Fig. 6.2 Glycerophosphonolipids: (a) phosphatidylethylamine; (b) an analogue, which is a derivative of chimyl alcohol rather than a diacylphosphonolipid. It is named l-hexadecyl-2-acyl-sn-glycero(3)-2-phosphonoethylamine.
Table 6.4 Major glycosylacylglycerols of plant and algal photosynthetic membranes
Structure and chemical name
Source and fatty acid composition
Plant sulpholipid (sulphoquinovosyl-diacylglycerol; SQDG)
0-ch, chocor1 oh ch2ocor2 l,2-diacyl-[a-d-galactopyranosyl-(l/^6/)-ß-d-galactopyranosyl-(r^3)]-sn-glycerol h,c—s03H
oh o-ch, chocor1
1,2-diacyl-[6-sulpho-a-d-quinovopyranosyl-(1^3)-sn-glycerol. [d-quinovose is 6-deoxy-d-glucose. Note the carbon-sulphur bond]
Especially abundant in plant leaves and algae; mainly in chloroplast. Contains a high proportion of polyunsaturated fatty acids. Chlorella vulgaris MGDG has mainly 18:1,18:2 when dark grown but 20% 18:3 when grown in the light. Euglena gracilis MGDG has 16:4. Spinach chloroplast MGDG has 25% 16:3, 72% 18:3. Also found in the central nervous systems of several animals in small quantity.
Usually found together with MDGD in chloroplasts of higher plants and algae. Not quite so abundant as MGDG. Also has high proportion of polyunsaturated fatty acids, especially 18:3. In both lipids the glycerol has the same configuration as in the phospholipids.
Usually referred to as a 'sulpholipid' as distinct from a 'sulphatide', which is reserved for cerebroside sulphates. Typical lipid of chloroplast membranes but present elsewhere in some marine algae. Also found in cyanobacterial photosynthetic membranes and, to a lesser extent, in purple photosynthetic bacteria. Contains more saturated fatty acids (mainly palmitic) than the galactolipids, e.g. spinach leaf sulpholipid has 27% 16:0, 39% 18:2, 28% 18:3.
of its total acyl groups as this one component in some plants! The reason for this exceptional enrichment is not known although speculations have been made (Section 6.5.11). Moreover, a unifying theory connected with photosynthesis is not possible since many cyanobacteria do not make a-linolenate and marine algae contain little of the acid. In the plant sulpholipid the most usual combination of acyl groups is palmitate/a-linolenate. Interestingly, unlike animal lipids (Section 6.2.1) most palmitate is esterified at position 2 and most of the a-linolenate at position 1. This distribution of fatty acids stems from the special features of plant fatty acid metabolism (Section 2.2.5) and because the sulpholipid is assembled via the 'prokaryotic pathway' of metabolism. Since the position at which saturated and unsaturated fatty acids are attached to the glycerol has a marked effect on their melting properties (Section 6.5.9), fatty acid distribution may have functional significance. It has also been speculated that the different distribution of acyl moieties between plants and animals may be related to specific interactions with membrane proteins (Section 6.5.11).
Bacteria contain a number of phosphatidylgly-colipids. These compounds are confined to certain types of Gram-positive organisms such as the streptococci or mycoplasmas. In addition, different species of algae and bacteria (including archae-bacteria) contain various sulphoglycolipids -
Table 6.5 Some glycosylglycerides found in bacteria
Structure of glycoside moiety
ß-d-Glucopyranosyl-(1^6)-0-ß-d-glucopyranoside a-d-Glucopyranosyl-(1^2)-0-a-d-glucopyranoside a-d-Mannopyranosyl-(1^3)-0-a-d-mannopyranoside
Lactobacillus usually with the sulphur present in a sulphate ester attached to the carbohydrate moiety. Lipids containing taurine seem to be significant components of some protozoa.
6.2.4 Betaine lipids are important in some organisms
Betaine ether-linked glycerolipids are naturally occurring lipids which are important in many lower plants (e.g. bryophytes), algae, fungi and protozoa. Because of their structure (Fig. 6.3) and properties, they are often envisaged as substituting for phosphatidylcholine in membranes. A number of structures has been identified but diacylglycer-yltrimethylhomoserine (DGTS) is the most common. Typically, it represents 10-20% of all membrane lipids but, in some species of mosses, it may be around 40% of the total.
6.2.5 Ether-linked lipids and their bioactive species
Apart from the betaine ether-linked glycerolipids (Section 6.2.4), there are many lipids that have ether links to carbon position 1 (and sometimes position
Table 6.6 Fatty acid compositions of glycosylglycerides in two plants
(% total fatty acids)
Spinach MGDG trace
Pea MGDG 4
'16:3-plants' contain hexadecatrienoate in their monogalactosyldiacylglycerol while
'18:3-plants' contain a-linolenate instead. The reason for this is provided by the differences in fatty acid metabolism between these two types of plants (see Section 2.2.5).
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