Biosynthesis of Lipid-linked Oligosaccharides

Dolichols are long-chain poly(isoprenol)s found in eukaryotes only, and consist of 13-22 isoprene units. There are two internal, trails-iso-prene units, the other internal units being ct.s-isoprene residues.1 

Characteristically, the a-isoprene unit is saturated, unlike bacterial undecaprenol, in which it is unsaturated, The dolichol mono- and di-phosphoric esters function as glycosylated intermediates2 in the synthesis of glycoproteins, keratan sulfate, cellulose, and yeast mannan, and that is why they are discussed here. 

The initial demonstration that isopentenyl diphosphate may be used in the biosynthesis of dolichol phosphate in animals and plants 

A particulate, enzyme preparation from human lymphocytes contains a phosphatase that converts dolichol diphosphate into dolichol phosphate and phosphate.15 This alkaline phosphatase may play a role in the recycling of dolichol phosphate, if this compound is to act as a coenzyme. However, the specificity of the phosphatase for dolichol diphosphate, 2,3-dehydrodolichol diphosphate, or dolichol diphosphate-linked oligosaccharides27 has not yet been determined. 

The finding that dolichols differ in chain length has, of course, raised the question of the polyprenyl specificity of the various glyco-syltransferases. The idea that different (that is, not miscible) pools of dolichol phosphate exist for different glycosyltransferases has also emerged, and was discussed in a review by Elbein.35 Conflicting evidence has been obtained,36-38 and this point needs further investigation. 

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Fig. 45.—Pathway of the Biosynthesis of Lipid-linked Oligosaccharide Precursor of the N-Acetyl-lactosaminic Type of Glycans, and of the Intermediary Glycopeptide. (See general reviews 126-136, 226, and 227.)...

Structures of dolichol phosphate, GlcNAc-P-P-Dol, and Man-P-Dol. GlcNAc-P-P-Dol is the first compound formed in the biosynthesis of the lipid-linked oligosaccharide shown in Figure 16-5. Replacement of GlcNAc by -OH in GIcNAc-P-P-Dol yields dolichol diphosphate (dolichol pyrophosphate). Replacement of Man by Glc in Man-P-Dol yields Glc-P-Dol. 

Figure 16.17 Biosynthesis of the lipid-linked oligosaccharide intermediate. 

Membrane preparations from immature stages of the fruit fly Ceratitis capitata catalyse the transfer of o-mannose into lipid-linked oligosaccharides. The lipid-linked trisaccharide has been partially characterized and tentatively assigned the structure D-mannosyl -> NN diacetylchitobiosyl-poly(prenyl-pyrophosphate). These insect poly(prenyl)-oligosaccharides may participate in glycoprotein biosynthesis. 

In Section II (on the biosynthesis of oligosaccharides), emphasis is placed on the biosynthesis of the dolichol-linked sugars, because the inhibitors mainly interfere with steps in the dolichol cycle of protein glycosylation. Modification of the oligosaccharide side-chain, once it has been transferred from the carrier lipid to the protein, is not discussed in any great detail. 

In bacterial lipopolysaccharides, O-specific chains composed of repeating, or modified repeating, units are linked to a unique oligosaccharide sequence of the core region which is connected to a lipid A fragment serving as a hydrophobic anchor embedded in the bacterial outer-membrane. Biosynthesis of O-specific chains was found to occur independently on formation of other structural fragments of the lipopolysaccharide molecule. Both block and monomeric mechanisms were demonstrated for the biosynthesis of these polymers. 

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