G-protein-coupled receptors (GPCRs) as indicated by their name are coupled to trimeric G-proteins. Upon ligand binding to the receptor, the trimeric G-proteins dissociate into a Gα-subunit and a Gβ/γ-heterodimer. Both the α-subunit and the Gβ/γ-dimer then activate further signalling, for instance, by regulating the production of second messengers or the activity of ion channels. In the following we will discuss the history of the discovery of GPCR signalling, second messengers like cAMP and phospholipids and crucial target enzymes of these second messengers such as protein kinase A and protein kinase C.
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