G protein-coupled receptors (GPCRs), also known as seven transmembrane domain receptors, 7TM receptors, heptahelical receptors, and G protein-linked receptors (GPLR), comprise a large protein family of transmembrane receptors that sense molecules outside the cell and activate inside signal transduction pathways and, ultimately, cellular responses. Proteins are large Organic compounds made of Amino acids arranged in a linear chain and joined together by Peptide bonds between the Carboxyl Transmembrane receptors are Integral membrane proteins which reside and operate typically within a cell's Plasma membrane, but also in the membranes of In Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by The cell is the structural and functional unit of all known living Organisms It is the smallest unit of an organism that is classified as living and is often called In Biology, signal transduction refers to any process by which a cell converts one kind of signal or stimulus into another G protein-coupled receptors are found only in eukaryotes, including yeast, plants, choanoflagellates, and animals. Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex The ligands that bind and activate these receptors include light-sensitive compounds, odors, pheromones, hormones, and neurotransmitters, and vary in size from small molecules to peptides to large proteins. In Biochemistry, a ligand ( latin ligare = to bind is a substance that is able to bind to and form a complex with a Biomolecule A pheromone (from Greek φέρω phero "to bear" + ‘ορμόνη " Hormone " is a Chemical that triggers a natural Hormones (from Greek ὁρμή - "impetus" are chemicals released by cells that affect cells in other parts of the body See Chemical synapse for an introduction to concepts and terminology used in this article Peptides (from the Greek πεπτίδια, "small digestibles" are short Polymers formed from the linking in a defined order of α- Amino Proteins are large Organic compounds made of Amino acids arranged in a linear chain and joined together by Peptide bonds between the Carboxyl G protein-coupled receptors are involved in many diseases, but are also the target of around half of all modern medicinal drugs. 
The very large rhodopsin A group has been further subdivided into 19 subgroups (A1-A19). Rhodopsin-like receptors are the largest group of G-protein coupled receptors G-protein-coupled receptors GPCRs constitute a vast protein family that encompasses Secretin family of 7 transmembrane receptors is a family of evolutionarily related proteins. The class C G-protein-coupled receptors ( are a class of G-protein coupled receptors that include the Metabotropic glutamate receptors ( and several additional The metabotropic glutamate receptors, or mGluR s are a type of Glutamate receptor which are active through an indirect metabotropic process Fungal pheromone mating factor receptors form a distinct family of G-protein coupled receptors. Cyclic AMP receptors from Slime molds are a distinct family of G-protein coupled receptors. Frizzled is a family of G protein-coupled receptor proteins that serve as receptors in the Wnt signaling pathway and other signaling pathways Smoothened is a G protein-coupled receptor protein encoded by the gene of the hedgehog pathway conserved from flies to humans  More recently, an alternative classification system called GRAFS (Glutamate, Rhodopsin, Adhesion, Frizzled/Taste2, Secretin) has been proposed. The metabotropic glutamate receptors, or mGluR s are a type of Glutamate receptor which are active through an indirect metabotropic process Rhodopsin, also known as visual purple, is a Pigment of the Retina that is responsible for both the formation of the Photoreceptor cells and the Frizzled is a family of G protein-coupled receptor proteins that serve as receptors in the Wnt signaling pathway and other signaling pathways A Taste receptor is a type of receptor which facilitates the sensation of Taste. Human secretin receptor (gene name SCTR) is a G protein-coupled receptor which binds Secretin and is the leading member ( i 
GPCRs are involved in a wide variety of physiological processes. Some examples of their physiological roles include:
GPCRs are integral membrane proteins that possess seven membrane-spanning domains or transmembrane helices (Figure 1). An Integral Membrane Protein ( IMP) is a Protein Molecule (or assembly of proteins that is permanently attached to the Biological membrane. Transmembrane domain usually denotes a single transmembrane Alpha helix of a Transmembrane protein. The extracellular parts of the receptor can be glycosylated. Glycosylation is the enzymatic process that links Saccharides to produce glycans, either free or attached to Proteins and Lipids This enzymatic These extracellular loops also contain two highly-conserved cysteine residues that form disulfide bonds to stabilize the receptor structure. Not to be confused with Cystine, its oxidized dimer Cysteine (abbreviated as Cys or C) is an α- Amino acid with In Chemistry, a disulfide bond is a single Covalent bond derived from the coupling of Thiol groups Some seven transmembrane helix proteins (such as channelrhodopsin) that resemble GPCRs may contain different functional groups, such as entire ion channels, within their protein. Channelrhodopsins are Light-gated ion channels They are useful molecules enabling the use of light to control intracellular acidity calcium influx and electrical excitability
