Enzymes glycosyl transferase

While in the synthesis of naturally occurring oligosaccharides the use of enzymes (glycosyl transferases) proved very useful due to their substrate specificity and stereospecificity and the possibility to work without protective groups, the synthesis of unnatural oligosaccharides still relies on synthetic methods. ... 

High selectivity and substrate specificity of glycosyl transferases make them valuable catalysts for special linkages in polymer-supported synthesis. There is, however, still a rather limited set of enzymes available to date, and the need to synthesize a variety of natural and non-natural oligosaccharides prevails. Particularly with regard to combinatorial approaches, chemical solid-phase oligosaccharide synthesis promises to meet the demands most effectively. 

The first ten years of this work has produced a plethora of novel, interesting and biologically active compounds. The focus has been on gaining an understanding of the mechanism of action of various -glucan endohydrolases. We hope, in the next decade, to be able to transfer this experience emd knowledge to other important enzymes, such as glycosyl transferases. 

The glycosyl transferases (E.C. 2.4 group) usually show high specificity towards the structure of the glycosyl acceptor. Furthermore, the enzyme specificity seems to determine which of the available nucleophilic acceptor-groups participates in the reaction, and also determines the stereospecificity of the glycosylation. 

It serves as a membrane anchor for the growing polysaccharide. We have already discussed one example in the hypothetical cellulose synthase mechanism of Fig. 20-5. For some polysaccharides the mechanism is better established. The synthetic cycles all resemble that of Fig. 20-5 and can be generalized as in Eq. 20-20. Here NDP-Glx is a suitable nucleotide disphosphate derivative of sugar Glx, and Z-Glx is the repeating unit of the polysaccharide formed by the action of glycosyl-transferases and other enzymes. 

Action of various glycosyl transferases and other enzymes to build a modified sugar or oligosaccharide... 

Figure 3-8. Biosynthesis of anthocyanins and condensed tannins. The enzymes involved in this pathway are (a) anthocyanidin synthase (E.C. 1.14.11.19), (b) anthocyanin 3-glycosyl transferase, and (c) BANYULS.

Enzymes Aldolases Transketolases Pyrophospho -rylases Glycosyl -transferases Glycosidases... 

In summary, many important in situ regeneration cycles for nucleotide sugars have been established for the synthesis of glycoconjugates with glycosyl-transferases. However, more detailed investigations will be needed to define the inhibition, the stability and the kinetics of the enzymes involved, and to achieve synthesis conditions with high yields and high enzyme productivities. 

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