Polyprenol-P-activated sugars

Not all donor substrates of glycosyltransferase reactions are nucleotide-activated sugars. Polyprenol-based lipids are commonly used in bacteria, archaea and eukaryotes to build donor substrates. The glycosyltransferases using polyprenol-based substrates are mainly hydrophobic proteins with multiple trans-membrane domains. The number of isoprene repeats varies according to the organisms, prokaryotes having 11-12 isoprene units, yeasts 14-16 units, animals 16-21 units and plants 20-24 units. In Dol, the last isoprene unit is reduced. This reduction is required for the subsequent phosphorylation step. All polyprenol-based substrates are phosphorylated with the exception of the undecaprenol substrates involved in the elongation of the LPS outer core, which carry a pyrophosphate group.

Figure 19. Polyprenol-linked substrates in all domains of life. Dolichol has a saturated α-isoprene unit whereas prokaryotic polyprenol have an unsaturated bond.

Polyprenol and Dol share a common biosynthesis pathway with cholesterol up to the formation of farnesyl-PP. The basic isoprene unit is synthesized from acetyl-CoA, which is condensed to 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) and further reduced to mevalonate. The HMG-CoA reductase activity is rate limiting in the pathway, as it is regulated at the transcription and protein levels by sterol concentrations. The HMG-CoA reductase enzyme is also the target of inhibitors called statins, which are commonly prescribed as cholesterol-lowering drugs.

Figure 20. Mevalonate and dolichol biosynthesis pathway.

After reaching farnesyl-PP, the growing polyprenols are anchored in membranes and elongated by cis-prenyltransferase enzymes, also called dehydrodolichyl diphosphate synthase (DHDDS). The DHDDS enzymes are responsible for the variable chain length of polyprenols observed between prokaryotes and eukaryotes. Before receiving a monosaccharide unit, Dol undergo three modifications. First, the pyrophosphate group is removed by phosphatases, then the last isoprene unit is reduced and finally a phosphate group is added again by a dedicated kinase. The monosaccharides Man and Glc are attached to Dol-P in eukaryotes and various exotic sugars like GalA and Ara4N to polyprenols in bacteria. Dol-P-Man and –Glc are substrates in the final assembly steps of lipid-linked oligosaccharides in the N-linked glycosylation pathway, in the biosynthesis of the glycosylphosphatidylinositol (GPI) anchor, in O-mannosylation and in C-mannosylation. In bacteria, polyprenol-P linked sugars are substrates for the modification of the lipid A core and for the synthesis of arabinogalactan and lipoarabinomannan in mycobacteria. Deficiencies in cis-prenyltransferase, Dol reductase, Dol kinase and Dol-P-Man synthase lead to a shortage of Dol-P linked substrates and thus to disorders of glycosylation.