Early structural models for GPCRs were based on their weak analogy to bacteriorhodopsin for which a structure had been determined by both electron diffraction (PDB 2BRD, ) and X ray-based crystallography ( ). Bacteriorhodopsin is a protein used by Archaea, most notably Halobacteria. The Protein Data Bank ( PDB) is a repository for 3-D structural data of Proteins and Nucleic acids These data typically obtained by X-ray crystallography X-ray crystallography is a method of determining the arrangement of Atoms within a Crystal, in which a beam of X-rays strikes a crystal and scatters  In 2000, the first crystal structure of a mammalian GPCR, that of bovine rhodopsin ( ), was solved. Rhodopsin, also known as visual purple, is a Pigment of the Retina that is responsible for both the formation of the Photoreceptor cells and the  While the main feature, the seven transmembrane helices, is conserved, the relative orientation of the helices differ significantly from that of bacteriorhodopsin. In 2007, the first structure of a human GPCR was solved ( , ).  This was followed immediately by a higher resolution structure of the same receptor ( ).  This human β2-adrenergic receptor GPCR structure, proved to be highly similar to the bovine rhodopsin in terms of the relative orientation of the seven transmembrane helices. The beta-2 adrenergic receptor (β2 adrenoreceptor also known as ADRB2, is an Beta-adrenergic receptor, and also denotes the human Gene encoding However the conformation of the second extracellular loop is entirely different between the two structures. Since this loop constitutes the "lid" that covers the top of the ligand binding site, this conformational difference highlights the difficulties in constructing homology models of other GPCRs based only on the rhodopsin structure. In Protein structure prediction, homology modeling, also known as comparative modeling, is a class of methods for constructing an atomic-resolution model of a
G protein-coupled receptor are activated by an external signal in the form of a ligand or other signal mediator. This creates a conformational change in the receptor, causing activation of a G protein. GTP chemical structurepng|thumb|180px| Guanosine triphosphate]] G proteins short for guanine nucleotide-binding proteins, are a family of Proteins involved Further effect depends on the type of G protein.
GPCRs include receptors for sensory signal mediators (e. g. , light and olfactory stimulatory molecules); adenosine, bombesin, bradykinin, endothelin, γ-aminobutyric acid (GABA), hepatocyte growth factor, melanocortins, neuropeptide Y, opioid peptides, opsins, somatostatin, tachykinins, vasoactive intestinal polypeptide family, and vasopressin; biogenic amines (e. Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700 Olfaction (also known as olfactics or smell) refers to the Sense of smell. Adenosine is a Nucleoside composed of a Molecule of Adenine attached to a Ribose sugar molecule ( Ribofuranose) moiety via a β-N9- Bombesin is a 14 Amino acid Peptide originally isolated from the Skin of a Frog. Bradykinin is a Peptide that causes blood vessels to enlarge (dilate and therefore causes blood pressure to lower Endothelins are proteins that constrict blood vessels and raise blood pressure Gamma-aminobutyric acid (GABA is the chief inhibitory Neurotransmitter in the Mammalian Central nervous system. Hepatocyte growth factor/scatter factor (HGF/SF is a Paracrine cellular growth motility and morphogenic factor. Melanocortins are a group of Pituitary Peptide hormones that include adrenocorticotropin ( ACTH) and the alpha beta and gamma Melanocyte-stimulating Neuropeptide Y (NPY is a 36 Amino acid peptide Neurotransmitter found in the Brain and Autonomic nervous system. An opioid is a chemical Substance that has a Morphine -like action in the body Opsins are a group of light-sensitive 35-55 kDa membrane-bound G protein-coupled receptors of the Retinylidene protein family found in Photoreceptor cells Somatostatin (also known as growth hormone inhibiting hormone ( GHIH) or somatotropin release-inhibiting factor ( SRIF) is a Peptide hormone Tachykinin peptides are one of the largest family of Neuropeptides found from Amphibians to Mammals They were so named due to their ability to rapidly Vasoactive intestinal peptide ( VIP, also polypeptide) is a Peptide hormone containing 28 Amino acid residues and is produced in many Arginine vasopressin ( AVP) also known as vasopressin, argipressin or antidiuretic hormone ( ADH) is a Hormone found in A biogenic amine is a Biogenic substance with an Amine group Examples Some prominent