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Glycosylphosphatidylinositol or glycophosphatidylinositol (GPI) is a phosphoglyceride that can be attached to the C-terminus of a protein during posttranslational modification. The resulting GPI-anchored proteins play key roles in a wide variety of biological processes. GPI is composed of a phosphatidylinositol group linked through a carbohydrate-containing linker and via an ethanolamine phosphate (EtNP) bridge to the C-terminal amino acid of a mature protein. The two fatty acids within the hydrophobic phosphatidyl-inositol group anchor the protein to the cell membrane.
Phosphatidylinositol consists of a family of lipids made of a phosphate group, two fatty acid chains, and one inositol molecule. They represent a class of the phosphatidylglycerides. Typically phosphatidylinositols form a minor component on the cytosolic side of eukaryotic cell membranes. The phosphate group gives the molecules a negative charge at physiological pH.
Glycerophospholipids or phosphoglycerides are glycerol-based phospholipids. They are the main component of biological membranes. Two major classes are known: those for bacteria and eukaryotes and a separate family for archaea.
Phosphoinositide phospholipase C is a family of eukaryotic intracellular enzymes that play an important role in signal transduction processes. These enzymes belong to a larger superfamily of Phospholipase C. Other families of phospholipase C enzymes have been identified in bacteria and trypanosomes. Phospholipases C are phosphodiesterases.
Glycosylphosphatidylinositol phospholipase D (EC 3.1.4.50, GPI-PLD, glycoprotein phospholipase D, phosphatidylinositol phospholipase D, phosphatidylinositol-specific phospholipase D) is an enzyme with systematic name glycoprotein-phosphatidylinositol phosphatidohydrolase. This enzyme catalyses the following chemical reaction
The enzyme phosphatidylinositol diacylglycerol-lyase catalyzes the following reaction:
The enzyme glycerophosphoinositol inositolphosphodiesterase (EC 3.1.4.43) is an enzyme that catalyzes the chemical reaction
The enzyme phosphatidate phosphatase (PAP, EC 3.1.3.4) is a key regulatory enzyme in lipid metabolism, catalyzing the conversion of phosphatidate to diacylglycerol:
In enzymology, a N-acetylglucosaminylphosphatidylinositol deacetylase (EC 3.5.1.89) is an enzyme that catalyzes the chemical reaction
In enzymology, a diglucosyl diacylglycerol synthase is an enzyme that catalyzes the chemical reaction
In enzymology, a monogalactosyldiacylglycerol synthase is an enzyme that catalyzes the chemical reaction
In enzymology, a phosphatidylinositol N-acetylglucosaminyltransferase is an enzyme that catalyzes the chemical reaction
In enzymology, a CDP-diacylglycerol—glycerol-3-phosphate 3-phosphatidyltransferase is an enzyme that catalyzes the chemical reaction
In enzymology, a CDP-diacylglycerol—inositol 3-phosphatidyltransferase is an enzyme that catalyzes the chemical reaction
In enzymology, a CDP-diacylglycerol—serine O-phosphatidyltransferase is an enzyme that catalyzes the chemical reaction
Phospholipase C (PLC) is a class of membrane-associated enzymes that cleave phospholipids just before the phosphate group (see figure). It is most commonly taken to be synonymous with the human forms of this enzyme, which play an important role in eukaryotic cell physiology, in particular signal transduction pathways. Phospholipase C's role in signal transduction is its cleavage of phosphatidylinositol 4,5-bisphosphate (PIP2) into diacyl glycerol (DAG) and inositol 1,4,5-trisphosphate (IP3), which serve as second messengers. Activators of each PLC vary, but typically include heterotrimeric G protein subunits, protein tyrosine kinases, small G proteins, Ca2+, and phospholipids.
Glypiation is the addition by covalent bonding of a glycosylphosphatidylinositol (GPI) anchor and is a common post-translational modification that localizes proteins to cell membranes. This special kind of glycosylation is widely detected on surface glycoproteins in eukaryotes and some Archaea.
Diacylglycerol kinase (CTP dependent) (EC 2.7.1.174, DAG kinase, CTP-dependent diacylglycerol kinase, diglyceride kinase) is an enzyme with systematic name CTP:1,2-diacyl-sn-glycerol 3-phosphotransferase. This enzyme catalyses the following chemical reaction
Diacylglycerol diphosphate phosphatase (EC 3.1.3.81, DGPP phosphatase, DGPP phosphohydrolase, DPP1, DPPL1, DPPL2, PAP2, pyrophosphate phosphatase) is an enzyme with systematic name 1,2-diacyl-sn-glycerol 3-phosphate phosphohydrolase. This enzyme catalyses the following chemical reaction
Variant surface glycoprotein (VSG) is a ~60kDa protein which densely packs the cell surface of protozoan parasites belonging to the genus Trypanosoma. This genus is notable for their cell surface proteins. They were first isolated from Trypanosoma brucei in 1975 by George Cross. VSG allows the trypanosomatid parasites to evade the mammalian host's immune system by extensive antigenic variation. They form a 12–15 nm surface coat. VSG dimers make up ~90% of all cell surface protein and ~10% of total cell protein. For this reason, these proteins are highly immunogenic and an immune response raised against a specific VSG coat will rapidly kill trypanosomes expressing this variant. However, with each cell division there is a possibility that the progeny will switch expression to change the VSG that is being expressed. VSG has no prescribed biochemical activity.
References
- Hereld D, Krakow JL, Bangs JD, Hart GW, Englund PT (1986). "A phospholipase C from Trypanosoma brucei which selectively cleaves the glycolipid on the variant surface glycoprotein". J. Biol. Chem. 261 (29): 13813–9. doi: 10.1016/S0021-9258(18)67092-9 . PMID 3759991.
- Carnall N, Webb H, Carrington M (1997). "Mutagenesis study of the glycosylphosphatidylinositol phospholipase C of Trypanosoma brucei". Mol. Biochem. Parasitol. 90 (2): 423–32. doi:10.1016/S0166-6851(97)00177-1. PMID 9476790.
- Armah DA, Mensa-Wilmot K (2000). "Tetramerization of glycosylphosphatidylinositol-specific phospholipase C from Trypanosoma brucei". J. Biol. Chem. 275 (25): 19334–42. doi: 10.1074/jbc.M001798200 . PMID 10764777.
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