Ho Oh

Sugar Nucleotide Donor

Retaining

Glycosyltransferase

Inverting

Glycosyltransferase

Oligosaccharide

Oligosaccharide

HO OH Nucleotide r O

HO OH Nucleotide

Figure 1 Reactions catalyzed by Leloir glycosyltransferases (B = uracil, guanine, cytosine). The acceptor glycosylation site is highlighted.

With advances in molecular biology and biotechnology, more than 30 glycosyltransferases from mammalian [9,14], bacterial [15-17], and even viral [18] sources have been cloned, and many are now readily available in large multiunit quantities. For synthetic applications, the major sugar nucleotide donor substrates for mammalian glycosyltransferases are commercially available, and cost limitations for large-scale synthesis are rapidly being overcome with nucleotide donor recycling systems [12,19] and pathway engineering [20]. Glycosyltransferase-catalyzed synthesis of both natural and unnatural oligosaccharides is an important component of glycochemistry, as evidenced by the numerous comprehensive reviews that have appeared [21 -25].

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