Chemoenzymatic Synthesis of Tri-antennary N-Glycans Terminating in Sialyl-Lewis(x) Reveals the Importance of Glycan Complexity for Influenza A Virus Receptor Binding.

Sialyl-Lewis(x) (SLe(x)) is involved in immune regulation, human fertilization, cancer, and bacterial and viral diseases. The influence of the complex glycan structures, which can present SLe(x) epitopes, on binding is largely unknown. We report here a chemoenzymatic strategy for the preparation of a panel of twenty-two isomeric asymmetrical tri-antennary N-glycans presenting SLe(x)-Le(x) epitopes on either the MGAT4 or MGAT5 arm that include putative high-affinity ligands for E-selectin. The N-glycans were prepared starting from a sialoglycopeptide isolated from egg yolk powder and took advantage of inherent substrate preferences of glycosyltransferases and the use of 5'-diphospho-N-trifluoracetylglucosamine (UDP-GlcNHTFA) that can be transferred by branching N-acetylglucosaminyltransferases to give, after base treatment, GlcNH(2)-containing glycans that temporarily disable an antenna from enzymatic modification. Glycan microarray binding studies showed that E-selectin bound equally well to linear glycans and tri-antennary N-glycans presenting SLe(x)-Le(x). On the other hand, it was found that hemagglutinins (HA) of H5 influenza A viruses (IAV) preferentially bound the tri-antennary N-glycans. Furthermore, several H5 HAs preferentially bound to N-glycan presenting SLe(x) on the MGAT4 arm. SLe(x) is displayed in the respiratory tract of several avian species, demonstrating the relevance of investigating the binding of, among others IAVs, to complex N-glycans presenting SLe(x).