

Full agonists bind to the receptor and elicit a maximum possible response, similar to what an endogenous agonist would elicit. Partial agonists are weaker and are only capable of generating a sub-maximal response, regardless of the amount consumed. Inverse agonists', on the other hand, suppress the baseline activity of a receptor. They have the opposite effect of agonists, reducing the receptor's activity below its normal basal level.

What is the difference between competitive and non-competitive antagonists?

Competitive antagonists bind to the same site as an agonist but do not activate the receptor, thus preventing agonist binding and activity. Non-competitive antagonists bind to a different site (allosteric site) and inhibit the receptor, preventing the binding of agonists regardless of their concentration. An example of a non-competitive antagonist is ketamine at the NMDA-glutamate receptor.