Y

CH2OP-OCH2CH-CH2OH

Animals, higher plants, microorganisms.

Animals; small amounts in yeast; plants.

Animals; small amounts in yeast; plants.

Micro-organisms (M. phlei, M. tuberculosis).

Mainly higher plants and micro-organisms.

Diphosphatidylglycerol

(bisphosphatidylglycerol, cardiolipin)

Animal; higher plants; micro-organisms.

The natural lipid is found as a derivative of myo-inositol-1-phosphate only. The major inositol-containing phosphoglyceride (about 80% of total in animals). 3-Phosphate derivatives of inositol phosphoglycerides present in very small amounts.

Mainly nervous tissue, but also plasma membranes of other cells.

Distribution as above. Both compounds have very high rates of turnover.

'Free' glycerol has opposite stereochemical configuration to the acylated glycerol, i.e. 1,2-diacyl-1-sn-glycerol. Probably most abundant phospholipid because of its prevalence in photosynthetic membranes.

Major component of many bacteria, localized in inner mitochondrial membrane of eukaryotes.

Phosphatidylglycerol tional 'X' groups are esterified to the phosphate moiety the lipids are called phosphatidyl-X. Major types of such diacylphosphoglycerides are shown in Table 6.3 where relevant comments about their distribution and properties are made also.

6.2.2 Phosphonolipids constitute a rare class of lipids found in few organisms

In 1953, Rouser and his associates first identified a phosphonolipid in biological extracts - from a sea anemone. Subsequent work has shown that such lipids, which contain a C-P bond, are significant constituents of lower animals such as molluscs, coelenterates and protozoa. Phosphonolipids can be detected in bacteria and mammals, but only in very low quantities. Two types of glycerophos-phonolipids have been found (Fig. 6.2) and there are also other structures that contain a sphingosine (and, sometimes, a galactosyl) residue instead of glycerol. Usually, organisms tend to accumulate phosphonolipids based on either glycerol or on sphingosine rather than both types. In protozoa such as Tetrahymena, the glycerophosphonolipids are concentrated in the ciliary membranes. This is perhaps because the phosphonolipids are particularly resistant to chemical as well as enzymic attack.

6.2.3 Glycosylglycerides are particularly important components of photosynthetic membranes

Glycolipids that are based on glycerol have been found in a wide variety of organisms. However, whereas in animals they are only found in very small quantities, they are major constituents of some micro-organisms and are the main lipid components of the photosynthetic membranes of algae (including the blue-greens or cyanobacteria) and plants (Table 6.4). Their structure is analogous to that of glycerophospholipids with the sugar(s) attached glycosidically to position 3 of glycerol and fatty acids esterified at the other two positions.

Since the membranes of higher plant chloroplasts are the most prevalent on land, and the photo-synthetic membranes of marine algae are the most common in the seas and ocean, it follows that these glycosylglycerides are the most abundant mem brane lipids in Nature, in spite of the sparse attention paid to them in most standard biochemistry textbooks! The two galactose-containing lipids -monogalactosyldiacylglycerol and digalactosyldi-acylglycerol - represent about 40% of the dry weight of photosynthetic membranes of higher plants. Position 1 of the galactose ring has a (3-link to glycerol whereas, in digalactosyldiacylglycerol there is an a,1-6 bond between the sugars. Whereas galactose is almost the only sugar found in the glycosylglycerides of higher plants, other sugars such as glucose may be found in algae, particularly marine species. In bacteria several combinations of residues may be found in diglycosyldiacylglycerols (Table 6.5). The most common combinations are two glucose, two galactose or two mannose residues linked a,1-2 or (3,1-6. Such glycosylglycerides do not form a large proportion of the total lipids in bacteria, but are found more frequently in the Gram-positives or photosynthetic Gram-negatives. In addition, bacteria may contain higher homologues with up to seven sugar residues.

Apart from the galactose-containing lipids, a third glycosylglyceride is found in chloroplasts (Table 6.4). This is the plant sulpholipid. It is more correctly called sulphoquinovosyldiacylglycerol and contains a sulphonate constituent on carbon 6 of a deoxyglucose residue. This sulphonic acid group is very stable and also highly acidic so that the plant sulpholipid is a negatively charged molecule in Nature. Although this sulpholipid occurs in small amounts in photosynthetic bacteria and some fungi, it is really characteristic of the photosynthetic membranes of chloroplasts and cyanobacteria.

All three chloroplast glycolipids usually contain large amounts of a-linolenic acid (Table 6.6). In fact, monogalactosyldiacylglycerol may have up to 97%

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