Nucleoside diphosphate sugar synthesis

Synthesis. The synthases are present at the endomembrane system of the cell and have been isolated on membrane fractions prepared from the cells (5,6). The nucleoside diphosphate sugars which are used by the synthases are formed in the cytoplasm, and usually the epimerases and the other enzymes (e.g., dehydrogenases and decarboxylases) which interconvert them are also soluble and probably occur in the cytoplasm (14). Nevertheless some epimerases are membrane bound and this may be important for the regulation of the synthases which use the different epimers in a heteropolysaccharide. This is especially significant because the availability of the donor compounds at the site of the transglycosylases (the synthases) is of obvious importance for control of the synthesis. The synthases are located at the lumen side of the membrane and the nucleoside diphosphate sugars must therefore cross the membrane in order to take part in the reaction. Modulation of this transport mechanism is an obvious point for the control not only for the rate of synthesis but for the type of synthesis which occurs in the particular lumen of the membrane system. Obviously the synthase cannot function unless the donor molecule is transported to its active site and the transporters may only be present at certain regions within the endomembrane system. It has been observed that when intact cells are fed radioactive monosaccharides which will form and label polysaccharides, these cannot always be found at all the membrane sites within the cell where the synthase activities are known to occur (15). A possible reason for this difference may be the selection of precursors by the transport mechanism. 

Glycosyl—Enzyme Complex Intermediates in Biosynthesis of Complex Saccharides. The synthesis of nucleoside diphosphate sugars involves the transfer of a nucleotidyl group from a nucleoside triphosphate to a sugar 1-phosphate with the simultaneous release of pyrophosphate according to the following general reaction (11) ... 

The metabolic routes leading to polyglucan synthesis were elucidated after the discovery of nucleoside-diphosphate sugars by L. F. Leloir and co-workers in 1955. This finding led to the conclusion that biosynthesis and degradation of glycogen and starch occur by different pathways. 

Synthesis of glycopyranosylphosphonate analogues of certain natural nucleoside diphosphate sugars as potential inhibitors of gly-cosyltransferases. J. Med. Chem. 1987 30 1383-1391. 42. 

C) they require nucleoside diphosphate sugars for their synthesis... 

The first seven sugars are transferred to dolichol phosphate from nucleoside diphosphate sugars, UDP-N-acetylglucosamine and GDP-mannose. Each reaction is catalyzed by a separate glycosyltransferase. The antibiotic, tunicamycin, inhibits synthesis of all N-linked glycoproteins by inhibiting the first enzyme in the process. 

Synthesis of Nucleoside Triphosphates (XTPs). In vivo, the nucleoside diphosphate sugars are synthesized from the sugar-1-phosphate and the appropriate nucleoside triphosphates (Scheme 14). 

A polyprenol phosphate such as dolichol phosphate (10.63) can react with a nucleoside diphosphate sugar in one of two ways (11.119) and (11.120). As an example of a synthesis proceeding... 

Cytidine phosphates cytidine S -monophosphate (CMP, cytidylic acid, M, 323.2), cytidine 5 -diphos-phate (CDP, M, 403.19) and cytidine 5 -triphosphate (CTP, M, 483.16). For structure, see e. g. Pyrimidine biosynthesis. CTP is a precursor of RNA synthesis, while deoxy-CTP is a precursor of DNA synthesis. CDP may be regarded as the coenzyme of phospholipid biosynthesis (see Membrane lipids) (activated choline is CDP-choline). Glycerol and the sugar alcohol, ribitol, are also activated by bonding to CDP (see Nucleoside diphosphate sugars). Reduction of ribose... 

Whitesides and coworkers have carried out a comparison of enzymic and chemical routes to CTP, GTP and UTP on a 10-gram scale. They concluded that CTP and GTP were best made enzymically, and UTP by reaction of CTP with nitrous acid. The triphosphates were then employed for the enzymic synthesis of UDP-Glucose, UDP-Glucuronic acid, and GDP-Mannose.i94 Cytidine diphosphate sugars have been prepared from the 3,6-dideoxyhexoses paratose and abequose,193 and all four nucleoside diphosphate sugars of 6-sulpho-a-D-quinovose have been synthesized for studies of sulpholipid biosynthesis in chloroplasts.196 The stable analogue (138) of CMP-KDO has been prepared by a triester approach, but was only a weak inhibitor of KDO incorporation into lipopolysaccharides.197 A reference to acetylated forms of UDPGlc is mentioned in Chapter 7. 

GDP-fucose is a glycosyl donor substrate for fucosyltransferases. Although it has been synthesized by many chemical methods [47], the use of fucosyl phosphite in the synthesis seems the most practical [48]. A recent improvement of this method includes the addition of tetrazole to the morpholidate coupling reaction [48b]. This method might also be applicable to the synthesis of other nucleoside diphosphate sugars (Figure 18). 

Choline and ethanolamine are activated in much the same way as are sugars. For example, choline can be phosphorylated using ATP (Eq. 17-58, step a) and the phosphocholine formed can be further converted (Eq. 17-58, step b) to cytidine diphosphate choline. Phosphocholine is transferred from the latter onto a suitable acceptor to form the final product (Eq. 17-58, step c). Tire polymerization pattern differs from that for polysaccharide synthesis. When the sugar nucleotides react, the entire nucleoside diphosphate is eliminated (Eq. 17-56), but CDP-choline and CDP-ethanolamine react with elimination of CMP (Eq. 

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