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In biochemistry and pharmacology, a ligand is a substance that forms a complex with a biomolecule to serve a biological purpose.
Ligands include:
Substrates
Inhibitors
Activators
Signaling lipids
Neurotransmitters
A ligand that can bind to and alter the function of the receptor that triggers a physiological response is called a receptor agonist.
Endogenous agonists:
In general, receptors for small molecule neurotransmitters such as serotonin will have only one endogenous agonist, but often have many different receptor subtypes (e.g. 13 different receptors for serotonin). On the other hand, neuropeptide receptors (e.g. opioid receptors) tend to have fewer subtypes, but may have several different endogenous agonists. This allows for a high degree of complexity in the body's signalling system, with different tissues often showing quite distinct responses to a particular ligand.
Exogenous agonists:
- Drugs
- Full agonists bind to and activate a receptor with the maximum response that an agonist can elicit at the receptor. One example of a drug that can act as a full agonist is isoproterenol, which mimics the action of adrenaline at β-adrenoreceptors (see Beta-1 Adrenergic receptor. Another example is morphine, which mimics the actions of endorphins at μ-opioid receptors throughout the central nervous system.
A physiological agonist is a substance that creates the same bodily responses but does not bind to the same receptor.
Examples of agonists:
- acts as a 1alpha,25(OH)(2)D(3) superagonist of Vitamin D on Vitamin D receptor and exhibits both antiproliferative and prodifferentiating properties in vitro.
- in Transport inhibitor response 1 (3c6o). Water molecules are shown as red spheres.
- of human Bile acid receptor ligand-binding domain (deeppink) complex with non-steroidal agonist, nuclear receptor coactivator 1 peptide (cyan) and sulfate ions (PDB entry 3ruu). [1]
Ligands that bind to a receptor but fail to activate the physiological response are receptor antagonists.
- participates directly in agonist/competitive antagonist binding, affects activation gating, and is the portion that forms the 'middle' layer.
- in the structure of Glutamate receptor (GluA2).
- The small molecule [2], was studied as a treatment for stroke because it had demonstrated neuroprotective efficacy in experimental models of stroke and tolerability in healthy volunteers; however, in a multicenter, double-blind, randomized, placebo-controlled phase II trial, it was found to have significant sedative effects in patients with acute stroke which precludes its further development as a neuroprotective agent[3].
