Archaebacterial lipids

Symmetric amphiphilic molecules, in which two hydrophilic residues are linked by hydrophobic segments, are generally known as bola-lipids based on their resemblance to an old South American hunting weapon. Well-characterized bola amphiphiles are archaebacterial lipids, which usually consist of two glycerol backbones connected by two hydrophobic... 

Chain elongation during polymerization of prenyl units can be terminated in one of a number of ways. The pyrophosphate group may be hydrolyzed to a monophosphate or to a free alcohol. Alternatively, two polyprenyl compounds may join "head to head" to form a symmetric dimer. The C30 terpene squalene, the precursor to cholesterol, arises in this way from two molecules of famesyl diphosphate as does phy-toene, precursor of the Qo carotenoids, from E,E,E geranylgeranyl diphosphate. The phytanyl groups of archaebacterial lipids (p. 385) arise rather directly from geranylgeranyl diphosphate by transfer of the poly-... 

Present nomenclature of these core lipids and their variants or derivatives is cumbersome and trivial names appear to be required. Nishihara et al. [26] have suggested that diphytanylglycerol diether (structure 1, Fig. 1) and its variants (lA-lE, Fig. 1) be called archaeol (modified by adding the appropriate alkyl group designations, such as C20, C25, 3-hydroxy, etc.), and dibiphytanyldiglycerol tetraether (2) and its variant (2A) be called caldarchaeol and nonitolcaldarchaeol , respectively. This forms the basis of a useful and practical system of nomenclature for archaebacterial lipids, which will be followed here. 

It is clear that detailed studies with whole-cell and cell-free systems are now required to elucidate the novel mechanisms underlying the complex biosynthetic pathways of archae-bacterial lipids, which have been outlined in the foregoing sections. Perhaps the time is now ripe for a concerted effort to be made to clone the genes involved in archaebacterial lipid biosynthesis, thereby aiding greatly in identifying the enzymes involved. 

Furthermore, the unnatural -l configuration of the backbone glycerol would impart resistance to attack by phospholipases released by other organisms and would thus have a survival value for the archaebacteria. More specific functions of archaebacterial lipids will now be considered. 

The archaebacterial lipids have been used to prepare liposomes. They form monolayer lipid membranes70 rather than the normal bilayer type because there are polar groups on both ends of the a,oo-dicarboxylic acids. Membrane lipids extracted from these bacteria have also been used to form liposomes. Physical studies showed positive results such as extra stability at high temperature, at high or low pH, and in other harsh conditions.71-74... 

The potential applications of archaebacterial lipid liposomes (archaeosomes) as novel vaccine and drug delivery systems have been reviewed recently.88-90 In general, this type of liposomes exhibited higher stability to oxidation, high temperature, alkaline pH, and action of phopholipases. The safety profile study of these type of liposomes had shown that they are well tolerated after intravenous or oral delivery in mice. The stability and safety profile of these liposomes indicated that they may offer a superior alternative to the use of conventional liposomes. 

Figure 19.4. Structures of two archaebacterial lipid models (8, 9) and a synthetic extremophile lipid analog with PE-type head groups (10).

The use of stereospecitic numbering (sn) method for the glycerol-based lipids (e.g., glycerolipids and glycerophospholipids) [3], Acyl or alkyl chains are typically linked to the i -l and/or sn-2 positions of glycerol, with the exception of some lipids that contain three acyl or alkyl chains or contain more than one glycerol group and archaebacterial lipids in which sn-2 and/or sn-3 modification occurs. 

Angehni, R., Babudri, F., Lobasso, S. and Corcelli, A. (2010) MALDI-TOF/MS analysis of archaebacterial lipids in lyophilized membranes dry-mixed with 9-aminoacridine. J. Lipid Res. 51, 2818-2825. 

Lobasso, S. Saponetti, M. S. Polidoro, K Lopalco, P Urbanija, J. Kralj-Iglic, V. Corcelli, A. Archaebacterial lipid membranes as models to study the interaction of 10-N-nonyl acridine orange with phospholipids. Chem. Phys. Lipids 2009, 157, 12-20. 

De Rosa, M., Gambacorta, A., and Gliozzi, A. (1986) Structure, biosynthesis, and physicochemical properties of archaebacterial lipids. Microbial Rev., 50, 70-80. 

Berkowitz, W.E. and Yanzhong, W. (1997a) A Cio chrron applicable to the synthesis of archaebacterial lipids./. Org Chem., 62,1536-1539. 

Raguse, B., Culshaw, P.N., and Raval, K. (2000) The synthesis of archaebacterial lipid analogues. Tetrahedron Lett., 41, 2971-2974. 

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