In biochemistry, a receptor is a protein on the cell membrane or within the cytoplasm that binds to a specific factor (a ligand), such as a neurotransmitter, hormone, or other substance, and initiates the cellular response to the ligand. As all receptors are proteins, their structure is encoded into the DNA. Most hormone genes contain a short sequence that signals to the cell whether it needs to be transported to the cell membrane or it is to remain in the cytoplasm.
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.
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, embedded in the lipid bilayer of cell membranes. These receptors are either coupled to G proteins or possess enzyme or ion channel activity that allow the activation of signal transduction pathways in response to the activation by the ligand.
- 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.
Peripheral membrane protein receptors
G protein-coupled receptors (GPCRs)
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 (³-Aminobutyric acid or GABA)
- Angiotensin receptors (Angiotensin)
- Cannabinoid receptors (Cannabinoids)
- Cholecystokinin receptors (Cholecystokinin)
- Dopamine receptors (Dopamine)
- Glucagon receptors (Glucagon)
- 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)
- many more ...
Tyrosine kinase receptors
- Erythropoietin receptor (Erythropoietin)
- Insulin receptor (Insulin)
- various 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 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:
- Thyroid hormone receptor
- Retinoid receptor (vitamin A and related compounds);
- Peroxisome proliferator-activated receptors (PPARs)