Polyprenyl sugar phosphate

The )8-(1 2)- and a-(1 5)-linked arabinose residues are incorporated into the polymer from the activated polyprenyl sugar phosphate 10, which is in turn synthesized from glucose via 5-phosphoribose pyrophosphate (pRpp) [46-48]. Elongation of the polymer chain is believed to involve a family of arabinosyltransferases (AraT s) that recognize both 10 and arabinofuranoside-based acceptors of differing structures (Fig. 5) [18,19,49,50]. In AG biosynthesis, the entire polysaccharide appears to be assembled as a polyprenol diphosphate intermediate, which is transferred to peptidoglycan prior to the addition of the mycolate esters [18]. In LAM biogenesis, the arabinan portion is believed to be synthesized as a polyprenol phosphate that is transferred to lipomannan [51]. 

It is difficult to identify the lipid moiety as a polyprenol, because of the small amounts present in tissues. Nevertheless, some information can be obtained by using glycosylated lipids labelled in the sugar moiety. The first indication of the presence of polyprenyl glycosyl phosphates comes from study of the properties just mentioned, namely, lability to mild acid, stability to mild alkali, and acidic properties. Information on the polyprenyl nature of the lipid may be obtained by labelling the lipid with radioactive 2,4-dideoxyO-C-methyl-D-g/ycero-pentono-l.S-lactone (mevalonic acid ). 

FIGURE 8.18 Dolichol phosphate is an initiation point for the synthesis of carbohydrate polymers in animals. The analogous alcohol in bacterial systems, undecaprenol, also known as bactoprenol, consists of 11 isoprene units. Undecaprenyl phosphate delivers sugars from the cytoplasm for the synthesis of cell wall components such as peptidoglycans, lipopolysaccharides, and glycoproteins. Polyprenyl compounds also serve as the side chains of vitamin K, the ubiquinones, plastoquinones, and tocopherols (such as vitamin E). 

The polyprenols obtained from tobacco and Arum maculatum (solanesol and spadicol), which are all E, are the exceptions. Another interesting fact is that, in most seed-plants, the polyprenols are allylic, as is bacterial undecaprenol. Saturated or slightly modified polyprenols, such as animal dolichol, seem to be present only in fungi. Any of those plant polyprenols, in a phosphorylated form, can be potential sugar acceptors for transfer reactions. Experiments were performed with phosphorylated allylic polyprenols and membrane preparations from Phaseolus aureus. D-Mannose from GDP-D-mannose was incorporated into the exogenous, polyprenyl phosphates,29,49 52 but it was not possible to establish unequivocally the role of these polyprenols as lipid intermediates. 

The first evidence showing that plant polyprenols involved in glycosyl-transfer reactions are not allylic, but a-saturated, like animal dolichol (2), came from Pont Lezica and coworkers.23 The authors postulated that the presence of sugar acceptors having the properties of a-saturated polyprenyl phosphates may be a general feature of eukaryotic cells, in contrast to the a-unsaturated polyprenyl phosphates characteristic of prokaryotic cells. 

Another possible regulatory mechanism has been pointed out by Elbein for plant systems. His results implied either that the carrier lipid for different sugars are different polyprenyl phosphates, or that the glycosyltransferases have their own pools of lipid carriers, which do not mix.16 The results, compared to those for other plant and animal systems, suggested that the polyprenyl phosphates are the same for different sugars in each system, thus ruling out the first hypothesis, and reinforcing the concept of different pools.30... 

The synthesis of XXI was accomplished by direct condensation of XIX with moraprenyl phosphoimidazo1id-ate XX in a THF-DMSO-mixture under mild conditions (27). This new method makes it possible to prepare polyprenyl pyrophosphate oligosaccharides directly from unprotected phosphates of oligosaccharides, thus avoiding the need for de-protection procedures which may be accompanied by extensive destruction of polyprenyl pyrophosphate sugars. 

L. L. Danilov, S. D. Maltsev, V. N. Shibaev, and N. K. Kochetkov, Synthesis of polyprenyl pyrophosphate sugars from unprotected mono- and oligo-saccharide phosphates, Carbohydr. Res., 88 (1981) 203-211. 

The initiation reaction for O antigen subunit biosynthesis occurs at the interface of the plasma membrane and the cytosol where the nucleotide sugar precursors are available. The reaction involves the formation of a phosphodiester bond between a membrane-associated polyprenyl phosphate and a cytosolic UDP-sugar with the release of UMP. Depending on the specific microorganism, this reaction is catalyzed by two different... 

The formation of lipid-linked sugars in vitro by Bacillus stearothermophilus. Salmonella typhimurium, and Shigella jlexneri requires an unsaturated a-isoprenyl residue in a polyprenyl phosphate, in contrast to th- t in rat-liver microsomes which requires a saturated a-isoprenyl residue. o-Glucopyranosyl polyprenyl phosphate was formed on incubation of UDP-D-[ C]glucose with particulate preparations from B. stearothermophilus, and it may serve as a D-glucosyl donor in the synthesis of an extracellular polysaccharide. The properties of the membrane-associated sialyltransferase complexes of Escherichia coli have been examined. These complexes catalyse the incorporation of iV-acetylneuraminic... 

Two glycoproteins (mol. wt. 1.8 x 10 and 3.4 x 10 ) have been identified in both the nuclei and nuclear membranes of rat liver. These glycoproteins, which bind in concanavalin A-ferritin complexes, are located exclusively on the cisternal surface of the membranes. Rough- and smooth-microsomal fractions and Golgi membranes of rat liver have been shown to contain different types and amounts of polyprenyl phosphates and exhibit specificity as sugar acceptors from sugar nucleotides. ... 

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