GDP-Man

Mannose Hexose Man GDP-Man Common sugar in N-linked glycoproteins. 

Five Man residues are added, using GDP-man-nose as the donor. 

How, exactly, retinol affects protein glycosylation is not yet known, but the formation of retinol phosphate, the D-mannosylation of retinol phosphate by GDP-Man, and the transfer of Man from D-mannosyl (retinol phosphate) to pronase-sensitive glycoconjugates are well established.5857,59 On the other hand, the formation and role of other gly-cosyl esters of retinol phosphate are still controversial.2,80 It is very probable that the D-mannosylation of retinol phosphate, and also the transfer of Man from this lipid derivative to protein, are catalyzed by enzymes different from those involved in the dolichol pathway,81-83 and this finding points to the different physiological roles of Man-P-Dol and D-mannosyl (retinol phosphate). Thus, whereas Man-P-Dol can serve to D-mannosylate dolichol diphosphate-linked oligosaccharides (see Section III), such a role has not been found for D-mannosyl (retinol phosphate).81,83... 

That some cell-free systems were incomplete became evident when the role of UDP-Glc was appreciated (see Refs. 127-135). Only in the presence of UDP-Glc (in addition to UDP-GlcNAc and GDP-Man, when no endogenous acceptors are present) was a lipid-linked oligosaccharide obtained that had the same chemical properties as the D-... 

As already indicated, control points in the regulation of the rate of glyeosylation of proteins may be (a) the availability of Dol-P (Section 11,1,a), (b) the D-glucosylation of the lipid-linked oligosaccharide, or its transfer to protein, or both (Section 1,2,a), and, as already discussed, (c) the metabolic fate of GDP-Man. Furthermore, GDP-Man inhibits fonnation of Glc-P-Dol in cell-free preparations from liver,157 and activates formation of GlcNAe-PP-Dol in cell-free preparations from tissues of chick embryo.156... 

At concentrations of amphomycin that completely block the formation of Man-P-Dol, the incorporation ofa-linked D-mannose into lipid-linked oligosaccharides is not completely inhibited.347 As was mentioned in Sections II,2,a and II,2,b, EDTA in vitro and 2-deoxy-2-fluoro-D-glucose in vivo have a similar effect, namely, the formation of a heptasaceharide-lipid, Man5(GlcNAc)2-PP-Dol, is still possible in the absence of Man-P-Dol. The question as to whether these D-man-nosyl residues come directly from GDP-Man is discussed in Section II,2,b. 

CDG-Ii GDP-Man ManiGlcNAc2-PP-dolichol mannosyl-transferase hALG2 9q22 607906... 

CDG-Ik GDP-Man GlcNAc2-PP-dolichol mannosyltrans-ferase hALGl 16pl3 608540... 

Various mammalian-tissue preparations were also found224 to catalyze the formation of GDP-Fuc from GDP-Man both NAD+ and an NADPH-generating system were necessary for maximum activity. 

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