Classification and Nomenclature of Glycerophospholipids

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.

sn-1

sn-1

O O—X sn-1-alkyl-sn-2acyl species

Phospholipid

Phosphatidylcholine (PC) Phosphatidylethanolamine (PE)

Phosphatidylserine (PS)

Phosphatidylinositol (PI)

Phosphatidylglycerol (PG) Phosphatidic acid (PA)

sn-1

sn-1

Headgroup(X)

Headgroup(X)

.nh2

Figure 1 Molecular structures of phospholipids. The class of phospholipid is defined by the nature of the nitrogenous base or polyol esterified to the phosphate group (X). The species distribution within any phospholipid class is determined by the fatty acyl substituents at the sn-1 and sn-2 positions of the glycerol backbone. The dipalmitoyl species would be designated PC16:0/16:0 if X was choline. If arachidonic acid was esterified at sn-2, then the molecule would be designated as PC1 6:0/20:4. In the diacyl species, fatty acids are attached by ester linkages. For sn-1-alkyl-sn-2-acyl species, the sn-1 fatty acid is attached by an ether bond. For sn-1-alkenyl-sn-2-acyl species, the sn-1 fatty acid is attached by a vinyl ether linkage.

In addition to diacyl species, with both fatty acids attached by ester bonds, there are a number of species with ether-linked fatty acids, principally in the sn-1 position. These ether phospholipids include 1-alkyl-2-acyl species, largely present in PC, and 1-alk-1-enyl species (plasmalogens), largely present in PE. These ether lipids are major components of many cell membranes, particularly neuronal and inflammatory cells, and there have been significant advances in understanding the biochemical pathways for their synthesis and catabolism. Some alkyl acyl PC species are substrates for the generation of the potent bioactive lipid platelet-activating factor (1-alkyl-2-acetyl-glycero-3-phosphocholine (PAF)), but the function of most ether lipids is largely unclear. One possibility is that generation of 1-alkyl-2-acyl-glycerol as a second messenger rather than diacylglycerol may contribute to differential regulation of protein kinase C isoforms, and antioxidant properties have been reported for plasmalogens.

N Lipid rafts

Outside cell

Plasma ;; '{¡; ;; ;; ¡; u :< ^ ;; u n :.; ■■! :i ;; ;'

membrane;; y. :■) il i; )■ >) !1« )) )) i; ¡1 K V: ¡; ¡1 !! ;( Cytoplasm

Neutral phospholipid \\ Acidic phospholipid SI (uncharged) - PC, PE ' (negatively charged - PS, PI, PA)

Sphingomyelin VI Glucosylphosphatidylinositol-linked (rigid, uncharged) protein (GPI)

Figure 2 Topology of distribution of phospholipids within the plasma membrane of eukaryotic cells. The outer leaflet of the membrane bilayer is enriched in neutral PC and rigid components, such as sphingolipids and glucosylphosphatdylinositol-linked proteins. The distribution of charged acidic phospholipids, such as PS and PI, to the inner leaflet of the membrane is actively regulated by a combination of enzymes called flipases and scramblases.

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