Three important classes of membrane lipid are widely distributed - glycerolipids, sphingolipids and steroids. Of these, glycerolipids are quantitatively by far the most important group. They can be conveniently divided into two main groups - those containing phosphorus (phosphoglycerides) and those without phosphorus but containing a sugar constituent (glycosylglycerides). Confusingly, some compounds (e.g. sphingomyelin) can be classified in more than one group.
6.2.1 Phosphoglycerides are the major lipid components of most biological membranes
The stereochemistry of phosphoglycerides was discussed in Chapter 1. The phosphoglycerides comprise a very widespread and diverse group of structures. In most membranes they are the main lipid components and, indeed, the only general exceptions to this statement are the photosynthetic membranes of plants, algae and cyanobacteria and the archaebacterial membranes.
Usually, phosphoglycerides contain fatty acids esterified at positions 1 and 2 of glycerol. They are, thus, diacylphosphoglycerides. These lipids are named after the moiety which is attached to the phosphate esterified at position 3 of glycerol. Thus, the compounds can be thought of as derivatives of diacylglycerols in which the hydroxyl on carbon atom 3 is esterified with phosphoric acid, which in turn is esterified with a range of molecules -organic bases, amino acids, alcohols.
The simplest phosphoglyceride contains only phosphoric acid attached to diacylglycerol and is called phosphatidic acid (Table 6.3). Where addi-
Table 6.3 Structural variety of different diacylglycerophospholipids
Name of phospholipid Source
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