Activated monosaccharides, synthesis using

Activated monosaccharides, synthesis using Leloir-pathway glycosyltransfeiases,... 

The actual mechanisms for the formation of the glycosidic bond(s) between the two or more monosaccharide residues cannot be accomplished by the simple Fischer type of synthesis that is used for the formation of the alcohol glycosides. Nature uses relatively elaborate, enzymatically catalyzed reactions with high-energy, activated monosaccharide units (sometimes di-, tri-, and tetrasaccharide units) to achieve the synthesis of these combined monosaccharides (see Chapter 10 for some of these biological syntheses). 

Rare or unnatural monosaccharides have many useful applications as nonnutritive sweeteners, glycosidase inhibitors and so on. For example, L-glucose and L-fructose are known to be low-calorie sweeteners. In addition, rare or unnatural monosaccharides are potentially useful as chiral building blocks for the synthesis of biologically active compounds. Therefore, these compounds have been important targets for the development of enzymatic synthesis based in the use of DHAP-dependent aldolases alone or in combination with isomerases. Fessner et al. showed that rare ketose-1-phosphates could be reached not only by aldol addition catalyzed by DHAP-dependent aldolases, but by enzymatic isomerization/ phosphorylation of aldoses [35]. Thus, for example, L-fructose can be prepared... 

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. 

Nucleoside 5 -phosphates are employed as starting materials to prepare the activated derivatives (56). Their synthesis has been discussed in a Chapter in this Series.266 The methods used most frequently for preparing glycosyl phosphates involve the interaction of protected glycosyl bromides with diphenyl phosphate and subsequent removal of the protective groups,267 or by fusion of the peracetylated monosaccharides with anhydrous phosphoric acid.268... 

The chiral synthesis,219-221 by the use of monosaccharides, of optically active natural products having mainly branches of the B-type has now become common in organic chemistry. The reduction of alkylidene derivatives was actually used for the synthesis of thromboxane B2,222 cana-densolide,223 and a degradation product from boromycin.224... 

The aim of this contribution is to present experimental data concerning the synthesis, physicochemical properties and bio-activity of several nanosilica-based composites tested by interaction with native red blood cells (RBCs). The investigations include (i) synthesis of nanocomposites by adsorption of BSA, fructose and glucose on a fumed silica surface, and (ii) analysis of the adsorption properties of composites and their bio-activity. It is known that both glucose and fructose do not individually adsorb on fumed silica therefore, bovine serum albumin was used as a mediator to bind the monosaccharides to the fumed silica surface. 

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