glycoconjugates include chemical synthesis, enzymatic and chemoenzymatic synthesis, and microbial production (Boltje et al. 2009; Kiessling and Splain 2010; Hsu et al. 2011; Schmaltz et al. 2011). The next sections cover the scope and limitations of these methodologies. Despite the shortcoming of these technologies, they have been instrumental in addressing a number of important problems in glycobiology research and for the discovery of vaccines and therapeutics. In particular, the Consortium for Functional Glycomics, funded by the National Institute of General Medical Sciences, has employed a chemoenzymatic approach for the preparation of a collection of approximately 600 glycans derived from N- and O-linked glycoproteins and glycolipids (Stevens et al. 2006; Rillahan and Paulson 2011). These compounds are modified with an artificial aminopropyl linker, which allows covalent attachment to N-hydroxysuccinimide-activated glass slides. The resulting microarrays have found wide utility for integrating binding specificities of a diverse range of glycan-binding proteins, determining dissociation constants and dissecting binding energies, and analyzing multivalent and hetero-ligand binding. The species-specific nature of the interaction between virus and host glycans and determination of ligand specificities of monoclonal antibodies have allowed use of glycan arrays in rapid assessment of influenza virus receptor specificity. A significant barrier to widespread use of glycan arrays, however, is the limited availability of well-defined oligosaccharides, and current arrays contain only a fraction of naturally occurring oligosaccharides. Also, very similar arrays displaying very similar glycans can, nevertheless, provide significantly different results with regard to GBP binding. There are exciting challenges ahead before glycan arrays can become a standardized method of analysis.

Development of a fully synthetic heparin fragment for treatment of deep vein thrombosis exemplifies the importance of the organic synthesis of glycans. Heparin and heparan sulfate are naturally occurring linear polysaccharides that are modified by sulfate esters. A heparin-derived pentasaccharide that can bind to antithrombin III (AT II) and that exhibits anticoagulant activity has been identified. A fully synthetic analog (fondaparinux) of this domain has been developed, which is being produced on a multikilogram scale to treat deep vein thrombosis (Petitou and van Boeckel 2004). In contrast to porcine mucosal tissue-derived heparin, the synthetic compound is easy to characterize and has a much-improved subcutaneous bioavailability. The importance of synthetic oligosaccharides for anticoagulation therapy was highlighted by the recent discovery of batches of heparin that caused hypotension and resulted in nearly 100 deaths. These reactions resulted from contamination with oversulfated chondroitin sulfate, which is a popular shellfish-derived oral supplement for the treatment of arthritis (Guerrini et al. 2008). The ability to synthe-

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