Glycosyl Transfer Reactions

More than one hundred glycosyl transferases have been identified to date and each one appears to specifically catalyze the formation of a unique glycosidic linkage [1622]. 

Enzymatic methods [1630] of preparing the NDP-sugars have several advantages over the chemical methods [1631]. For example, the NDP-sugar may be generated in situ (e.g., by epimerization of UDP-Glc to UDP-Gal), making it possible to drive unfavorable equilibria in the required direction. Purification steps may be eliminated because the byproducts of enzyme-catalyzed methods do not interfere with further enzymatic steps. 

The point of interest to synthetic chemists is the range of acceptors and donors that can be used in glycosyl transferase-catalyzed reactions. Fortunately, the specificity of glycosyl transferases is high but not absolute. 

UDP-galactosyl (UDP-Gal) transferase is the best-studied transferase in terms of specificity for the acceptor sugar. It has been demonstrated that this enzyme catalyzes the transfer of UDP-Gal to a remarkable range of acceptor substrates of the carbohydrate-type [1622, 1632-1635]. Other glycosyl transferases, although less well-studied than UDP-Gal transferase, also appear to tolerate various acceptors as substrates [1636-1639] (Table 2.9). 

The liberated UDP is recycled back to the respective triphosphate by p5mivate kinase at the expense of phosphoenol pyruvate. The overall yield of this sequence was in the range of 70% when performed on a scale greater than 10 g. 

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For pectinolytic enzymes no glycosyl transfer reaction have been reported except for D-galacturonan digalacturonohydrolase (EC 3.2.1.82) from Selenotnonas ruminantium [4]... 

The lack of glycosyl transfer reaction is the class of pectinolytic hydrolases is in agreement with the observed inversion of the anomeric configuration of the newly formed reducing ends of the products. All three polygalacturonases studied here utilize the single displacement mechanism of hydrolysis. 

The first evidence showing that plant polyprenols involved in glycosyl-transfer reactions are not allylic, but a-saturated, like animal dolichol (2), came from Pont Lezica and coworkers.23 The authors postulated that the presence of sugar acceptors having the properties of a-saturated polyprenyl phosphates may be a general feature of eukaryotic cells, in contrast to the a-unsaturated polyprenyl phosphates characteristic of prokaryotic cells. 

Further work in plant systems confirmed, by different methods (biosynthesis from mevalonate,27 28 mass spectrometry,33 and the techniques already mentioned30), that the polyprenyl phosphate involved in glycosyl-transfer reactions is dolichyl phosphate. Table II shows different sources from which dolichyl phosphate has been identified in plants. 

DeLuca H (1977) The direct involvement of vitamin A in glycosyl transfer reactions of mammalian membranes. Vitamins and Hormones 35,1-57. 

Hanessian, S. Lou, B. Stereocontrolled Glycosyl Transfer Reactions with Unprotected Glycosyl Donors, Chem. Rev 2000, 100,4443 1463. 

There is one common feature of all transfer studies. If a substance can function as an acceptor, it must evidently have a higher affinity for the enzyme than has water, because, in all these reactions, water (used as the solvent) is present in large excess. This holds for other glycosyl transfer reactions as well. 

Regarded as glycosyl-transfer reactions, various features of these hydrolyses are readily understood. 

In the next step, a 2-acetamido-2-deoxy-D-glucosyl group is transferred from its UDP derivative onto the monosaccharide-lipid intermediate, to form the disaccharide-lipid intermediate (see Reaction II of Scheme 3). This is a glycosyl-transfer reaction of the more usual type, and UDP is released as a product of the reaction. ... 

Seibel et al. [36] reported, for the first time, the use of microwave heating in the glycosylation of amino acids. They performed glycosyl transfer reactions of perace-tylated monosaccharides (glucose and galactose) and disaccharides (maltose and lactose) with N-9-fluorenylmethoxycarbonyl-L-serine benzyl ester in the presence of iron trichloride with short reaction times (4 min, compared with 5-10 h by conventional heating) and with improved yields (52-61% compared with 10-31% by conventional heating). It is worthy of note that in these reactions heavy metal compounds, for example silver trifluoromethanesulfonate, mercury dibromide and di-cyanide, or boron trifluoride-diethyl etherate, can be replaced by the environmentally safe promoter iron trichloride. 

Enzymatic approaches to glycoside synthesis may circumvent these problems. Enzymes (i.e. nucleoside glycosyl transferases) are stereospecific and regioselective catalyzing glycosyl transfer reaction under mild conditions. Extensive protection-deprotection schemes are unnecessary and the control of anomeric configuration is managed. 

Scheme 9 Fluorescence sensing of the glycosyl-transfer reaction based on ratiometric detection by 33...

Glycosylation transfer reaction catalyzed by GGTase is divided into four types hydrolysis reaction, cychzation reaction, couphng reaction and disproportionation reaction, in which the coupling reaction in some documents is also known as the open-loop reaction. The direction of glycosylation reaction catalyzed by the GGTase depends on the reaction rate constant of the four types of reaction. Typically, the rate constant of cychzation reaction is greater than that of the hydrolysis reaction, and therefore the direction of the reaction is mainly to produce GD (Fig. 2.3). 

In the detailed description of glycosyl transfer reactions and polymer... 

Pigman, eds). Academic Press, New York, pp. 256-84 De Luca, S. (1977a) Incorporation of Mannose and Glucose into Prenylphosphate Sugars in Isolated Human Platelet Membranes , Biochimica et Biophysica Acta, 498, 341-8 De Luca, L.M. (1977b) The Direct Involvement of Vitamin A in Glycosyl Transfer Reactions of Mammalian Membranes , Vitamins and Hormones, 35,1-57 De Luca, L.M. (1978) Vitamin A , in Handbook of Lipid Research (H.F. De Luca, ed.),... 

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