Enzyme-coupled Receptor

Enzyme-coupled receptors are a major kind of cell membrane receptors characterized either by having an intrinsic enzymatic activity on their cytosolic domain or by being coupled with an enzyme. They have two main domains: ligand binding site on the outside and an enzyme binding (catalytic) domain on the inside of the cell. Enzyme-coupled receptors are single-pass transmembrane proteins. These receptors are very diverse but they can be classified in six main classes:

• Receptor tyrosine kinases (RTKs) usually phophorylate some of their own tyrosine residues. Once phosphorylated they become docking sites for other intracellular signalling proteins. RTKs include the insuline receptor and many growth factor receptors.
  
• Tyrosine-kinase-associated receptors are coupled to tyrosine kinases (such as Src or JAK kinases) on the cytosolic side of the membrane. Receptors for the growth hormone, prolactin, erythropoietin and alpha and gamma interferons are part of this group.
  
• Receptor serine/threonine kinases phosphorylate serine and threonine residues on themselves or associated proteins. The receptor for the transforming growth factor β (TGFβ) acts as a serine/threonine kinase).
  
• Histidine-kinase-associated receptors, important in bacterial chemotaxis.
• Receptor guanylyl cyclases act through the production of the second messenger cGMP.
• Receptorlike (unknown ligand) tyrosine phosphatases have intrinsic phosphatase