There are two strategies for the enzymatic synthesis of oligosaccharides; the use of glycosidases and the use of trans-ferases. The reaction using glycosidases can be further classified into two methods; the transglycosylation and the reverse hydrolysis reaction. In this review, I would like to summarize and discuss these by focusing on the regioselectivity in the synthesis of oligosaccharides by the transglycosylation and reverse hydrolysis reaction.

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A wealth of knowledge has been amassed on the diverse roles played by oligosaccharides in biological systems. To further understand and exploit the activities of oligosaccharides, access is required to natural and nonnatural oligosaccharides, yet the synthesis of oligosaccharides remains one of the most challenging areas of chemistry. Glycosynthases represent a new class of mutant enzymes that are available to complement chemical synthesis and the use of glycosyl transferases as routes to-wards oligosaccharides. Glycosynthases are mutant glycosidases with a non-nucleophilic amino acid replacing the catalytic nucleophile. They are hydrolytically inactive, but when presented with an activated donor sugar, that mimics the covalent glycosyl-enzyme intermediate, they efficiently condense donor sugar and acceptor sugar in high yield. This review illustrates the progress made since the discovery of the first glycosynthase in the author's laboratory (JACS, 1998, 120, 5583-5584). Several different glycosidases have been engineered to transfer a range of different donor sugars onto a wide variety of acceptor sugars, providing new synthetic routes to oligosaccharides. Advances have also been made in increasing the rates of transglycosylation of particular glycosynthases. This, in turn, has broadened the range of oligosaccharides accessible via glycosynthase methodology.

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We do not have a powerful enzyme to utilize the biomass, though cellulose degradation by enzymes has been investigated for a long time. Many cellulase systems from fungi, bacteria, plants and even some animals have been studied, and the biological function of cellulase has been discussed. The important role of cellulase for bacteria and fungi should be to degrade cellulose and serve the soluble sugars for assimilation by the cell. However, for the plant it may be to modify cellulose assemblies that are the dominant component of the cell wall, whereby organisms create the varieties of cellulases and related enzymes for processing different types of celluloses. In this report we introduce the cellulose degradation of enzyme systems produced by fungi, especially white-rot fungus Irpex lacteus.

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Phosphorolytic enzymes that act on carbohydrates are interesting enzymes not only in terms of their basic properties but also in regard to their various applications. However, they have not been studied in as much detail as other types of enzymes such as the hydrolases and the synthases. Phosphorolytic enzymes have attracted attention because of their application in the synthesis of carbohydrate chains, exploiting their strict regiospecificities. In addition, a number of new phosphorolytic enzymes have been discovered recently. In this review, the characteristics and applications of the phosphorolytic enzymes are described in an overview of the current research that has been performed on this interesting class of enzymes.

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