Divergent Chemoenzymatic Synthesis of Sulfated Ganglio-Oligosaccharides for Probing Ligand Requirements of Glycan Binding Proteins.

Sulfoglycolipids are an important class of acidic glycosphingolipids that have been implicated in a multitude of biological processes. Little is known about the interactome of sulfated gangliosides, and it is not well understood how a possible interplay between sialylation and sulfation influences molecular recognition. Here, we describe a chemoenzymatic strategy that readily provided a panel of twenty-one sulfated and sialylated ganglio-oligosaccharides. It is based on the chemical synthesis of a core tetra- and pentasaccharide that are equipped with the orthogonal protecting groups allyloxycarbonate (Alloc), levulinate ester (Lev) and t-butyldimethylsilyl ether (TBS). Selective removal of one or more protecting groups followed by sulfation of the resulting alcohol(s) and deprotecting gave several ganglio-oligosaccharides. Compound lacking an internal sialic acid could be prepared by exposing several derivatives to a sialidase. Compounds having an unmodified terminal galactoside could enzymatically be sialylated to give hybrid structures, which could be further extended to provide 2,8-sialosides. The synthetic glycans were printed as a microarray which was used to examine ligand requirements of a series of glycan-binding proteins including antibodies, toxins, galectins and siglecs. It was found that sulfation regulates protein binding in complex manners and in general sulfation and sialylation of C-3 of the terminal Gal influences protein binding in different ways.