GDP-D-mannose

Hassid and coworkers293 reported that this mung-bean enzyme-system catalyzes the incorporation of D-glucose from GDP-D-glucose into a polysaccharide having the characteristics of cellulose and that, in the presence of GDP-D-mannose, this incorporation was stimulated. The same enzyme-system, when provided with GDP-D-mannose as the sole substrate, catalyzed the incorporation of GDP-D-mannose into a glucoman-... 

Heller and Villemez299,300 solubilized, but did not separate, the D-mannosyltransferase and D-glucosyltransferase activities from this particulate-enzyme system, and found that, when only GDP-D-mannose is provided as the substrate, a/ -(1— 4)-linked D-mannan is synthesized and, when only GDP-D-glucose is present, ap-(l— 4)-linked D-glucan is produced. They further noted, with this transferase system, that incorporation of D-glucose from GDP-D-glucose into polysaccharide is a shortlived reaction that can be greatly extended if GDP-D-mannose is also included in the reaction mixture. 

The reversible, enzymic formation of this D-mannolipid involves the transfer of the D-mannosyl group from GDP-D-mannose to undecaprenyl... 

Higher plants make large amounts of L-ascorbate, which in leaves may account for 10% of the soluble carbohydrate content.28 However, the pathway of synthesis differs from that in Fig. 20-2. Both D-mannose and L-galactose are efficient precursors. The pathway in Eq. 20-4, which starts with GDP-d-mannose and utilizes known enzymatic processes, has been suggested.28 29 The GDP-D-mannose-3, 5-epimerase is a well documented but poorly understood enzyme. Multistep mechanisms related to that of UDP-glucose 4-epimerase (Eqs. 20-1,15-14) can be envisioned. 

A separate enzyme, a 3,5-epimerase catalyzes inversion at both C-3 and C-5 (step d).59e Finally, a third enzyme is needed for a second reduction (step e) using NADPH.59f Tire biosynthesis of GDP-L-fiicose from GDP-D-mannose occurs by a parallel sequence.60 61b... 

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 transfer of the oligosaccharide to protein is followed in vivo by the excision of the D-glucosyl residues the nine D-mannosyl residues present in the transferred oligosaccharide probably remain in the inner core of the D-mannan. However, the possibility that some D-mannosyl residues could be almost simultaneously removed and added cannot be precluded.117,167 Finally, terminal, < -d-(1 - 3)-linked D-mannosyl groups are added to the trimmed, protein-bound oligosaccharide.118 The outer chain of the d-mannan is formed after the inner-core oligosaccharide has been transferred to protein and processed, apparently by direct transfer of D-mannosyl groups from GDP-D-mannose.168 The similarity of the structures of the outer chain... 

The fact that glucomannan is obtained from GDP-D-mannose as substrate suggests that the particulate enzyme contains an epimerase which converts GDP-D-mannose to GDP-D-glucose. However, such an epimerase has not been observed. In this connection cellulose isolated from wood contains a considerable amount of D-mannose. 

The conversion of D-mannose 6-phosphate into D-mannosyl phosphate is catalyzed by phosphomannomutase, an enzyme distinct from phosphoglucomutase. Both enzymes have been detected by Mathe-son705 in cassia seeds, mung beans, orchid tubers, and pea seedlings. These enzymes from cassia seeds have been separated from one another by chromatography on DEAE- and O-phosphono-cellulose columns, and further characterized. Phosphomannomutase from animal sources requires the presence of either D-galactose 1,6-bisphosphate or D-mannose 1,6-bisphosphate for activity.708 D-Mannosyl phosphate may then be enzymically transformed into GDP-D-mannose in the... 

Colitose is biosynthesized from GDP-D-mannose,191 while the other 3,6-di-deoxy hexoses are made from CDP-D-glucose.192... 

It has been suggested that a similar mechanism of D-galactosyl transfer may be involved in the biosynthesis of galactomannan, with the participation of GDP-D-mannose.143,146 Courtois and coworkers145 have, in fact, detected UDP-D-galactose and GDP-D-mannose in fenugreek seeds. 

UDP-D-xylose, GDP-D-mannose, GDP-L-Fucose, or CMP-D-A-acetylneuraminic acid. Mammalian GTs also use dolichol-diphosphate-GlcNAc2Man9GlC3, dolichol-phosphate-mannose and dolichol-phosphate-glucose (Fig. 2). 

D-mannose and L-galactose are efficient precursors for ascorbate synthesis, that proceeds through a pathway involving GDP-D-mannose, GDP-L-galactose, L-galactose, and L-galac-tono-1,4-lactone [223],... 

In subsequent steps this intermediate is likely converted to an activated nucleoside-diphosphate, (NDP)-mannose, followed by dehydration to generate an NDP-4-keto-6-deoxy-D-maimose, a reaction catalyzed by GDP-D-mannose-4,6-dehydratase (MDH). This intermediate would then be converted to GDP-L-hicose by an NDP-L-fiicose synthetase, which catalyzes epimerization at the C-3 and C-S positions of the hexose ring and an NADPH-dependent reduction at the C-4 position (22, 23). In the proposed pathway (Figure 2), conversion of the NDP-L-fiicose (a pyranose) to the fiiranose, and oxidation of the S-keto group would provide the activated S-dehydro-a-L-fiicofuranose for attachment to the C4 hydroxyl of ( )-3-(3,4-dihydroxyphenyl)-2-methylacrylic acid-derived moiety of hygromycin A. 

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