examples of biogenic amines include Histamine g. , dopamine, epinephrine, norepinephrine, histamine, glutamate (metabotropic effect), glucagon, acetylcholine (muscarinic effect), and serotonin); chemokines; lipid mediators of inflammation (e. Dopamine is a Hormone and Neurotransmitter occurring in a wide variety of animals including both vertebrates and invertebrates Norepinephrine ( INN) (abbreviated norepi or NE) or noradrenaline ( BAN) (abbreviated NA or NAd) is a Histamine is a Biogenic amine involved in local immune responses as well as regulating physiological function in the gut and acting as a Neurotransmitter Glutamic acid (abbreviated as Glu or E) is one of the 20 Alpha Amino acids It is not among the human Essential amino acids Its Metabotropic receptor is a subtype of membrane receptors at the surface or in vesicles of Eukaryotic cells Glucagon is an important Hormone involved in Carbohydrate metabolism. The Chemical compound acetylcholine (often abbreviated ACh) is a Neurotransmitter in both the Peripheral nervous system (PNS and Central Muscarinic receptors, or mAChRs, are Acetylcholine receptors that form Metabotropic Ion channels in the Plasma membranes of certain Serotonin (ˌsɛrəˈtoʊnən ( 5-hydroxytryptamine, or 5-HT) is a Monoamine Neurotransmitter synthesized in serotonergic Neurons Chemokines are a family of small Cytokines, or Proteins secreted by cells Proteins are classified as chemokines according to shared structural characteristics Lipids are broadly defined as any fat- Soluble ( lipophilic) naturally-occurring Molecule, such as fats oils waxes cholesterol sterols fat-soluble Inflammation ( Latin, inflamatio, to set on fire is the complex biological response of vascular tissues to harmful stimuli such as Pathogens g. , prostaglandins, prostanoids, platelet-activating factor, and leukotrienes); and peptide hormones (e. A prostaglandin is any member of a group of Lipid compounds that are derived enzymatically from Fatty acids and have important functions in the Animal body Prostanoid is the term used to describe a subclass of Eicosanoids consisting of the Prostaglandins (mediators of inflammatory and anaphylactic Platelet-activating factor, also known as a PAF, PAF-acether or AGEPC (acetyl-glyceryl-ether-phosphorylcholine is a potent Phospholipid activator Leukotrienes are naturally produced Eicosanoid lipid mediators, which may be responsible for the effects of an inflammatory response Peptide hormones are a class of peptides that are secreted into the blood stream and have Endocrine functions in living animals g. , calcitonin, C5a anaphylatoxin, follicle-stimulating hormone (FSH), gonadotropic-releasing hormone (GnRH), neurokinin, thyrotropin-releasing hormone (TRH), and oxytocin). Calcitonin is a 32-amino acid linear polypeptide Hormone that is produced in Humans primarily by the parafollicular (also known as C-cells of the Anaphylatoxins, or anaphylotoxins, are fragments ( C3a, C4a and C5a) that are produced during the pathways of the Complement system Gonadotropin-releasing hormone ( GNRH) also known as Luteinizing-hormone releasing hormone ( LHRH) is a tropic Peptide Hormone Tachykinin peptides are one of the largest family of Neuropeptides found from Amphibians to Mammals They were so named due to their ability to rapidly Thyrotropin-releasing hormone ( TRH) also called thyrotropin-releasing factor ( TRF) thyroliberin or protirelin, is a tropic Oxytocin ( IPA: /ˌɔksɪˈtoʊsɪn/ (Greek "quick birth" is a Mammalian Hormone that also acts as a Neurotransmitter in the GPCRs that act as receptors for stimuli that have yet to be identified are known as orphan receptors. An orphan receptor is an apparent receptor that has a similar structure to other identified receptors but whose Endogenous ligand has not yet been identified
Whereas, in other types of receptors that have been studied, ligands bind externally to the membrane, the ligands of GPCRs typically bind within the transmembrane domain. In Biochemistry, a ligand ( latin ligare = to bind is a substance that is able to bind to and form a complex with a Biomolecule
The transduction of the signal through the membrane by the receptor is not completely understood. In Biology, signal transduction refers to any process by which a cell converts one kind of signal or stimulus into another It is known that the inactive G protein is bound to the receptor in its inactive state. GTP chemical structurepng|thumb|180px| Guanosine triphosphate]] G proteins short for guanine nucleotide-binding proteins, are a family of Proteins involved Once the ligand is recognized, the receptor shifts conformation and thus mechanically activates the G protein, which detaches from the receptor. In Chemistry, conformational isomerism is a form of Stereoisomerism in which Molecules with the same Structural formula (same connectivity The receptor can now either activate another G protein or switch back to its inactive state. This is an overly simplistic explanation, but suffices to convey the overall set of events.
