Cell membrane receptors are classified according to the signal transduction mechanism involved into:
• Ion channel-linked (ionotropic) receptors, which are coupled directly to ligand-gated ion channels. Examples include nicotinic acetylcholine receptors, ionotropic glutamate receptors, and gamma-aminobutyric acid (GABA) A receptors.
• Catalytic receptors, which possess a cytoplasmic catalytic region that usually behaves as a tyrosine kinase.
• G-protein-linked receptors, which are further discussed in the chapter on endocrine physiology (see p. 258).
Cell membrane receptors structurally comprise the following groups, depending on the number of times they span the membrane:
• Single trans-luminal domain receptors, which are directly or indirectly coupled to intracellular kinase enzymes.
Tyrosine kinase receptors consist of a tyrosine kinase domain, a hormone-binding domain, and a carboxy-terminal segment with multiple tyrosines for auto-phosphorylation.
The activation of signalling by tyrosine kinase receptors involves: Ligand-induced oligomerisation of the receptor; Trans-phosphorylation of the activation loop;
Phosphorylation of additional sites and recruitment of proteins to the receptor complex; Phosphorylation of substrates.
Tyrosine kinase receptor family, which includes epithelial growth factors, fibroblast growth factors and insulin-like growth factors;
Cytokine receptor superfamily;
Serine-threonine kinase receptor family;
Guanyl cyclase receptor family;
Phosphotyrosine phosphatase family.
* Seven transmembrane domain receptors, associated with GTP-activated protein (G-protein)-coupled receptors.
* Four transmembrane domain receptors, that form ligand- or transmitter-gated ion channels.
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