Synthesis of Homo- and Hetero-Oligosaccharides from Underivatized Sugars Using Glycosidases

Progress in the development of knowledge of “glycobiology” is seriously impeded by the lack of quantities of characterized oligosaccharides available at reasonable prices. Specialist companies offer chemically synthesized materials, but prices tend to be high because of the complexity of the synthetic methods. Oligosaccharides may be isolated from natural sources but, again, prices are high. An alternative approach to oligosaccharide synthesis is to use glycosidases to catalyze the condensation of underivatised sugars in conditions of low water activity. This approach has the instant attraction that both enzymes and substrates are inexpensive and readily available: The obvious drawback is that, at equilibrium, the amounts of uncondensed substrates in the reaction mixture will exceed the amounts of oligosaccharide product(s) so isolation of the product(s) may not be straightforward. The history of oligosaccharide production is reviewed by Rastall and Bucke (1 ). Serious studies commenced in the late 1980s by Hedbys et al. (2 ) and Johansson et al. (3 ). These showed that condensation reactions retained the anomeric specificity of the enzymes used but that there was little regiospecificity of synthesis, so that equilibrium reaction mixtures contained several different oligosaccharides. For instance, at equilibrium, the reaction mixture of D-mannose incubated with jack bean α-mannosidase contained at least the 1,6-, 1,3-, 1,2-, and 1-1 linked mannobioses, with the 1,6-linked disaccharide predominating (3 ).