Chemoenzymatic Synthesis and Biological Recognition of a Sulfonate Isostere of 6-Sulfo-sialyl Lewisx

Sulfation of N-acetylglucosamine (GlcNAc) moieties of glycans is a common modification that has been implicated in many biological and disease processes. Glycans having sulfate replaced by a stable analogue may find use as glycomimetic drugs. Here, we describe the synthesis of analogues of UDP-GlcNAc in which C-6 hydroxyl is replaced by a thiol or disulfide-protected thiol. It was found that UDP-GlcNAc-6-deoxy-6-SH is a donor substrate for GCNT1 and UDP-GlcNAc-6-deoxy-6-S-SMe for B3GnT2, allowing the preparation of glycopeptides and oligo-LacNAc derivatives having a GlcNAc-6-deoxy-6-S-R moiety, respectively. The disulfide can be reduced to a thiol, which can easily be oxidized to the corresponding sulfonates. Furthermore, oligosaccharides having a sulfonate or disulfide at C-6 are appropriate substrates for glycosyltransferases, providing access to a panel of glycomimetics. The sulfonates and several reference glycans and glycopeptides were printed as a glycan microarray that was used to examine binding selectivities of several lectins, Siglecs, and hemagglutinins of influenza A viruses. It was found that sulfonates can either be tolerated, enhance binding as in the case of Siglec-4, or abolish recognition as for Siglec-9. Molecular modeling studies of the complex of Siglec-4 with sulfated and sulfonated sialyl LacNAc indicate that plasticity of the binding site of the protein and a great charge on oxygens of a sulfonate are responsible for the higher binding affinity. Introduction of a 6-sulfonate gives better step economy than conventional enzymatic sulfation, is simpler to operate, provides compounds resistant to hydrolysis by sulfatases, and can modulate binding selectivities.