Small-angle X-ray scattering to obtain models of multivalent lectin-glycan complexes

Recent advances in small-angle X-ray scattering (SAXS) have led to the ability to model the glycans on glycoproteins and to obtain the low-resolution solution structures of complexes of lectins bound to multivalent glycan-presenting scaffolds. This progress in SAXS can respond to the increasing interest in the biological action of glycoproteins and lectins and in the design of multivalent glycan-based antagonists. Carbohydrates make up a significant part of the X-ray scattering content in SAXS and should be included in the model together with the protein, whose structure is most often based on a crystal structure or NMR ensemble, to give a far-improved fit with the experimental data. The modeling of the spatial positioning of glycans on proteins or in the architecture of lectin-glycan complexes delivers low-resolution structural information hitherto unmatched by any other method. SAXS data on the bacterial lectin FimH, strongly bound to heptyl α-D-mannose on a sevenfold derivatized β-cyclodextrin, permitted determination of the stoichiometry of the complex and the geometry of the lectin deposition on the multivalent β-cyclodextrin. The SAXS methods can be applied to larger complexes as the technique imposes no limit on the size of the macromolecular assembly in solution.