Nucleoside diphosphate sugars from

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 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. 

Nucleoside Diphosphate Sugars.—The biosynthesis of saccharides from sugar nucleotides has recently been reviewed. The incorporation of monosaccharides into bacterial cell walls involves the participation of... 

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). 

Important derivatives of aldose-l-phosphates are the nucleoside diphosphate sugars. In most cases they are built de novo from aldose-l-phosphates and nucleotide triphosphates (Fig. 27). The nucleotide most frequently used is uridine triphosphate. Other nucleotide triphosphates, however, may react as well... 

Starting from sucrose, one-pot reactions are suitable to work as regeneration cycles for nucleotide sugars as demonstrated for UDP-Glc, UDP-Gal, and dTDP-deoxy-sugars [98-103] (Scheme 6.4). Sucrose synthase (SuSy), known for the reverse reaction of a GT, is used in combination with different enzyme module systems. Nucleoside diphosphate (NDP) from GT modules are substrates of SuSy catalyzed synthesis of activated glucose (UDP-Glc, dTDP-Glc) which enter enzyme modules to generate the donor substrates of GTs. 

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. 

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. 

Fig. 4 Synthesis of nucleotide sugars from sucrose and nucleoside diphosphates (NDPs) by a sucrose synthase...

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