It is believed that a receptor molecule exists in a conformational equilibrium between active and inactive biophysical states.  The binding of ligands to the receptor may shift the equilibrium toward the active receptor states.  Three types of ligands exist: agonists are ligands that shift the equilibrium in favour of active states; inverse agonists are ligands that shift the equilibrium in favour of inactive states; and neutral antagonists are ligands that do not affect the equilibrium. In Pharmacology, an inverse agonist is an agent which binds to the same receptor binding-site as an Agonist for that receptor and reverses constitutive It is not yet known how exactly the active and inactive states differ from each other.
If a receptor in an active state encounters a G protein, it may activate it (Figure 2, blue protein in part B). GTP chemical structurepng|thumb|180px| Guanosine triphosphate]] G proteins short for guanine nucleotide-binding proteins, are a family of Proteins involved GTP chemical structurepng|thumb|180px| Guanosine triphosphate]] G proteins short for guanine nucleotide-binding proteins, are a family of Proteins involved Some evidence suggests that receptors and G proteins are actually pre-coupled. For example, binding of G proteins to receptors affects the receptor's affinity for ligands. Activated G proteins are bound to GTP. Guanosine-5'-triphosphate ( GTP) is a Purine Nucleotide. One role is as substrate for the synthesis of RNA during transcription.
Further signal transduction depends on the type of G protein. The enzyme adenylate cyclase (Figure 2, green protein in panel C) is an example of a cellular protein that can be regulated by a G protein, in this case the G protein Gs. Adenylate cyclase ( also known as adenylyl cyclase or AC) is a Lyase Enzyme. The Gs alpha subunit (or Gs protein) is a Heterotrimeric G protein subunit which activates Adenylate cyclase. Adenylate cyclase activity is activated when it binds to a subunit of the activated G protein (Figure 2, Panel D). Activation of adenylate cyclase ends when the G protein returns to the GDP-bound state (Figure 2, panels E and A). Guanosine diphosphate, abbreviated GDP, is a Nucleotide. It is an Ester of Pyrophosphoric acid with the Nucleoside Guanosine
In the late 1990s, evidence began accumulating to suggest that some GPCRs are able to signal without G proteins. The ERK2 mitogen-activated protein kinase, a key signal transduction mediator downstream of receptor activation in many pathways, has been shown to be activated in response to cAMP-mediated receptor activation in the slime mold D. See also Extracellular signal-regulated kinases Mitogen-activated discoideum despite the absence of the associated G protein α- and β-subunits.
In mammalian cells, the much-studied β2-adrenoceptor has been demonstrated to activate the ERK2 pathway after arrestin-mediated uncoupling of G-protein-mediated signaling. It therefore seems likely that some mechanisms previously believed to be purely related to receptor desensitisation are actually examples of receptors switching their signaling pathway rather than simply being switched off.
In kidney cells, the bradykinin receptor B2 has been shown to interact directly with a protein tyrosine phosphatase. Bradykinin receptor B2, also known as BDKRB2, is a Bradykinin receptor, but also denotes the human Gene encoding it The presence of a tyrosine-phosphorylated ITIM (immunoreceptor tyrosine-based inhibitory motif) sequence in the B2 receptor is necessary to mediate this interaction and subsequently the antiproliferative effect of bradykinin. An immunoreceptor tyrosine-based inhibition motif (ITIM is a conserved sequence of Amino acids (S/I/V/LxYxxI/V/L that is found in the Cytoplasmic tails of many inhibitory 
GPCRs become desensitized when exposed to their ligand for a prolonged period of time. There are two recognized forms of desensitization: 1) homologous desensitization, in which the activated GPCR is downregulated; and 2) heterologous desensitization, wherein the activated GPCR causes downregulation of a different GPCR. Desensitisation of receptors is a process wherby after prolonged Agonist exposure the receptor is uncoupled from its signalling cascade and thus the biological effect The key reaction of this downregulation is the phosphorylation of the intracellular (or cytoplasmic) receptor domain by protein kinases. Phosphorylation is the addition of a Phosphate (PO4 group to a Protein molecule or a small molecule The cytoplasm is the contents of a cell that is enclosed within the Plasma membrane. A protein kinase is a Kinase Enzyme that modifies other Proteins by chemically adding Phosphate groups to them ( Phosphorylation)
