Polar Lipid Structure, Composition and Biosynthesis in Extremely Halophilic Bacteria

Abstract

The polar lipids of extremely halophilic bacteria are derived from the saturated dialkylglycerol ether, sn-2,3-diphytanylglycerol, abbreviated “archaeol”, and consist of phospholipids (archaeol analogues of phosphatidylglycerol, PG, phosphatidylglycerophosphate, PGP, and phosphatidylglycerosulfate, PGS) and glycolipids (sulfated triglycosyl-archaeol, S-TGA- I, triglycosylarchaeol, TGA-2, sulfated diglycosyl-archaeol, S-DGA- 1, and others). The polar lipid composition, particularly that of the glycolipids, appears to be correlated with the taxonomic classification of the extreme halophiles on the level of the genera so far distinguished: Halobacterium, Haloarcula,Halo ferax, Halococcus, Natronobacterium and Natronococcus. Biosynthesis of these archaeol analogues of phospholipids and glycolipids proceeds by complex pathways in a multienzyme, membrane-bound system, absolutely dependent on high salt concentration. The first step is the alkylation of a glycerol derivative (presumably dihydroxyacetone) with a C₂₀-isoprenylpyrophosphate. The alkylated product, containing ether-linked C₂₀-isoprenyl groups, serves as precursor for both the isoprenylarchaeol phospholipids and the glycolipids. Stepwise reduction of the isoprenyl chains to phytanyl chains then gives the final saturated archaeol analogues of phospholipids and glycolipids.