Structure and sugar-specificity of basic winged-bean lectin: structures of new disaccharide complexes and a comparative study with other known disaccharide complexes of the lectin

Abstract

Crystal structures of the complexes of basic winged-bean agglutinin with the disaccharides Galα1-4Gal (galabiose), Galα1-6Glc (mellibiose) and Galα1-4Galβ-Et have been determined and the complex with Galα1-2Gal has been modelled. The interactions of the nonreducing Gal with the lectin at the primary site are the same as those in the known complexes with disaccharides having the α1 → 3 linkage. The second residue in Galα1-4Gal and Galα1-6Glc forms a water bridge to the lectin, while the ethyl group in Galα1-4Galβ-Et makes nonpolar interactions. In complexes involving disaccharides with α1-3 linkages, which form part of the A and B blood-group substances, the second sugar residue forms a direct hydrogen bond to the variable loop in the binding site of the lectin. This in part explains the specificity of the lectin for the blood-group substances and also the higher affinity of α1 → 3-linked disaccharides for the lectin compared with disaccharides involving other linkages. Including those reported here, 14 crystal structures involving the lectin, accounting for 54 crystallographically independent subunits, are available. A comparative study of these structures shows that the region involving the curved β-sheet which nestles the metal ions is relatively rigid. The carbohydrate-binding region is perched on this region. The flat β-sheet, which is involved in oligomerization and exhibits considerable variability in legume lectins, is relatively flexible. Indeed, the structures of basic winged-bean lectin have been of critical importance in establishing legume lectins as a family of proteins in which small alterations in essentially the same tertiary structure lead to large variations in quaternary association. They have also provided a structural explanation of the blood-group specificity of the lectin.