Cyclic AMP-dependent protein kinases (protein kinase A) are activated by the signal chain coming from the G protein (that was activated by the receptor) via adenylate cyclase and cyclic AMP (cAMP). In Cell biology, protein kinase A, refers to a family of Enzymes whose activity is dependent on the level of Cyclic AMP (cAMP in the cell Adenylate cyclase ( also known as adenylyl cyclase or AC) is a Lyase Enzyme. Cyclic adenosine monophosphate ( cAMP, cyclic AMP or 3'-5'-cyclic Adenosine monophosphate) is a Second messenger that is important in many biological In a feedback mechanism, these activated kinases phosphorylate the receptor. The longer the receptor remains active, the more kinases are activated, the more receptors are phosphorylated. In β2-adrenoceptors, this phosphorylation results in the switching of the coupling from the Go class of G-protein to the Gi class. The beta-2 adrenergic receptor (β2 adrenoreceptor also known as ADRB2, is an Beta-adrenergic receptor, and also denotes the human Gene encoding Gi alpha subunit (or Gi/G0 or Gi protein) is a Heterotrimeric G protein subunit which inhibits the production of cAMP-dependent PKA mediated phosphorylation is also known as heterologous desensitisation, because it is not specific to ligand bound receptor. In fact any receptor causing in a increase in PKA activity will cause increased amounts of this type of desensitisation of other receptors coupled to Go (e. g. , dopamine receptor D2 activation may lead to β2-adrenoceptor desensitisation of this type). Dopamine receptor D2, also known as DRD2, is a Protein that is a receptor 
The G protein-coupled receptor kinases (GRKs) are protein kinases that phosphorylate only active GPCRs. G-Protein coupled receptor kinases ( GRKs) regulate the activity of G-protein coupled receptors by Phosphorylating their intracellular domains after their
Phosphorylation of the receptor can have two consequences:
It is generally accepted that G-protein-coupled receptors can form homo- and/or heterodimers and possibly more complex oligomeric structures, and indeed heterodimerization has been shown to be essential for the function of receptors such as the metabotropic GABA(B) receptors. A dimer is a Chemical or Biological entity consisting of two subunits called Monomers which are held together by either Intramolecular forces A dimer is a Chemical or Biological entity consisting of two subunits called Monomers which are held together by either Intramolecular forces In Chemistry, an oligomer consists of a limited number of Monomer units (ολιγος or oligos is Greek for "a few" in contrast to a However, it is presently unproven that true heterodimers exist. Present biochemical and physical techniques lack the resolution to differentiate between distinct homodimers assembled into an oligomer or true 1:1 heterodimers. It is also unclear what the functional significance of oligomerization might be, although it is thought that the phenomenon may contribute to the pharmacological heterogeneity of GPCRs in a manner not previously anticipated. In Chemistry, an oligomer consists of a limited number of Monomer units (ολιγος or oligos is Greek for "a few" in contrast to a Heterogeneous is an adjective used to describe an object or system consisting of multiple items having a large number of structural variations This is an actively-studied area in GPCR research.
The best studied example of receptor oligomerisation are the metabotropic GABAB receptors. GABAB receptors (GABABR are Metabotropic Transmembrane receptors for Gamma-aminobutyric acid (GABA that are linked via G-proteins These receptors are formed by heterodimerization of GABABR1 and GABABR2 subunits. Gamma-aminobutyric acid (GABA B receptor 1, also known as GABBR1, is a human Gene. Gamma-aminobutyric acid (GABA B receptor 2, also known as GABBR2, is a human Gene. Expression of the GABABR1 without the GABABR2 in heterologous systems leads to retention of the subunit in the endoplasmic reticulum. The endoplasmic reticulum (Greek endo = "within" (prefix plásma = "formed entity" Latin reticulum = "little net" or ER, is an Organelle Expression of the GABABR2 subunit alone, meanwhile, leads to surface expression of the subunit, although with no functional activity (i. e. , the receptor does not bind agonist and cannot initiate a response following exposure to agonist). Expression of the two subunits together leads to plasma membrane expression of functional receptor. It has been shown that GABABR2 binding to GABABR1 causes masking of a retention signal of functional receptors. 
GCR2 is a G-protein-coupled receptor for the plant hormone abscisic acid that has been identified in Arabidopsis thaliana. Plant hormones (also known as phytohormones) are chemicals that regulate plant growth Abscisic acid (ABA also known as abscisin II and dormin, is a Plant hormone. Arabidopsis thaliana ( A-ra-bi-dóp-sis tha-li-á-na; thale cress, mouse-ear cress or Arabidopsis) is a small Another putative receptor is GCR1 for which no ligand has been identified yet. 
A novel GPCR containing a lipid kinase domain has recently been identified in Dictyostelium that regulates cell density sensing. The dictyostelids are a group of cellular Slime molds or social Amoebae When food (normally bacteria is readily available they take the form of individual amoebae 
Medical Subject Headings ( MeSH) is a huge Controlled vocabulary (or metadata system for the purpose of indexing journal articles and books Orientations of Proteins in Membranes (OPM database provides spatial positions of protein three-dimensional structures with respect to the Lipid bilayer.