Enzymatic and chemo-enzymatic synthesis of carbohydrates

Carbohydrate-mediated biochemical recognition processes are potential targets for drug development. The synthesis of complex carbohydrates and related structures, however, has been a difficult task, especially from practical point of view. New solutionand solidphase methods based on enzymatic and a combined enzymatic and chemical procedure have recently been developed for the practical synthesis of various carbohydrate-related structures. Monosaccharides, azaand thiosugars, and their analogs can be effectively prepared based on aldolase-catalyzed reactions. Many oligosaccharides can now be synthesized on multi-gram or kilogram scales based on recombinant glycosyltransferases with in situ regeneration of sugar nucleotides. Serine proteases can be engineered to peptide ligases for glycopeptide coupling in aqueous solution, and further enzymatic incorporation of sugars to form complex glycopeptides can be achieved. Phospholipids containing sugars and azasugars can be prepared using phospholipase D-catalyzed exchange reaction followed by self-assembly to form liposome-like structures with sugars displayed on the surface. These newly developed chemo-enzymatic methods have been applied to the synthesis of specific inhibitors of glycosidases and glycosyltransferases, and of receptors of carbohydrate-mediated cell adhesion. Introduction Carbohydrates on cell surfaces are effective information molecules (1). Although only seven to eight different monosaccharides are commonly used in mammalian systems as building blocks (2,3), the multifunctionality of these monomers and the number of possibilities for their linkage can lead to the assembly of an immense variety of complex structures. Multimillion different tetrasaccharide structures, for example, can be constructed from this small number of building blocks, when considering the branching, stereochemistry of glycosidic linkages, and modification of hydroxyl and amino groups. Oligosaccharides therefore represent a class of effective biomolecules coding for a vast amount of information required in various biological recognition processes, such as bacterial and viral infection, cell adhesion, signal transduction, differentiation, development, regulation and many other intercellular communications (1). It seems that carbohydrate-mediated recognition is a fascinating area for drug discovery. The pace of development of carbohydrate-based pharmaceuticals has, however, been much slower than that of other classes of biomolecules. Part of the reason is due to the lack of significant glycobiology information as well as the difficulty encountered in the synthesis, especially from a practical point of view, of complex carbohydrates. There is no method available for the amplification of oligosaccharides to facilitate sequence analysis. There is no machine available for a rapid synthesis of oligosaccharides for the study of structure-function relationship. Furthermore, there are also concerns about the poor oral activity of carbohydrates and the weak binding of carbohydrates to their receptors. New d' iscovenes in ~1In the past five years, however, several new exciting discoveries in glycobiology have renewed our interest in carbohydrate research, and the one of particular interest to us is sialyl Lewis x (SLeX) mediated adhesion of leukocytes to E-selectin containing endothelial cells in response to inflammatory reactions (4). Since the tetrasaccharide SLex was determined to be the ligand for Eselectin (9, it has been considered to be potentially useful as an inhibitor of E-selectin and therefore as a new antiinflammatory agent. In addition to E-selectin, Pand L-selectins and their ligands have