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Encyclopedia > G protein

G-proteins, short for guanine nucleotide binding proteins, are a family of proteins involved in second messenger cascades. They are so-called because of their signaling mechanism, which uses the exchange of Guanosine diphosphate (GDP) for Guanosine triphosphate (GTP) as a molecular "switch" to allow or inhibit biochemical reactions inside the cell. Alfred Gilman and Martin Rodbell were awarded the Nobel Prize in Physiology or Medicine in 1994 for their discovery and research on G-proteins. A representation of the 3D structure of myoglobin, showing coloured alpha helices. ... In biology, second messengers are low-weight diffusible molecules that are used in signal transduction to relay a signal within a cell. ... GDP (guanosine diphosphate) is a chemical compound essential to signal transduction in living cells. ... Guanosine triphosphate (GTP) is also known as guanosine-5-triphosphate, G3P, and 9-ß-D-ribofuranosylguanine-5-triphosphate (or, equivalently, 9-ß-D-ribofuranosyl-2-amino-6-oxo-purine-5-triphosphate). ... Biochemistry is the chemistry of life. ... Cells in culture, stained for keratin (red) and DNA (green) The cell is the structural and functional unit of all living organisms, sometimes called the building blocks of life. ... Alfred Goodman Gilman (b. ... Martin Rodbell won a Nobel Prize in 1994 Martin Rodbell (December 1, 1925- December 7, 1998) was an American biochemist and molecular endocrinologist who is best known for his discovery of G-proteins. ... List of Nobel Prize laureates in Physiology or Medicine from 1901 to the present day. ... 1994 was a common year starting on Saturday of the Gregorian calendar, and was designated the International year of the Family. ...

Contents


General properties

G-proteins belong to the larger grouping of GTPases. "G-protein" usually refers to the membrane-associated heterotrimeric G-proteins, sometimes referred to as the "large" G-proteins. These proteins are activated by G-protein coupled receptors and are made up of alpha (α), beta (β) and gamma (γ) subunits. There are also "small" G proteins or small GTPases like ras that are monomeric and not membrane-associated, but also bind GTP and GDP and are involved in signal transduction. GTPases are a large family of enzymes that can bind and hydrolyze GTP. The GTP binding and hydrolysis takes place in the highly conserved G domain common to all GTPases. ... In cell biology, G-protein-coupled receptors, also known as GPCR, seven transmembrane receptors, heptahelical receptors, or 7TM receptors, are a class of transmembrane receptors. ... In structural biology, a protein subunit or subunit protein is a single protein molecule that assembles (or coassembles) with other protein molecules to form a multimeric or oligomeric protein. ... In biology, small GTPases are small (20-25 KDa) proteins that bind to guanosine triphosphate GTP. This family of proteins are homologous to Ras GTPases and also called the Ras superfamily GTPases. ... Ras (in molecular biology) are very important molecular switches for a wide variety of signal pathways that control such processes as cytoskeletal integrity, proliferation, cell adhesion, apoptosis, and cell migration. ... In biology, signal transduction is any process by which a cell converts one kind of signal or stimulus into another. ...


G-proteins are perhaps the most important signal transducing molecules in cells. In fact, diseases such as diabetes, alcoholism, and certain forms of pituitary cancer, among many others, are thought to have some root in the malfunction of G-proteins, and thus a fundamental understanding of their function, signaling pathways and protein interactions may lead to eventual treatments and possibly the creation of various preventive approaches. This article is about the disease that features high blood sugar. ... Polish propaganda poster saying: Stop drinking! Come with us build happy tomorrows. ... When normal cells are damaged or old they undergo apoptosis; cancer cells, however, avoid apoptosis. ...


