Glycerophospholipid classes are commonly referred to as phosphatidylcholine, phosphatidylethanola-mine, etc. They are composed of a spectrum of molecular species (phospholipid molecular species are the individual different molecules within any different class of phospholipid determined by the combination of fatty acids esterified to the glycerol backbone. Any given mammalian cell contains up to 1000 individual phospholipid molecular species) defined by the substituent fatty acid groups attached to the sn-1 and sn-2 positions of the glycerol backbone. For example, the individual molecular species palmitoyloleoyl phosphatidylcholine can be named formally as either glycerol 1-hexadecanoate 2-9-octadecaenoate 3-phosphocholine or 1-hexadeca-noyl-2-octadeca-9-enoyl-3-glycerophosphocholine. One shorthand designation for this molecule, adopted in this article, is PC16:0/18:1, where PC designates the phospholipid class, in this case phos-phatidylcholine, and 16:0 and 18:1 (fatty acid nomenclature is based on total number of carbon atoms in the acyl chain, followed by total number of double bonds. For instance, 16:0 is saturated 16-carbon palmitic acid, whereas 20:4 is poplyunsatu-rated 20-carbon arachidonic acid) designate the fatty acids esterified at the sn-1 and sn-2 positions.
For phospholipids from cell membranes, saturated fatty acids are generally located at the sn-1 and unsaturated fatty acids at the sn-2 position, with notable exceptions. For instance, dipalmitoyl PC (PC16:0/16:0) is a major component of lung and surfactant PC, whereas significant amounts of dido-cosahexaenoyl phosphatidylethanolamine (PE22:6/ 22:6) are present in retinal PE. In addition, PC species with 18:1n-9 at the sn-1 position are minor components of many cells.
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