Biomimetic Synthesis and Properties of Polyprenoid

The polyprenoids, which represent the largest family of natural products in the living world, are biogenic compounds that derive from the assemblage and modification of fivecarbon isoprene units. Some polyprenoids, such as cholesterol in animals, phytosterols in plants or hopanoids and -dihydroxylated carotenoids in Bacteria (Fig. 1), are of paramount importance in biological membranes, where they act as reinforcers (Ourisson & Nakatani, 1994). Without these reinforcers, the self-organization of phospholipid molecules would not resist from shear stresses. Membranes of Bacteria and Eukarya are formed by the self-assembly of amphiphilic phospholipids whose polar head-groups are linked to two fatty acid chains by ester bonds. The molecular dimensions of cholesterol, phytosterols and hopanoids approach closely those of hydrophobic parts of phospholipid molecules in their stretched form, and their hydrophobic tails are localized in the middle of the membrane (Yamamoto et al., 1993). Thus, these terpenoids reinforce the lipid bilayer by cooperative attractive van der Waals forces and modulate membrane rigidity and fluidity (Milon, Lazrak et al., 1986). In some Bacteria, carotenoids reinforce membrane by crossing both halves of the bilayer (Milon, Wolff et al., 1986). Archaea, the third major kingdom of living organisms, possess structurally unique lipids: their polar head groups are linked to polyprenyl chains by ether bonds, in contrast to the ester bonds and n-acyl chains in Eucarya and Procarya. These ether linkages, which are chemically stable, enable these organisms containing membranes to survive under extreme conditions of pH, temperature, pressure and salt concentration (Chong, 2010; Koga et al., 1993). Another striking feature of some archaeal lipids is the presence of 72-membered rings (Eguchi et al., 2000). Apart from this maintenance of the membrane integrity, membrane terpenoids fulfill many other functions. Dolichol phosphates are widely present in membranes and are involved in the N-glycosylation of proteins. Ubiquinones, which are composed of a 1,4-benzoquinone