Hamelryck T W, Loris R, Bouckaert J, Dao-Thi M H, Strecker G, Imberty A, Fernandez E, Wyns L, Etzler M E
Laboratorium voor Ultrastructuur, Vlaams Interuniversitair Instituur voor Biotechnologie, Vrije Universiteit Brussel, Paardenstraat 65, Sint-Genesius-Rode, B-1640, Belgium.
J Mol Biol. 1999 Mar 5;286(4):1161-77. doi: 10.1006/jmbi.1998.2534.
The seed lectin (DBL) from the leguminous plant Dolichos biflorus has a unique specificity among the members of the legume lectin family because of its high preference for GalNAc over Gal. In addition, precipitation of blood group A+H substance by DBL is slightly better inhibited by a blood group A trisaccharide (GalNAc(alpha1-3)[Fuc(alpha1-2)]Gal) containing pentasaccharide, and about 40 times better by the Forssman disaccharide (GalNAc(alpha1-3)GalNAc) than by GalNAc. We report the crystal structures of the DBL-blood group A trisaccharide complex and the DBL-Forssman disaccharide complex.A comparison with the binding sites of Gal-binding legume lectins indicates that the low affinity of DBL for Gal is due to the substitution of a conserved aromatic residue by an aliphatic residue (Leu127). Binding studies with a Leu127Phe mutant corroborate these conclusions. DBL has a higher affinity for GalNAc because the N-acetyl group compensates for the loss of aromatic stacking in DBL by making a hydrogen bond with the backbone amide group of Gly103 and a hydrophobic contact with the side-chains of Trp132 and Tyr104. Some legume lectins possess a hydrophobic binding site that binds adenine and adenine-derived plant hormones, i.e. cytokinins. The exact function of this binding site is unknown, but adenine/cytokinin-binding legume lectins might be involved in storage of plant hormones or plant growth regulation. The structures of DBL in complex with adenine and of the dimeric stem and leaf lectin (DB58) from the same plant provide the first structural data on these binding sites. Both oligomers possess an unusual architecture, featuring an alpha-helix sandwiched between two monomers. In both oligomers, this alpha-helix is directly involved in the formation of the hydrophobic binding site. DB58 adopts a novel quaternary structure, related to the quaternary structure of the DBL heterotetramer, and brings the number of know legume lectin dimer types to four.
豆科植物双花扁豆的种子凝集素（DBL）在豆科凝集素家族成员中具有独特的特异性，因为它对N-乙酰半乳糖胺（GalNAc）的偏好高于半乳糖（Gal）。此外，含有五糖的A血型三糖（GalNAc(α1-3)[Fuc(α1-2)]Gal）对DBL沉淀A+H血型物质的抑制作用略强于GalNAc，而福斯曼二糖（GalNAc(α1-3)GalNAc）的抑制作用约为GalNAc的40倍。我们报道了DBL-A血型三糖复合物和DBL-福斯曼二糖复合物的晶体结构。与结合Gal的豆科凝集素的结合位点进行比较表明，DBL对Gal的低亲和力是由于一个保守的芳香族残基被一个脂肪族残基（Leu127）取代。对Leu127Phe突变体的结合研究证实了这些结论。DBL对GalNAc具有更高的亲和力，因为N-乙酰基团通过与Gly103的主链酰胺基团形成氢键以及与Trp132和Tyr104的侧链形成疏水接触，弥补了DBL中芳香族堆积的损失。一些豆科凝集素具有一个疏水结合位点，可结合腺嘌呤和腺嘌呤衍生的植物激素，即细胞分裂素。这个结合位点的确切功能尚不清楚，但结合腺嘌呤/细胞分裂素的豆科凝集素可能参与植物激素的储存或植物生长调节。DBL与腺嘌呤复合物以及来自同一植物的二聚体茎叶凝集素（DB58）的结构提供了关于这些结合位点的首个结构数据。两种寡聚体都具有不寻常的结构，其特征是一个α-螺旋夹在两个单体之间。在这两种寡聚体中，这个α-螺旋直接参与疏水结合位点的形成。DB58采用了一种与DBL异四聚体四级结构相关的新型四级结构，使已知的豆科凝集素二聚体类型增加到四种。
