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In biochemistry, a receptor is a protein on the cell membrane or within the cytoplasm or cell nucleus that binds to a specific molecule (a ligand), such as a neurotransmitter, hormone, or other substance, and initiates the cellular response to the ligand. Ligand-induced changes in the behavior of receptor proteins result in physiological changes that constitute the biological actions of the ligands.
- 1 Binding and activation
- 2 Agonists versus antagonists
- 3 Overview
- 4 Peripheral membrane protein receptors
- 5 Transmembrane receptors
- 6 Intracellular receptors
- 7 Role in Genetic Disorders
- 8 Receptor Regulation
- 9 See also
- 10 External links
Binding and activation
- (the brackets stand for concentrations)
A measure of how well a certain molecule fits into a given receptor is the binding affinity which is measured as the dissociation constant Kd (good fit means high affinity and a low Kd). The activation of the second messenger cascade and the final biological response is achieved only when at a certain time point a significant number of receptors are activated by bound ligands.
Agonists versus antagonists
Not every ligand that binds to a receptor l also activates the receptor. The following classes of ligands exist:
- (Full) agonists are able to activate the receptor and result in a maximal biological response. Most natural ligands are full agonists
- Partial agonists are not able to activate the receptor maximally, resulting in a partial biological response compared to a full agonist.
- Antagonists bind to the receptor but do not activate it. This results in a receptor blockade that inhibits the binding of agonists.
- Inverse agonists are antagonists that are able to further reduce the receptor activation by decreasing its basal activity
The shapes and actions of receptors are newly investigated by the X-ray crystallography and computer modelling. This increases the current understanding of drug action at binding sites on the receptors.
Receptors exist in different types, dependent on their ligand and function:
- Some receptor proteins are peripheral membrane proteins;
- Many hormone receptors and neurotransmitter receptors are transmembrane proteins: transmembrane receptors are embedded in the lipid bilayer of cell membranes, that allow the activation of signal transduction pathways in response to the activation by the binding molecule, or ligand.
- Another major class of receptors are intracellular proteins such as those for steroid and intracrine peptide hormone receptors. These receptors often can enter the cell nucleus and modulate gene expression in response to the activation by the ligand.
Peripheral membrane protein receptors
G protein-coupled receptors
These receptors are also known as seven transmembrane receptors or 7TM receptors.
- Muscarinic acetylcholine receptor (Acetylcholine and Muscarine)
- Adenosine receptors (Adenosine)
- Adrenoceptors (also known as Adrenergic receptors, for adrenaline, and other structurally related hormones and drugs)
- GABA receptors, Type-B (γ-Aminobutyric acid or GABA)
- Angiotensin receptors (Angiotensin)
- Cannabinoid receptors (Cannabinoids)
- Cholecystokinin receptors (Cholecystokinin)
- Dopamine receptors (Dopamine)
- Glucagon receptors (Glucagon)
- Metabotropic glutamate receptors (Glutamate)
- Histamine receptors (Histamine)
- Olfactory receptors (for the sense of smell)
- Opioid receptors (Opioids)
- Rhodopsin (a photoreceptor)
- Secretin receptors (Secretin)
- Serotonin receptors, except Type-3 (Serotonin, also known as 5-Hydroxytryptamine or 5-HT)
- Somatostatin receptors (Somatostatin)
- Calcium-sensing receptor (Calcium)
- Chemokine receptors (Chemokines)
- many more ...
Receptor tyrosine kinases
These receptors detect ligands and propagate signals via the tyrosine kinase of their intracellular domains. This family of receptors includes;
- Erythropoietin receptor (Erythropoietin)
- Insulin receptor (Insulin)
- Eph receptors
- Insulin-like growth factor 1 receptor
- various other receptors for growth factors & cytokines
Guanylyl cyclase receptors
- GC-A & GC-B: receptors for Atrial-natriuretic peptide (ANP) and other natriuretic peptides
- GC-C: Guanylin receptor
- Nicotinic acetylcholine receptor (Acetylcholine, Nicotine)
- Glycine receptor (GlyR) (Glycine, Strychnine)
- GABA receptors: GABA-A, GABA-C (GABA)
- Glutamate receptors: NMDA receptor, AMPA receptor, and Kainate receptor (Glutamate)
- 5-HT3 receptor (Serotonin)
The entire repertoire of human plasma membrane receptors is listed at the Human Plasma Membrane Receptome (http://receptome.stanford.edu).
- sigma1 (neurosteroids))
- IP3 receptor (inositol triphosphate, IP3)
- G protein-coupled receptors PMID 16902576
Role in Genetic Disorders
Many genetic disorders involve hereditary defects in receptor genes. Often, it is hard to determine whether the receptor is nonfunctional or the hormone is produced at decreased level; this gives rise to the "pseudo-hypo-" group of endocrine disorders, where there appears to be a decreased hormonal level while in fact it is the receptor that is not responding sufficiently to the hormone.
Cells can increase (upregulate) or decrease (downregulate) the number of receptors to a given hormone or neurotransmitter to alter its sensitivity to this molecule. This is a locally acting feedback mechanism.
- Signal transduction
- Schild regression for ligand receptor inhibition
- Ki Database
- Wikipedia:MeSH D12.776#MeSH D12.776.543.750 --- receptors.2C cell surface
- IUPHAR GPCR Database and Ion Channels Compendium
- Cell+surface+receptors at the US National Library of Medicine Medical Subject Headings (MeSH)