Saturation transfer difference NMR and computational modeling of a sialoadhesin–sialyl lactose complex

Abstract

The siglecs are a family of I-type lectins binding to sialic acids on the cell surface. Sialoadhesin (siglec-1) is expressed at much higher levels in inflammatory macrophages and specifically binds to α-2,3-sialylated N-acetyl lactosamine residues of glycan chains. The terminal disaccharide α- d-Neu5Ac-(2 → 3)-β- d-Gal is thought to be the main epitope recognized by sialoadhesin. To understand the basis of this biological recognition reaction we combined NMR experiments with a molecular modeling study. We employed saturation transfer difference (STD) NMR experiments to characterize the binding epitope of α-2,3-sialylated lactose, α- d-Neu5Ac-(2 → 3)-β- d-Gal-(1 → 4)- d-Glc 1 to sialoadhesin at atomic resolution. The experimental results were compared to a computational docking model and to X-ray data of a complex of sialyl lactose and sialoadhesin. The data reveal that sialoadhesin mainly recognizes the N-acetyl neuraminic acid and a small part of the galactose moiety of 1 . The crystal structure of a complex of sialoadhesin with sialyl lactose 1 was used as a basis for a modeling study using the FlexiDock algorithm. The model generated was very similar to the original crystal structure. Therefore, the X-ray data were used to predict theoretical STD values utilizing the CORCEMA-STD protocol. The good agreement between experimental and theoretical STD values indicates that a combined modeling/STD NMR approach yields a reliable structural model for the complex of sialoadhesin with α- d-Neu5Ac-(2 → 3)-β- d-Gal-(1 → 4)- d-Glc 1 in aqueous solution.