Glycerophospholipids or phosphoglycerides are glycerol-based phospholipids. They are the main component of biological membranes in eukaryotic cells. They are a type of lipid, of which its composition affects membrane structure and properties. Two major classes are known: those for bacteria and eukaryotes and a separate family for archaea.
Structures Glycerophospholipids are derived from glycerol-3-phosphate in a de novo pathway. The term glycerophospholipid signifies any derivative of glycerophosphoric acid that contains at least one O-acyl, or O-alkyl, or O-alk-1'-enyl residue attached to the glycerol moiety. The phosphate group forms an ester linkage to the glycerol. The long-chained hydrocarbons are typically attached through ester linkages in bacteria/eukaryotes and by ether linkages in archaea. In bacteria and procaryotes, the lipids consist of diesters commonly of C16 or C18 fatty acids. These acids are straight-chained and, especially for the C18 members, can be unsaturated. For archaea, the hydrocarbon chains have chain lengths of C10, C15, C20 etc. since they are derived from isoprene units. These chains are branched, with one methyl substituent per C5 subunit. These chains are linked to the glycerol phosphate by ether linkages. The two hydrocarbon chains attached to the glycerol are hydrophobic while the polar head, which mainly consists of the phosphate group attached to the third carbon of the glycerol backbone, is hydrophilic. This dual characteristic leads to the amphipathic nature of glycerophospholipids. They are usually organized into a bilayer in membranes with the polar hydrophilic heads sticking outwards to the aqueous environment and the non-polar hydrophobic tails pointing inwards. Glycerophospholipids consist of various diverse species which usually differ slightly in structure. The most basic structure is a phosphatidate. This species is an important intermediate in the synthesis of many phosphoglycerides. The presence of an additional group attached to the phosphate allows for many different phosphoglycerides. By convention, structures of these compounds show the 3 glycerol carbon atoms vertically with the phosphate attached to carbon atom number three (at the bottom). Plasmalogens and phosphatidates are examples.
Nomenclature and stereochemistry In general, glycerophospholipids use an "sn" notation, which stands for stereospecific numbering. When the letters "sn" appear in the nomenclature, by convention the hydroxyl group of the second carbon of glycerol (2-sn) is on the left on a Fischer projection. The numbering follows the one of Fischer's projections, being 1-sn the carbon at the top and 3-sn the one at the bottom. The advantage of this particular notation is that the spatial configuration (D or L) of the glycero-molecule is determined intuitively by the residues on the positions sn-1 and sn-3. For example sn-glycero-3-phosphoric acid and sn-glycero-1-phosphoric acid are enantiomers. Most vegetable oils have unsaturated fatty acids in the sn-2 position, with saturated fatty acids in the 1-sn and/or 3-sn position. Animal fats more often have saturated fatty acids in the 2-sn, with unsaturated fatty acids in the 1-sn and/or 3-sn position.
Examples Plasmalogens Plasmalogens are a type of phosphoglyceride. The first carbon of glycerol has a hydrocarbon chain attached via an ether, not ester, linkage. The linkages are more resistant to chemical attack than ester linkages are. The second (central) carbon atom has a fatty acid linked by an ester. The third carbon links to an ethanolamine or choline by means of a phosphate ester. These compounds are key components of the membranes of muscles and nerves.
Phosphatidates Phosphatidates are lipids in which the first two carbon atoms of the glycerol are fatty acid esters, and the 3 is a phosphate ester. The phosphate serves as a link to another alcohol-usually ethanolamine, choline, serine, or a carbohydrate. The identity of the alcohol determines the subcategory of the phosphatidate. There is a negative charge on the phosphate and, in the case of choline or serine, a positive quaternary ammonium ion. (Serine also has a negative carboxylate group.) The presence of charges give a "head" with an overall charge. The phosphate ester portion ("head") is hydrophilic, whereas the remainder of the molecule, the fatty acid "tail", is hydrophobic. These are important components for the formation of lipid bilayers. Phosphatidylethanolamines, phosphatidylcholines, and other phospholipids are examples of phosphatidates. The prefix phosphatidyl-, invokes phosphatidate in the stricter sense where, like phosphatidic acid, phosphate is a simple group. Phosphatidylcholine, for example, is derived from (-yl-) the basic PA structure with the addition of choline.
Phosphatidylcholines Phosphatidylcholines are lecithins. Choline is the alcohol, with a positively charged quaternary ammonium, bound to the phosphate, with a negative charge. Lecithins are present in all living organisms. An egg yolk has a high concentration of lecithins, which are commercially important as an emulsifying agent in products such as mayonnaise. Lecithins are also present in brain and nerve tissue.
Phosphatidylinositol Phosphatidylinositol makes up a small component of the cytosol in eukaryotic cell membranes and gives molecules a negative charge. Its importance relies in its role in activating sensory receptors that correlate with taste functions.
Phosphatidylserine Phosphatidylserine is important in cell signaling, specifically apoptosis. Cells will use this phosphatidylserine to enter cells via apoptotic mimicry. The structure of this lipid differs in plants and animals, regarding fatty acid composition. In addition, phosphatidylserine plays an important role in the human brain content, as it makes up 13–15% of the phospholipids in the human cerebral cortex. This lipid is found in a wide range of places. For example, in the human diet, about 130 mg are derived from phosphatidylserine. This has been said to have a positive impact on the brain, as it helps with reduced stress and improved memory.
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