Glycerophospholipid biosynthetic pathway

Glycerophospholipids or phosphoglycerides, in which the hydrophobic regions are composed of two fatty acids joined to glycerol; and sphingolipids, in which a single fatty acid is joined to a fatty amine, sphingosine, are glycerol-based phospholipids and the main component of biological membranes. The hydrophilic moieties in these amphipathic compounds may be as a simple as a single -OH at one end of the sterol ring system, or they may be more complex. Glycerophospholipds, as well as sphingolipids, contained polar or charged alcohols at their polar ends; some also contain phosphate groups. In glycerophospholipids, two fatty acids are ester-linked to glycerol at C-1 and C-2, and a highly polar or charged (and therefore hydrophilic) head group is attached to C-3 through a phosphodiester bond. All glycerophospholipds are derivatives of phosphatidic acid and are named for their polar head groups (e.g., phosphatidylethanolamine and phosphatidylcholine). All have a negative charge on the phosphate group at pH 7.0. The head-goup alcohol may also contribute one or more charges at pH near 7.0. The fatty acids in glycerophospholipds can be any of a wide variety. They are different in different species, in different tissues of the same species, and in different types of glycerophospholipids in the same cell or tissue. In general, glycerophospholipids contain a saturated fatty acid at C-1 and an unsaturated fatty acid at C-2, and, in general terms, the fatty acyl groups are generally 16 or 18 carbons long. Eukaryotic membranes contain significant amounts of two other types of glycerophospholipids: Plasmalogens and Alkylacylglycerophospholipids. Plasmalogens contain a hydrocarbon chain linked to glycerol C-1 via vinyl ether linkage whereas alkylacylglycerophospholipids the alkyl substituent at glycerol C-1 is attached via an ether linkage. About 20% of mammalian glycerophospholipids are plasmalogens, this percentage varies both from species to species and from tissue to tissue within a given organism. While plasmalogens comprise only about 0.8% of the phospholipids in human liver, they account for around 23% of those in human nervous tissue. The alkylacylglycerophospholipids are less abundant than the plasmalogens, e.g., about 59% of ethanolamine glycerophospholipids of human heart are plasmalogens, whereas only 3.6% are alkylacylglycerophospholipids. However, in bovine erythrocytes, 75% of the ethanolamine glycerophospholipids are of alkylacyl type.

甘油磷脂或磷甘油酯，其疏水区域由两个与甘油结合的脂肪酸组成；而鞘脂则是由一个脂肪酸与脂肪胺——鞘氨醇结合而成，这些甘油磷脂均为以甘油为基础的磷脂，并构成生物膜的主要成分。这些两亲性化合物的亲水基团可能仅由甾体环系统一端的单个羟基构成，亦或更为复杂。甘油磷脂以及鞘脂在其极性端含有极性或带电的醇类；其中一些还含有磷酸基团。在甘油磷脂中，两个脂肪酸通过酯键连接到甘油的C-1和C-2位置，并通过磷酯键连接一个高度极性或带电（因此亲水）的头部基团。所有甘油磷脂均为磷脂酸的衍生物，并以它们的极性头部基团命名（例如磷脂酰乙醇胺和磷脂酰胆碱）。在pH 7.0时，所有甘油磷脂的磷酸基团均带负电荷。头部基团的醇类在pH接近7.0时也可能贡献一个或多个电荷。甘油磷脂中的脂肪酸可以是多种多样。它们在不同物种、同一物种的不同组织和同一细胞或组织中的不同类型的甘油磷脂中各不相同。一般来说，甘油磷脂在C-1位置含有一个饱和脂肪酸，在C-2位置含有一个不饱和脂肪酸，并且脂肪酸酰基通常由16或18个碳原子组成。真核生物膜中还含有两种其他类型的甘油磷脂：溶血磷脂和烷基酰基甘油磷脂。溶血磷脂含有通过乙烯基醚键连接到甘油C-1位置的烃链，而烷基酰基甘油磷脂的烷基取代基通过醚键连接到甘油的C-1位置。大约20%的哺乳动物甘油磷脂是溶血磷脂，这一比例在物种之间以及在同一生物体内不同组织之间都有所不同。虽然溶血磷脂在人类肝脏中的磷脂含量中仅占约0.8%，但它们在人类神经系统中的比例约为23%。烷基酰基甘油磷脂的丰度低于溶血磷脂，例如，人类心脏的乙醇胺甘油磷脂中约59%是溶血磷脂，而只有3.6%是烷基酰基甘油磷脂。然而，在牛的红细胞中，75%的乙醇胺甘油磷脂是烷基酰基型的。