Enzymes glycals

If, as it is usually done, the interaction of enzyme with glycal is studied in the presence of substrate S having Michaelis constant K , the observed rate constant k pp, for the approach to the steady-state inhibition has to be corrected for the competition of substrate for the free enzyme, in order to calculate the rate constants kp , kp, and k yj, from the experimental data. 

Glycal assembly on a solid support eliminates the repetitive purifications usually associated with oligosaccharide synthesis. As a method, it has a certain generality as it does not require any specific enzymes or complex starting materials. Both natural and nonnatural sugars may be used in the constructions. 

The haem peroxidases are a superfamily of enzymes which oxidise a broad range of structurally diverse substrates by using hydroperoxides as oxidants. For example, chloroperoxidase catalyses the regioselective and stereoselective haloge-nation of glycals, the enantioselective epoxidation of distributed alkenes and the stereoselective sulfoxidation of prochiral thioethers by racemic arylethyl hydroperoxides [62]. The latter reaction ends in (i )-sulfoxides, (S)-hydroperoxides and the corresponding (R)-alcohol, all In optically active forms. 

E. W. Holla, Enzymic synthesis of selectively protected glycals, Angew. Chem., Int. Ed. Engl, 28 (1989) 220-221. 

Thus, X may be a saccharide, a 1,2-glycal, a phosphate residue, a nucleoside diphosphate, or a polyprenol diphosphate and X the OH of water, an alcohol, and often a saccharide. It is convenient, however, to consider enzymes cleaving polysaccharide chains in one of four classes hydrolases, glycosyl transferases, lyases, and phosphorylases. These follow the general reactions discussed below. 

Within the present article, a few selected examples are provided ofboth classical and more recent glycosylation methods applied to the synthesis of sLcL sLe", or of sulfated Lewis determinants. Recent procedures involve the use of glycal donors as developed by Danishefsky and his associates, of glycosyl fluorides according to Mukaiyama and Nicolaou, of sulfoxide donors as reported by Kahne, and enzyme-catalyzed oligosaccharide syntheses as developed by the schools of Whitesides, Wong, and Paulson. ... 

As might be expected, in the absence of acceptors the 2 -deoxyribosyl-enzyme slowly hydrolyses intriguingly, however, the hydrolysis is associated with the production of substantial quantities of ribal, which is then slowly hydrolysed. Formation of ribal is clearly the near microscopic reverse of the first step in the hydration of glycals by retaining glycosidases (Figure 5.21), for some reason favoured with a 2-deoxy substrate. 

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