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.
Transmembrane receptor:E=extracellular space; I=intracellular space; P=plasma membrane
Receptors exist in different types, dependant 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.
- Metabotropic receptors receptors are coupled to G proteins and affect the cell indirectly through enzymes which control ion channels.
- Ionotropic receptors contain a central pore which functions as a ligand-gated ion channel.
- Another major class of receptors are intracellular proteins such as those for steroid hormone receptors. These receptors often can enter the cell nucleus and modulate gene expression in response to the activation by the ligand.
- 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.
These receptors are also known as seven transmembrane receptors or 7TM receptors.
- "Muscarinic" Acetylcholine receptors (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)
- many more ...
Receptor Tyrosine Kinases
These receptors detect ligands and propogate signals via the tyrosine kinase of their intracellular domains. This family of receptors includes;
- Erythropoietin receptor (Erythropoietin)
- Insulin receptor (Insulin)
- Eph receptors
- IGF-1 Receptor
- various other receptors for growth factors and cytokines * ...
Guanylyl Cyclase Receptors
- GC-A & GC-B: receptors for Atrial-natriuretic peptide (ANP) and other natriuretic peptides
- GC-C: Guanylin receptor
- Nicotinic acetylcholine receptors (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)
- Steroid hormone receptor:
- Sex hormone receptors (sex hormones)
- Estrogen receptor
- Androgen receptor
- Progesterone receptor
- Vitamin D receptor (vitamin D)
- Glucocorticoid receptor (glucocorticoids)
- Mineralocorticoid receptor (mineralocorticoids)
- Thyroid hormone receptor
- Retinoid receptor (vitamin A and related compounds);
- Peroxisome proliferator-activated receptors (PPARs)
- sigma1 (neurosteroids))
- IP3 receptor (inositol triphosphate, IP3)
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.