Receptor-activated G-proteins

Receptor activated G-proteins are bound to the inside surface of the cell membrane. They consist of the Gα and the tightly associated Gβγ subunits. When a ligand activates the G-protein coupled receptor, the G-protein binds to the receptor, releases its bound GDP from the Gα subunit, and binds a new molecule of GTP. This exchange triggers the dissociation of the Gα subunit, the Gβγ dimer, and the receptor. Both, Gα-GTP and Gβγ, can then activate different 'signalling cascades' (or 'second messenger pathways') and effector proteins, while the receptor is able to activate the next G-protein. The Gα subunit will eventually hydrolize the attached GTP to GDP by its inherent enzymatic activity, allowing it to reassociate with Gβγ and starting a new cycle. In cell biology, G-protein-coupled receptors, also known as GPCR, seven transmembrane receptors, heptahelical receptors, or 7TM receptors, are a class of transmembrane receptors. ... Drawing of a cell membrane A component of every biological cell, the selectively permeable cell membrane (or plasma membrane or plasmalemma) is a thin and structured bilayer of phospholipid and protein molecules that envelopes the cell. ... In chemistry, a ligand is an atom, ion or functional group that is bonded to one or more central atoms or ions, usually metals generally through co-ordinate covalent bond. ... GTP (also known as guanylyl imidodiphosphate, guanosine-5-triphosphate, or guanosine triphosphate) is a chemical compound (nucleotide) that is incorporated into the growing RNA chain during synthesis of RNA and used as a source of energy during synthesis of proteins. ... Hydrolysis is a chemical process in which a molecule is cleaved into two parts by the addition of a molecule of water. ... Neuraminidase ribbon diagram An enzyme (in Greek en = in and zyme = leaven) is a protein, or protein complex, that catalyzes a chemical reaction and also controls the 3D orientation of the catalyzed substrates. ...


A well characterized example of a G-protein triggered signalling cascade is the cAMP pathway. The enzyme adenylate cyclase is activated by Gαs-GTP and synthesizes the second messenger cyclic adenosine monophosphate (cAMP) from ATP. Second messengers then interact with other proteins downstream to cause a change in cell behavior. Adenylate cyclase (EC 4. ... Structure of cAMP Cyclic adenosine monophosphate (cAMP, cyclic AMP or 3-5-cyclic adenosine monophosphate) is a molecule that is important in many biological processes; it is derived from adenosine triphosphate (ATP). ... Adenosine triphosphate (ATP) Adenosine triphosphate (ATP) is the nucleotide known in biochemistry as the molecular currency of intracellular energy transfer; that is, ATP is able to store and transport chemical energy within cells. ...


Alpha subunits

Gα subunits consist of two domains, the GTPase domain, and the alpha-helical domain. There exist at least 20 different alpha subunits, which are separated into several main families:

  • Gαs or simply Gs (stimulatory) - activates adenylate cyclase to increase cAMP synthesis
  • Gαi or simply Gi (inhibitory) - inhibits adenylate cyclase
  • Golf (olfactory) - couples to olfactory receptors
  • Gt (transducin) - transduces visual signals in conjunction with rhodopsin in the retina
  • Gq - stimulates phospholipase C
  • The G12/13 family - important for regulating the cytoskeleton, cell junctions, and other processes related to movements

Olfactory receptors are a type of G protein-coupled receptor in olfactory receptor neurons. ... Transducin is the name given to the G-protein alpha-subunits that are naturally expressed in vertebrate retina rods and cones. ... Categories: Biochemistry stubs | G protein coupled receptors | Sensory receptors | Pigments ... Human eye cross-sectional view. ... A phospholipase is an enzyme that converts phospholipids into fatty acids and other lipophilic substances. ... The cytoskeleton is a cellular scaffolding or skeleton contained, as all other organelles, within the cytoplasm. ...

Beta-gamma complex

The β and γ subunits are closely bound to one another and are referred to as the beta-gamma complex. The Gβγ complex is released from the Gα subunit after its GDP-GTP exchange. The free Gβγ complex can act as a signaling molecule itself, by activating other second messengers or by gating ion channels directly. For example, the Gβγ complex, when bound to histamine receptors, can activate phospholipase A2. Gβγ complexes bound to muscarinic acetylcholine receptors, on the other hand, directly open G-protein coupled inward rectifying potassium (GIRK) channels. Another, unrelated ion channeling process is part of ion implantation. ... Histamine is a monoamine chemical involved in local immune responses. ... A phospholipase is an enzyme that converts phospholipids into fatty acids and other lipophilic substances. ... Muscarine, L-(+)-muscarine, or muscarin is a natural product found in certain mushrooms, particularly in Inocybe and Clitocybe species. ... The chemical compound acetylcholine, often abbreviated as ACh, was the first neurotransmitter to be identified. ... General Name, Symbol, Number potassium, K, 19 Series alkali metals Group, Period, Block 1(IA), 4, s Density, Hardness 856 kg/m3, 0. ...


References

  • Kandel, Eric, James Schwartz, and Thomas Jessel. 2000. Principles of Neural Science. 4th ed. McGraw-Hill, New York.
  • Voet, Donald and Judith G. Voet. 1995. Biochemistry 2nd ed. John Wilely & Sons, New York.

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