Glycerophospholipid biosynthesis (Homo sapiens)
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Description
Glycerophospholipids are important structural and functional components of biological membranes and constituents of serum lipoproteins and the pulmonary surfactant. In addition, glycerophospholipids act as precursors of lipid mediators such as platelet-activating factor and eicosanoids. Cellular membranes contains a distinct composition of various glycerophospholipids such as phosphatidic acid (PA), phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidylglycerol (PG), phosphatidylinositol (PI), cardiolipin (CL), lysophosphatidic acid (LPA) and lysobisphosphatidic acid (also known as bis(monoacylglycerol) hydrogen phosphate - BMP).
Glycerophospholipids are first formed by the de novo (Kennedy) pathway using fatty acids activated as acyl-CoA donors. However, the acyl groups of glycerophospholipids are highly diverse and distributed in an asymmetric manner. Saturated and monounsaturated fatty acids are usually esterified at the sn-1 position, whereas polyunsaturated acyl groups are esterified at the sn-2 position. Subsequent acyl chain remodeling (Lands cycle) generates the diverse glycerophospholipid composition and asymmetry characteristic of cell membranes.
In the de novo pathway of glycerophospholipid biosynthesis, lysophosphatidic acid (LPA) is initially formed from glycerol 3-phosphate (G3P). Next, LPA is converted to PA by a LPA acyltransferase (AGPAT, also known as LPAAT), then PA is metabolized into two types of glycerol derivatives. The first is diacylglycerol (DAG) which is converted to triacylglycerol (TAG), PC, and PE. Subsequently, PS is synthesized from PC or PE. The second is cytidine diphosphate-diacylglycerol (CDP-DAG), which is processed into PI, PG, CL, and BMP. Each glycerophospholipid is involved in acyl chain remodeling via cleavage by phospholipases followed by reacylation by an acyltransferase.
Most of the glycerophospholipids are synthesized at the endoplasmic reticulum (ER), however, some, most notably cardiolipin, and BMP are synthesized in the mitochondrial and endosomal membranes respectively. Since the most of the glycerophospholipids are found in all membrane compartments, there must be extensive network of transport of glycerophospholipids from one membrane compartment to another via various mechanisms including diffusion through the cytosol, formation of transportation complexes, and diffusion via membrane contact sites (MCS) (Osman et al. 2011, Lebiedzinska et al. 2009, Lev 2010, Scherer & Schmitz 2011, Orso et al. 2011, Hermansson et al. 2011, Vance & Vance 2008). View original pathway at Reactome.
Glycerophospholipids are first formed by the de novo (Kennedy) pathway using fatty acids activated as acyl-CoA donors. However, the acyl groups of glycerophospholipids are highly diverse and distributed in an asymmetric manner. Saturated and monounsaturated fatty acids are usually esterified at the sn-1 position, whereas polyunsaturated acyl groups are esterified at the sn-2 position. Subsequent acyl chain remodeling (Lands cycle) generates the diverse glycerophospholipid composition and asymmetry characteristic of cell membranes.
In the de novo pathway of glycerophospholipid biosynthesis, lysophosphatidic acid (LPA) is initially formed from glycerol 3-phosphate (G3P). Next, LPA is converted to PA by a LPA acyltransferase (AGPAT, also known as LPAAT), then PA is metabolized into two types of glycerol derivatives. The first is diacylglycerol (DAG) which is converted to triacylglycerol (TAG), PC, and PE. Subsequently, PS is synthesized from PC or PE. The second is cytidine diphosphate-diacylglycerol (CDP-DAG), which is processed into PI, PG, CL, and BMP. Each glycerophospholipid is involved in acyl chain remodeling via cleavage by phospholipases followed by reacylation by an acyltransferase.
Most of the glycerophospholipids are synthesized at the endoplasmic reticulum (ER), however, some, most notably cardiolipin, and BMP are synthesized in the mitochondrial and endosomal membranes respectively. Since the most of the glycerophospholipids are found in all membrane compartments, there must be extensive network of transport of glycerophospholipids from one membrane compartment to another via various mechanisms including diffusion through the cytosol, formation of transportation complexes, and diffusion via membrane contact sites (MCS) (Osman et al. 2011, Lebiedzinska et al. 2009, Lev 2010, Scherer & Schmitz 2011, Orso et al. 2011, Hermansson et al. 2011, Vance & Vance 2008). View original pathway at Reactome.
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AcCho is synthesised in the cytoplasm of cholinergic neurons from acetyl-CoA and Cho by CHAT enzyme.
Acetylcholine from the synaptic cleft is degraded into inactive molecules, Cho and acetate by ACHE, which is located in the synaptic cleft (Weinstock & Groner 2008).
CTL1 is broadly expressed on leukocytes and endothelial cells (Wille et al. 2001). CTL2 is highly expressed in human inner ear and is the target of antibody-induced hearing loss (Nair et al. 2004).
Defects in AGK can cause mitochondrial DNA depletion syndrome 10 (MTDPS10 aka Sengers syndrome; MIM:212350), an autosomal recessive mitochondrial disorder characterized by congenital cataracts, hypertrophic cardiomyopathy, skeletal myopathy, exercise intolerance, and lactic acidosis. Mental development is normal, but affected individuals may die early from cardiomyopathy (Sengers et al. 1975, Mayr et al. 2012).
See recent review by Agarwal (2012, in press).
AGPAT1, 2, 3 and LPCAT1 have been characterized biochemically (AGPAT1, 2: Yamashita et al. 2007, West et al. 1997, Aguado and Campbell 1998, Gale et al. 2006; AGPAT3: Agarwal et al. 2006; LPCAT1: Nakanishi et al. 2006, Chen et al. 2006). Two additional proteins, AGPAT4 and AGPAT5, are inferred to have such activity based on studies of homologous mouse enzymes (Lu et al. 2005). These enzymes differ in their tissue specific patterns of expression in the body and in their preferences for specific acyl CoA molecules (Shindou and Shimizu 2009; Takeuchi and Reue 2009).