Glycosidases function

Fig. 11.1-11 Tunicamycin 10 is an inhibitor of N-glycan biosynthesis, while natural products such as 1-deoxymannojirimycin inhibit glycosidase function.

Over the past few decades the reactions of a number of retaining gly-cosylases have been reported to involve an oxocarbonium ion rather than a covalent intermediate. During the same period, however, a revised version of the traditional double-displacement mechanism has become widely accepted as the way retaining glycosidases function. Here, the required covalent intermediate is reached via an oxocarbonium ionlike transition state in which the anomeric C-l atom remains partly bonded to its original axial or equatorial substituent, with a second transition state intervening between the intermediate and cosubstrate. According to this model, the idea of an ion-pair intermediate is untenable—with the questionable exception of hen s egg-white lysozyme.13715... 

Rye CS, Withers SG (2000) Glycosidase mechanisms, CurrOpin Chem Biol 4 573-580 Saito M, Yu RK (1995) Biochemistry and function of sialidases. In Rosenberg A (ed) Biology of the sialic acids. Plenum, NY, pp 261-313... 

The principle of active-site-directed inactivation of glycosidases by gly-con-related epoxides can be extended to compounds having an exocyclic oxirane ring, either directly attached to the six-membered ring (32) or at some distance (33,34). Studies with -o-glucosidase from sweet almonds and intestinal sucrase-isomaltase revealed that, in spite of the higher intrinsic reactivity of these epoxides, this shift of the position of the epoxide function causes a 10- to 30-fold decrease of kj(max)/Ki, an effect which probably reflects the limited flexibility of the catalytic groups involved in the epoxide reaction. 

Table 47-5. Some glycosidases used to study the structure and function of glycoproteins. ...

The structures of many oligosaccharide chains can be elucidated by gas-liquid chromatography, mass spectrometry, and high-resolution NMR spectrometry. Glycosidases hydrolyze specific linkages in oligosaccharides and are used to explore both the structures and functions of glycoproteins. 

Recent developments in the enzymatic synthesis of carbohydrates can be classified into four approaches 1) asymmetric C-C bond formation catalyzed by aldolases (1-10 2) enzymatic synthesis of carbohydrate synthons (loll) 3) asymmetric glycosidic formation catalyzed by glycosidases (12.-17) and glycosyl transferases (18-23.) and 4) regioselective transformations of sugars and derivatives (24-25). These enzymatic transformations are stereoselective and carried out under mild conditions with minimum protection of functional groups. They hold promise in preparative carbohydrate synthesis. In connection with this book, we focus on the first two approaches. 

The area of covalent glycosidase inhibitors has recently been highlighted in an excellent review that also includes compounds containing such reporter groups as photolabile functional groups as photoaffinity probes, and others.161... 

From the available items of information, it can be concluded that deoxyfluoro derivatives of imino sugars and relatives are valuable tools for characterization of glycosidases as they pertain to the significance of individual functional groups around the ring, as well as considerations concerning the pH dependency of glycosidase and inhibitor activity. Their potential as leads toward pH-dependent, selective inhibitors has yet, however, to be realized in particular applications. 

Related imino alditols such as azepanes or lactam derivatives have been obtained and have shown to be glycosidase inhibitors [96, 100]. Both d- and l-gulonolactone have been converted into polyhydroxylated 1,6-aldonolactams of type 79 in a sequence of straightforward functional transformations, including sulfinylation of the corresponding aldonolactone-derived acetonides 76 that gave 5,6-cyclic sulfites 77 (Scheme 23) [101]. The latter reacted with sodium azide... 

Previous studies have shown that muscle lysosomal hydrolases are released early in the postmortem period due to a decrease in intracellular ATP concentrations. The decreased intracellular ATP level causes the rupture of the lysosomal membrane (14), releasing hydrolytic enzymes (proteases, lipases, and glycosidases) that further potentiate the weakening of membrane integrity and cellular function. Furthermore, as the acidosis increases (due to the anaerobic conditions associated with cellular death) the intramuscular pH to levels reach that which are optimal for the activity of several lysosomal thiol proteinases. 

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