Kulkarni Kiran A, Katiyar Samiksha, Surolia Avadhesha, Vijayan Mamannamana, Suguna Kaza
Molecular Biophysics Unit, Indian Institute of Science, Bangalore, India.
Proteins. 2007 Aug 15;68(3):762-9. doi: 10.1002/prot.21428.
Basic winged bean agglutinin binds A-blood group substance with higher affinity and B-blood group substance with lesser affinity. It does not bind the O substance. The crystal structures of the lectin, complexed with A-reactive and B-reactive di and tri saccharides, have been determined. In addition, the complexes of the lectin with fucosylated A-trisaccharides and B-trisaccharides and with a variant of the A-trisaccharide have been modeled. These structures and models provide valuable insights into the structural basis of blood group specificities. All the four carbohydrate binding loops of the lectin contribute to the primary combining site while the loop of variable length contributes to the secondary binding site. In a significant advance to the current understanding, the interactions at the secondary binding site also contribute substantially, albeit in a subtle manner, to determine the blood group specificity. Compared with the interactions of the B-trisaccharide with the lectin, the third sugar residue of the A-reactive trisacharide forms an additional hydrogen bond with a lysine residue in the variable loop. In the former, the formation of such a hydrogen bond is prevented by a shift in the orientation of third sugar resulting from an internal hydrogen bond in it. The formation of this bond is also facilitated by an interaction dependent change in the rotamer conformation of the lysyl residue of the variable loop. Thus, the difference in the interactions at the secondary site is generated by coordinated movements in the ligand as well as the protein. A comparison of the crystal structure and the model of the complex involving the variant of the A-trisaccharide results in the delineation of the relative contributions of the interactions at the primary and the secondary sites in determining blood group specificity.
基本四棱豆凝集素与A血型物质结合亲和力较高，与B血型物质结合亲和力较低，不与O型物质结合。已确定该凝集素与A反应性和B反应性二糖及三糖复合的晶体结构。此外，还构建了该凝集素与岩藻糖基化A三糖、B三糖以及A三糖变体的复合物模型。这些结构和模型为血型特异性的结构基础提供了有价值的见解。凝集素的所有四个碳水化合物结合环都对主要结合位点有贡献，而长度可变的环对次要结合位点有贡献。在对当前认识的一项重大进展中，次要结合位点的相互作用也以一种微妙的方式对确定血型特异性有很大贡献。与B三糖与凝集素的相互作用相比，A反应性三糖的第三个糖残基与可变环中的一个赖氨酸残基形成了一个额外的氢键。在前者中，由于其三糖内部氢键导致第三个糖的取向发生变化，从而阻止了这种氢键的形成。可变环中赖氨酰残基的旋转异构体构象的相互作用依赖性变化也促进了该键的形成。因此，次要位点相互作用的差异是由配体和蛋白质的协同运动产生的。对涉及A三糖变体的复合物的晶体结构和模型进行比较，从而确定了主要和次要位点的相互作用在决定血型特异性方面的相对贡献。
