Carbohydrate binding properties and carbohydrate induced conformational switch in sheep secretory glycoprotein (SPS-40): crystal structures of four complexes of SPS-40 with chitin-like oligosaccharides

Abstract

Crystal structures of four complexes of sheep secretory glycoprotein (SPS-40) with N-acetylglucosamine oligosaccharides (GlcNAc_{n, n=3-6}) have been determined at moderate resolutions. The binding studies of SPS-40 have been carried out using fluorescence spectroscopy and Surface Plasmon Resonance (SPR). Structure determinations of four complexes have shown a novel binding pattern of GlcNAc_n molecules to SPS-40. The results indicate that the most preferred recognition region in the carbohydrate binding groove in SPS-40 is at subsites -4 to -2 among which subsite -2 provides the maximum interactions with carbohydrate residues. These structures have also shown that the interactions of GlcNAc₃ and GlcNAc₄ do not perturb the protein structure and those of GlcNAc₅ induce partial conformational changes while in the case of GlcNAc₆ the partially closed binding groove opened up completely. As in other SPX-40 structures, SPS-40 structure contains three overlapping flexible surface segments, His188-His197, Phe202-Arg212 and Phe244-Pro260 with several charged residues protruding outwardly. It creates a cluster of positive charges with a flexible base thus indicating a good scope of promoting the intermolecular interactions. This protein is glycosylated at Asn39 and may recognize other receptors having sugar binding sites. It appears that SPS-40 may involve both carbohydrate and protein bindings. The systematic carbohydrate-binding studies and the detailed structural results of four protein-carbohydrate complexes provide an excellent insight into the mechanism of carbohydrate binding. These are the first studies of this kind on secretory glycoproteins and their interactions with carbohydrates.