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 transmembrane receptors that sensemolecules outside the cell and activate inside signal transduction pathways and, ultimately, cellular responses. G protein-coupled receptors are found only in eukaryotes, including yeast, choanoflagellates, and animals. 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. G protein-coupled receptors are involved in many diseases, and are also the target of approximately 40% of all modern medicinal drugs.
There are two principal signal transduction pathways involving the G protein-coupled receptors: the cAMP signal pathway and the Phosphatidylinositol signal pathway. 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 GDP for a 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, Gαi/o, Gαq/11, Gα12/13).
GPCRs are involved in a wide variety of physiological processes. Some examples of their physiological roles include:
- The visual sense: the opsins use a photoisomerization reaction to translate electromagnetic radiation into cellular signals. Rhodopsin, for example, uses the conversion of 11-cis-retinal to all-trans-retinal for this purpose
- The sense of smell: receptors of the olfactory epithelium bind odorants (olfactory receptors) and pheromones (vomeronasal receptors)
- Behavioral and mood regulation: receptors in the mammalian brain bind several different neurotransmitters, includingserotonin, dopamine, GABA, and glutamate
- Regulation of immune system activity and inflammation: chemokine receptors bind ligands that mediate intercellular communication between cells of the immune system; receptors such as histamine receptors bind inflammatory mediators and engage target cell types in the inflammatory response
- Autonomic nervous system transmission: both the sympathetic and parasympathetic nervous systems are regulated by GPCR pathways, responsible for control of many automatic functions of the body such as blood pressure, heart rate, and digestive processes
- Cell density sensing: A novel GPCR role in regulating cell density sensing.
- Homeostasis modulation (e.g., water balance).
A final common structural theme among GPCRs is palmitoylation of one or more sites of the C-terminal tail or the intracellular loops. Palmitoylation is the covalent modification of cysteine (Cys) residues via addition of hydrophobic acyl groups, and has the effect of targeting the receptor to cholesterol- and sphingolipid-rich microdomains of the plasma membrane called lipid rafts. As many of the downstream transducer and effector molecules of GPCRs (including those involved in negative feedback pathways) are also targeted to lipid rafts, this has the effect of facilitating rapid receptor signaling.
GPCRs respond to extracellular signals mediated by a huge diversity of agonists, ranging from proteins to biogenic aminesto protons, but all transduce this signal via a mechanism of G-protein coupling. This is made possible by virtue of a guanine-nucleotide exchange factor (GEF) domain primarily formed by a combination of IL-2 and IL-3 along with adjacent residues of the associated TM helices.
The G protein-coupled receptor is 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. 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 (HGF),melanocortins, neuropeptide Y, opioidpeptides, opsins, somatostatin, tachykinins, members of the vasoactive intestinal peptidefamily, and vasopressin; biogenic amines (e.g.,dopamine, epinephrine, norepinephrine,histamine, glutamate (metabotropic effect),glucagon, acetylcholine (muscarinic effect), and serotonin); chemokines; lipid mediators of inflammation (e.g., prostaglandins,prostanoids, platelet-activating factor, and leukotrienes); and peptide hormones (e.g.,calcitonin, C5a anaphylatoxin, follicle-stimulating hormone (FSH), gonadotropin-releasing hormone (GnRH), neurokinin, thyrotropin-releasing hormone (TRH), andoxytocin). GPCRs that act as receptors for stimuli that have not yet been identified are known as orphan receptors.
There are two principal signal transduction pathways involving the G protein-linked receptors: cAMP signal pathway and Phosphatidylinositol signal pathway.
- cAMP signal pathway
The cAMP signal transduction contains 5 main characters: stimulative hormone receptor (Rs) or inhibitory hormone receptor (Ri)
Stimulative regulative G-protein is a G protein-linked to stimulative hormone receptor (Rs) and its α subunit upon activation could stimulate the activity of an enzyme or other intracellular metabolism. On the contrary, inhibitory regulative G-protein is linked to an inhibitory hormone receptor and its α subunit upon activation could inhibit the activity of an enzyme or other intracellular metabolism.
The Adenylyl cyclase is a 12-transmembrane glucoprotein that catalyzes ATP to form cAMP with the help of cofactor Mg2+ or Mn2+. The cAMP produced is a second messenger in cellular metabolism and is an allosteric activator to Protein kinase A.
Protein kinase A is an important enzyme in cell metabolism due to its ability to regulate cell metabolism by phosphorylating specific committed enzymes in the metabolic pathway. It can also regulate specific gene expression, cellular secretion, and membrane permeability. The protein enzyme contains two catalytic subunits and two regulatory subunits. When there is no cAMP the complex is inactive. When cAMP binds to the regulatory subunits, their conformation is altered, causing the dissociation of the regulatory subunits, which activates protein kinase A and allows further biological effects.
cAMP phosphodiesterase is an enzyme that can degrade cAMP to 5'-AMP, which will terminate the signal.
- Phosphatidylinositol signal pathway
In the phosphatidylinositol signal pathway, the extracellular signal molecule binds with the G-protein receptor (Gq) on the cell surface and activates phospholipase C, which is located on the plasma membrane. The lipase hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP2) into two second messengers: Inositol 1,4,5-trisphosphate (IP3) and Diacylglycerol (DAG). IP3 binds with the receptor in the membrane of the smooth endoplasmic reticulum and mitochondria, help open the Ca2+ channel. DAG will help activate Protein Kinase C (PKC), which phosphorylates many other proteins, changing their catalytic activities, leading to cellular responses. The effects of Ca2+ is also remarkable: it cooperates with DAG in activating PKC and can activate CaM kinase pathway, in which calcium modulated protein calmodulin (CaM) binds Ca2+, undergoes a change in conformation, and activates CaM kinase II, which has unique ability to increase its binding affinity to CaM by autophosphorylation, making CaM unavailable for the activation of other enzymes. The kinase then phosphorylates target enzymes, regulating their activities. The two signal pathways are connected together by Ca2+-CaM, which is also a regulatory subunit of adenylyl cyclase and phosphodiesterase in cAMP signal pathway.