Introduction

Recent demonstrations that oligosaccharides play important roles in diverse biological events have resulted in renewed interest in the synthesis of oligosaccharides and their analogs. The availability of such molecules can facilitate studies on carbohydrate-protein recognition and help to elucidate molecular mechanisms of oligosac-charide-mediated biological processes [1,2] that could eventually lead to rationally designed carbohydrate-based therapeutics [3,4]. Despite many advances that have been made over the past decades, the chemical synthesis of oligosaccharides remains a challenge [5,6]. This is attributed to the inherent chemical difficulties presented by this class of molecules. Each monosaccharide carries at least three hydroxyl groups that must be protected and deprotected during synthesis. Also, glycosylation generates a new stereocenter at the anomeric carbon, and there are no general methods for the introduction of all types of glycosidic linkage in a manner that is both stereo-controlled and high yielding. The chemical synthesis of oligosaccharides is therefore very time-consuming and requires specialized expertise. The synthesis of oligosaccharide analogs containing modified sugars is an even more complex task than the preparation of natural structures. Almost twice as many steps are usually required for analog synthesis and the steps are more difficult, since most chemical protocols in the literature have been optimized for natural sugars.

In nature, glycosyltransferase enzymes accomplish the "daunting'' task of the construction of diverse and complex oligosaccharide [7-9]. These enzymes catalyze the transfer of a monosaccharide from a glycosyl donor (usually a sugar nucleotide) to a glycosyl acceptor in a regio- and stereospecific manner (Fig. 1). Pioneered by Barker [10,11], Whitesides [12] and Auge [13] and their colleagues, enzymatic synthesis by means of glycosyltransferases opened up a new venue for the preparation of oligosaccharides, avoiding many of the problems encountered in traditional chemical synthesis. Multistep protection and deprotection sequences are no longer required, since the glycosyl transfer catalyzed by glycosyltransferases is stereo- and regiospecific; thus the configuration of the newly formed anomeric center is absolute. Glycosyltransferases, unlike many catalysts used in organic synthesis, are environmentally benign and operate best in aqueous solution.

Although glycosyltransferases exhibit high specificity for both the sugar nucle-otide donor and acceptor, they have been shown to tolerate certain structural changes on both donor and acceptor substrates. An enzymatic approach to unnatural oligo-saccharides greatly simplifies the synthetic scheme because the requirement for the chemistry is then reduced to the synthesis of more readily accessible smaller "primers" (mono- to trisaccharides), which can be elongated in a regio- and stereospecific manner using glycosyltransferases without additional protection and deprotection steps.

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