Editing Neurotransmitter receptor (section)

==Metabotropic receptors: G-protein coupled receptors==
[[Image:Mu-opioid receptor (GPCR).png|thumb|A [[opioid receptor|mu-opioid G-protein-coupled receptor]] with its agonist]]

'''[[G protein-coupled receptors]]''' ('''GPCRs'''), also known as '''seven-transmembrane domain receptors''', '''7TM receptors''', '''heptahelical receptors''', '''serpentine receptor''', and '''G protein-linked receptors''' ('''GPLR'''), comprise a large [[protein]] family of [[membrane receptor|transmembrane receptor]]s that sense [[molecule]]s outside the [[Cell (biology)|cell]] and activate inside [[signal transduction]] pathways and, ultimately, cellular responses. G protein-coupled receptors are found only in [[eukaryote]]s, including yeast,  [[choanoflagellate]]s,<ref name="pmid12869759">{{cite journal |vauthors=King N, Hittinger CT, Carroll SB | title = Evolution of key cell signaling and adhesion protein families predates animal origins | journal = Science | volume = 301 | issue = 5631 | pages = 361–3 | year = 2003 | pmid = 12869759 | doi = 10.1126/science.1083853 | bibcode = 2003Sci...301..361K | s2cid = 9708224 }}</ref> and animals. The [[Ligand (biochemistry)|ligands]] that bind and activate these receptors include light-sensitive compounds, [[odor]]s, [[pheromone]]s, [[hormone]]s, and [[neurotransmitter]]s, and vary in size from small molecules to [[peptide]]s to large [[protein]]s. G protein-coupled receptors are involved in many diseases, and are also the target of approximately 30% of all modern medicinal drugs.<ref>{{cite journal | first=David | last=Filmore | pages=24–28 | title=It's a GPCR world | journal=Modern Drug Discovery | volume=2004 | year=2004 | issue=November | url=http://pubs.acs.org/subscribe/journals/mdd/v07/i11/html/1104feature_filmore.html}}</ref><ref name="pmid17139284">{{cite journal |vauthors=Overington JP, Al-Lazikani B, Hopkins AL | title = How many drug targets are there? | journal = Nat Rev Drug Discov | volume = 5 | issue = 12 | pages = 993–6 |date=December 2006 | pmid = 17139284 | doi = 10.1038/nrd2199 | s2cid = 11979420 }}</ref>

There are two principal signal transduction pathways involving the G protein-coupled receptors: the [[Cyclic adenosine monophosphate|cAMP]] signal pathway and the [[phosphatidylinositol]] signal pathway.<ref name="Gilman A.G. 1987 615–649">{{cite journal | author = Gilman A.G. | title = G Proteins: Transducers of Receptor-Generated Signals | journal = Annual Review of Biochemistry | year=1987 | volume=56 | pages=615–649 | doi = 10.1146/annurev.bi.56.070187.003151 | pmid = 3113327| s2cid = 33992382 }}</ref> When a ligand binds to the GPCR it causes a conformational change in the GPCR, which allows it to act as a [[guanine nucleotide exchange factor]] (GEF). The GPCR can then activate an associated [[G-protein]] by exchanging its bound [[guanosine diphosphate|GDP]] for a [[guanosine triphosphate|GTP]]. The G-protein's α subunit, together with the bound GTP, can then dissociate from the β and γ subunits to further affect intracellular signaling proteins or target functional proteins directly depending on the α subunit type ([[Gαs|G<sub>αs</sub>]], [[Gαi|G<sub>αi/o</sub>]], [[Gαq|G<sub>αq/11</sub>]], [[G12/G13 alpha subunits|G<sub>α12/13</sub>]]).<ref name="Wettschureck_2005">{{cite journal |vauthors=Wettschureck N, Offermanns S | title = Mammalian G proteins and their cell type specific functions | journal = Physiol. Rev. | volume = 85 | issue = 4 | pages = 1159–204 |date=October 2005 | pmid = 16183910 | doi = 10.1152/physrev.00003.2005 | s2cid = 24270725 }}</ref>{{rp|1160}}