Synthetic Carbohydrate Chemistry the Synthesis of Structurally Defined
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Synthetic Carbohydrate Chemistry The synthesis of structurally defined oligosaccharide is a rapidly developing domain and still a major challenge in the field of carbohydrate research. Novel synthetic tools and methodologies are required for the more efficient chemical and chemoenzymatic syntheses of complex carbohydrates and to further investigate their critical biological and pharmacological applications. In the Synthesis Subgroup, our project is the chemoenzymatic synthesis size- uniform, structurally defined heparin and heparan sulfate oligosaccharides. The structures of heparin and HS are diverse (Figure 1) and extremely difficult to synthesize by standard chemical synthesis. Thus, our laboratory takes an approach involving the integrated use of chemical synthesis and applied enzymology. Figure 1. Major disaccharide units of heparin (left) and HS (right) Chemoenzymatic synthesis of heparin oligosaccharides Heparin and heparan sulfate are glycosaminoglycans that play essential roles in the regulation of intracellular processes. Due to their structurally diverse nature, they are extremely difficult to prepare using conventional synthetic techniques. We have developed a chemoenzymatic approach (Figure 2) for the preparation of structurally defined heparin and heparan sulfate oligosaccharides to better understand the structure-function relationship of these important biomolecules. By preparing a library of various heparin and heparan sulfate structures, we will be able to elucidate the importance of sulfate pattern and sequence on the biological activities of heparin and heparan sulfate. Figure 2. Chemoenzymatic synthesis. Our approach involves the chemical synthesis of UDP-hexosamine and UDP-hexuronic acid donors as well as defined glycosyl acceptors. These donors and acceptors often have reactive or modifiable moieties that can be used for labeling, immobilization or can be transformed into other functional groups. Synthetically prepared acceptors are then glycosylated with these donors using recombinantly prepared glycosyltransferases. After the assembly of the oligosaccharide backbone, heparin/heparan sulfate modifying enzymes, such as sulfotransferases and epimerases are then used to construct useful natural and unnatural heparin and heparan sulfate sequences. Currently, all of our chemistry and enzymology is being performed in solution phase but we are now beginning to explore both the use of immobilized enzymes and the synthesis of heparin and heparan sulfate on solid phase supports. In collaboration with Professor Liu’s Laboratory at UNC, we recently successfully synthesized two ultra-low molecular weight heparins that exhibit similar anticoagulant properties as Arixtra, a commercial ultra-low molecular weight heparin (Figure 3). Figure 3. Chemoenzymatic synthesis of ultra-low molecular weight heparins Modification of ECM cross-linking with GAGs-containing Dendrons Dendrons, which have structurally multi-valent specificity, have been explored for potential applications such as drug delivery, target specific carrier, etc. We are currently investigating the unique properties and behavior of dendrons specifically designed for coupling heparin, heparin sulfate, low molecular weight heparin, and other active compound on the surface of the extracellular matrix (Figure 4). Modification of collagen-bearing heparin residues can offer us a good surface for cell attachment, recognition or antithrombin activity research. We are developing an efficient synthesis of heparin dendrons in which the high-density glycans are immobilized on the surface of collagen. Figure 4. Principle of coating glycans on the surface of ECM. .