Agonists are chemicals that bind to receptors on cell membranes and influence their activity. They can be endogenous or exogenous, and have varying levels of efficacy. Superagonists elicit a greater response than endogenous agonists, full agonists cause full activation, partial agonists cause partial activation, and inverse agonists cause the opposite effect. Receptor antagonists bind to receptors but do not affect their activity.
In biochemistry, agonists are all chemicals, including drugs, that show an affinity for a receptor on the cell membrane. Due to the affinity, the agonist can bind to the receptor and influence its activity within the cell. At the cellular level, receptors are found on the surface of the cell membrane and are usually an exposed part of a membrane protein. When a substance binds to a receptor, it causes a change in the receptor molecule, which can either initiate or inhibit its activity. Receptor agonists can positively or negatively affect the activity of the receptor to which they bind.
Inside the body, the receptors can be stimulated or inhibited by chemicals produced by the body, endogenous agonists; or those that are foreign or produced elsewhere, exogenous agonists. Examples of endogenous agonists include natural hormones, such as insulin and neurotransmitters. Neurotransmitters are chemicals produced by the body that are released by nerve cells to transmit nerve impulses from one nerve cell to another. Examples of neurotransmitters include adrenaline and dopamine.
The ability of receptor agonists to influence receptor activity is what makes them different from receptor antagonists. Receptor antagonists can also bind to receptors, but do not affect the receptor or its activity in any way. The amount to which a receptor agonist affects the activity of its target receptor is called efficacy. There is a broad spectrum of efficacy in receptor agonists.
In the spectrum of efficacy, there are four levels of different receptor agonists, which are ranked according to how much they affect the receptor when they bind to it. From most to least, the groups are: superagonist, full agonist, partial agonist, and inverse agonist. A superagonist is usually an exogenous receptor agonist. When it binds to and activates the receptor, it elicits a greater response than the endogenous agonist for that receptor. In other words, the response of the cell is greater than 100% when a superagonist binds to the target receptor.
Full receptor agonists cause the cell to become fully effective, or active, when they bind to the receptor. These types of agonists can be endogenous or exogenous. Examples of endogenous and exogenous agonists that are full agonists are endorphins and morphine, respectively. Endorphins are natural painkillers produced by the body, which bind to opioid receptors in the central nervous system. Morphine is a powerful pain reliever derived from opium poppies that mimics the action of endorphins and also activates opioid receptors.
Partial agonists bind to the target receptor, but cause only a partial increase in the cell’s activity compared to full or endogenous agonists. Finally, an inverse agonist binds to the receptor, but instead of activating it, it causes the opposite. Inverse agonists work in complete opposition to full or endogenous agonists causing the opposite effects to occur within a cell than when the full or endogenous agonist has activated the receptor.
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