Isoprenoid lipids

The separation of plant isoprenoid lipids, including ubiquinone, phylloquinone [=MK-4(II,III,IV-H6], and plastoquinone (160) by h.p.l.c. has been described. 

A large group of isoprenoid lipids, including sterols, terpenes, and carotenoid compounds, are... 

As there is competition for isoprenoid lipid between lipo- and exopolysaccharide, peptidoglycan, and teichoic acids, the availability of isoprenoid lipid phosphate is one of the most critical factors affecting the synthesis of exopolysaccharide 236... 

The way in which a polysaccharide is released from the isoprenoid lipid is not yet known. Most probably, enzymes are present that cleave the terminal, phosphate-linked, monosaccharide residue. The chain length of a polysaccharide may depend on the growth rate that is, a higher growth-rate might lead to a faster turnover of the carrier lipid and release of polysaccharide of lower molecular weight. Instances are known where variable chain-length results from the action of depolymerases.239... 

Hemmerlin, A., Gerber, E., Feldtrauer, J.R, Wentzinger, L., Hartmann, M.A., Tritsch, D., Hoeffler, J.F., Rohmer, M. and Bach, T.J. (2004) A review of tobacco BY-2 cells as an excellent system to study the synthesis and function of sterols and other isoprenoids. Lipids, 39, 723-35. 

Isoprenoid Lipids.—Lipids which contain a mono- or pyro-phosphate link between a polyisoprenoid alcohol and a carbohydrate residue are important intermediates in the biosynthesis of bacterial cell walls. The monophosphate derivatives are probably involved in the addition of single carbohydrate residues to existing polysaccharide chains while the pyrophosphates are probably intermediates in the biosynthesis of the main chain. Ficaprenyl (13 = 10), famesyl (13 = 2), dolichyl (14 n = 18), and citro-nellyl (14 n = 1) a-D-mannopyranosyl phosphates have now been... 

Amphomycin inhibits bacterial cell wall synthesis at the translocase step by binding to the large isoprenoid lipid called undecaprenylphosphate (173,174). In eukaryotes, in a similar manner, amphomycin binds to the large isoprenoid lipid called dolicholphosphate thus blocking the transfer of mannose from its uridinediphosphate derivative to this lipid (173—177). Amphomycin has been patented for use as a feed additive (178). 

C-terminal sequence covalently attached to isoprenoid lipids in some lipid-anchored proteins (e.g., Ras)... 

The formation of terpenes and other isoprenoid lipids involves the condensation of dimethylallyl pyrophosphate with isopentenyl pyrophosphate (29). In this case (Scheme 11), the leaving group is the pyrophosphate (PP) of an allylic pyrophosphate, such as dimethylallyl pyrophosphate, generating an allylic carbonium ion derivative (and inorganic pyrophosphate, which forms as an ion pair with the carbonium ion). The electron-rich agent that adds to the electron-deficient center to form the carbon-carbon bond is the rr-electron density of the isopentenyl pyrophosphate. The stereochemical course of the biosynthesis of complex natural products has been determined and is consistent with such a mechanism (30). 

Figure 28. Cyclic isoprenoid lipids and their relationships to squalene. (a) Squalene folded so as to demonstrate its relationship to a sterol carbon skeleton (b). In sterols that occur commonly, some of the methyl groups resulting from the cyclization of squalene move from one carbon to another and others are lost. The substituents marked R can be either methyl or H and the substituent marked R can be H, methyl, ethyl, or propyl.

The site of fatty-acid synthesis varies very significantly. If a cell has chloroplasts—if it is an algal unicell or a carbon-fixing cell within a differentiated photoautotroph—all fatty acids with 18 or fewer carbon atoms are produced within it (Cavalier-Smith 2000). If a cell lacks chloroplasts the fatty acids are produced in the cytosol using (in eukaryotic cells) acetate exported from the mitochondria. The consequences of these points can be discussed more completely and systematically in parallel with a consideration of isoprenoid lipids. 

Processes affecting the carbon-isotopic compositions of isoprenoid lipids. The isoprene carbon skeleton is indicated schematically in Figure 27. The corresponding biosynthetic reactant—equivalent in its role to acetyl-CoA—is isopentenyl pyrophosphate. As shown in Figure 29, this compound can be made by two different and fully independent pathways. The mevalonic-acid pathway was until recently thought to be the only route to isoprenoids. The deoxyxylulose-phosphate, or methylerythritol-phosphate, pathway was first discovered in Bacteria by Rohmer and coworkers (Flesch and Rohmer... 

Table 4. Pathways used for the biosynthesis of isoprenoid lipids.

FIGURE 3.5 Examples of isoprenoid lipids illustrating the structural diversity of the group, (a) Three isoprene units (2-methyl-1,3-butadiene) in various conformations (b) a-pinene (2,6,6-trimethylbicyclo[3.1.1]hept-2-ene) (c) P-farnescene (3,7,ll-trimethyl-l,3,10-dodec-atriene) (d) phytol (3,7,ll,15-tetramethyl-2-hexadecenol) (e) scalarin (12a-acetoxy-25a-hydroxyscalar-16-en-25,24-olide) (f) squalene (2,6,10,15,19,23-hexamethyl-2,6,10,14,18,22-tetracosahexaene) and (g) lanosterol (4,4,14a-trimethyl-5a-cholest-8,24-diene-3P-ol). 

Isoprenoid Lipids Classified in Terms of Terpenoid Class, with Common Examples... 

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