Glycosyl Hydrolases and Glycosyltransferases in the Synthesis of Oligosaccharides

Abstract

Glycobiology and related disciplines have received an enormous interest in recent years as they shed new light on the functional roles of carbohydrates in biological events leading to the understanding of mechanisms of important pathologies and to the development of new therapeutics. Although carbohydrate can be isolated from natural sources, the synthetic strategy plays its own role allowing access to larger quantities of structurally defined material and entry to analogs of naturally occurring structures. Nowadays, the bottleneck in this field is represented by some limitations of the potential of carbohydrate containing molecules because their complex structures make classical chemical synthesis very difficult. The synthesis of oligosaccharides have to face with three main snags: (i) reactivity of the leaving group on the monosaccharide acting as donor; (ii) regioselectivity towards a single hydroxyl group on the acceptor molecule; (iii) stereoselectivity in forming pure anomers of the new glycosidic product. The protecting-group manipulations that are needed for the stereocontrol of the products are demanding procedures thus yields and selectivity are lowered hindering the efficient production of oligosaccharides needed for biological testing. A particular benefit of the renewed interest for sugar chemistry has been the attention of scientific community to the biological methodologies in the production of oligosaccharides, an area which emerged in recent decades. In the carbohydrate field different enzymes were used for diverse purposes but enzymatic strategies for high-yield and stereospecific construction of glycosidic bonds are based on the action of two types of enzymes: glycosyl hydrolases (endo- and exo-glycosidases) and glycosyltransferases. Glycosyl hydrolases in the cells are responsible for the cleavage of glycosidic linkages; the exo-glycosidase are involved for glycan processing during in vivo glycoprotein synthesis. The glycosyltransferases are instead responsible in vivo for the synthesis of most cell-surface glycoconjugates. This review deals with the application of different types of glycosyl hydrolases. Elegant examples concerning the use of genetically modified representatives of glycosyl hydrolases (glycosynthases and thioglycoligases) will be also reported; some recent advances on the use of glycosyltransferases are also included. In compiling this review we were aware of the huge amount of excellent material published in the recent years on this topic, thus we will limit the covering of literature to the last decade. Attention will be devoted to the regioselectivity towards pyranosidic templates and to the yield of reactions. The molecular diversity obtained by the use of enzymes from different biological sources and the biological importance of the compounds synthesized will be focused as well. This review will put in evidence that glycosyl hydrolases and glycosyltransferases for the synthesis of the glycosidic linkages will play a relevant role in the next future as on the bench bio-catalysts for the chemists involved in the synthesis of oligosaccharides of biological interest.