Lipids of Archaebacteria

Abstract

The archaebacteria currently consist of several distinct subgroups including methanogens, extreme halophiles and certain thermoacidophiles. The lipids of archaebacteria are distinguished from those of other prokaryotes and eukaryotes by the absence of fatty acid glycerol ester lipids and the predominance of nonsaponifiable lipids. The lipid composition of the archaebacteria consists of isoprenoid and hydroisoprenoid hydrocarbons and isopranyl glycerol ether lipids.

The glycerol ethers of archaebacteria, which constitute the hydrophobic residues of the polar lipids and consequently the membrane interior are diphytanylglycerol diethers or dibiphytanyldiglycerol tetraethers. Either or both glycerol ether structures may be present, depending on genus. The tetraethers of the thermoacidophilic archaebacteria are more specialized in that the dibiphytanyl alkyl chains may contain 1 to 4 cyclopentyl rings. As a consequence of the presence of the tetraethers which can span the membrane, some archaebycterial membranes may exist as a lipid “monolayer” rather than the usual lipid bilayer. The structure of some diether-containing polar lipids of archaebacteria have been well established. The extent of the variety of tetraether containing polar lipid structures is still largely unknown, but both the symmetric and asymmetric substitution of polar head groups to the tetraether has been established in some instances. Among neutral lipids, squalenes and isoprenoid hydrocarbons appear to be universal. The exact pathways for the biosynthesis of the lipid components remain a challenge, but clearly the mevalonate pathway for isoprenoid biosynthesis is the major route of lipid synthesis in archaebacteria rather than the malonyl-CoA pathway for fatty acid biosynthesis in prokaryotes and eukaryotes.

The isopranyl glycerol ethers are distinctive, providing a useful taxonomic tool and molecular marker for the identification of archaebacteria. The lipids can also serve as useful biochemical “fossil” evidence for tracing the earlier existence of the organisms. Overall, the discontinuity of archaebacterial lipids formulates a point for delineating early stages of biological evolution and supports the concept that archaebacteria represent a third line of evolutionary descent.