Structure, Distribution and Biosynthesis of Glycerolipids from Higher Plant Chloroplasts

Summary

Galactolipids (MGDG and DGDG), sulfolipid and phosphatidylglycerol are the main constituents of plastid membranes. Glycerolipid biosynthesis requires first the assembly of glycerol and esterification by fatty acids at the sn-1 and sn-2 positions of the glycerol backbone. Then, the sn-3 position of phosphatidic acid or diacylglycerol is modified, for instance by addition of a third fatty acid for triacylglycerol, of a galactose for galactolipids, of a sulfoquinovose for sulfolipid, and phosphorylglycerol for phosphatidylglycerol. Directly or indirectly, the compounds used for the biosynthesis of glycerolipids derive from photosynthesis, i.e. from endogenous CO2 fixation by chloroplasts or from photosynthates produced in leaves. The two main MGDG molecular species found in chloroplasts have (a) 18:3 at both the sn-1 and sn-2 positions of the glycerol backbone, and (b) 18:3 and 16:3 respectively at the sn-1 and sn-2 positions of the glycerol backbone. The occurrence of such structures within plastid membranes reflects the existence of different pathways for the biosynthesis of these two types of molecules. Sulfolipid and phosphatidylglycerol molecular species also contain the typical structure of prokaryotic lipids with C16 fatty acids at the sn-2 position of glycerol. In contrast, a wide variety of diacylglycerol molecular species (i.e. with different acyl chain length and saturation levels at both sn positions) can be found in extremely variable amounts in envelope membranes where the synthesis of all typical plastid lipids takes place. Several enzymes, such as the inner envelope phosphatidate phosphatase and the outer envelope galactolipid:galactolipid galactosyltransferase, are involved in diacylglycerol formation, others, like the MGDG synthase or the sulfolipid synthase, use diacylglycerol within the inner envelope membrane as a substrate for the biosynthesis of membrane glycerolipids. A puzzling question is how such enzymes could be involved in the formation of the characteristic structural features of chloroplast glycerolipids and their final distribution within membranes. Although little molecular data are presently available on enzymes such as the phosphatidate phosphatase, the galactolipid:galactolipid galactosyltransferase or the MGDG synthase, detailed analysis of the biochemical properties of these key enzymes in galactolipid biosynthesis recently provided some clues to the problem. These observations suggest that the biochemical properties of the envelope MGDG synthase are highly responsible for the final MGDG molecular species found in plastid membranes.