Rao V S, Lam K, Qasba P K
Structural Glycobiology Section, Laboratory of Experimental and Computational Biology, National Cancer Institute, NCI-FCRDC, Frederick, Maryland 21702, USA.
J Biomol Struct Dyn. 1998 Apr;15(5):853-60. doi: 10.1080/07391102.1998.10508207.
Complexes of soybean agglutinin (SBA) with galactose (Gal) and N-acetyl galactosamine (GalNAc) have been modeled based on its homology to erythrina corallodendron (EcorL) lectin. The three dimensional structure of SBA-Gal modeled with homology techniques agrees well with SBA-(beta-LacNAc)2Gal-R complex determined by X-ray crystallographic techniques at the beta-sheet regions and the regions where Ca2+ and Mn2+ ions bind. However, significant deviations have been observed between the modeled and the X-ray structures, particularly at the loop regions where the polypeptide chain could not be unequivocally traced in the X-ray structure. The hydrogen bonding scheme, predicted from the homology model, shows that the invariant residues i.e. Asp, Gly, Asn, and aromatic residues (Phe) found in all other legume lectins, bind Gal, slightly in a different way than reported in X-ray structure of SBA-pentasaccharide complex. The higher binding affinity of GalNAc over Gal to SBA is due to additional hydrophobic interactions with Tyr107 rather than a hydrogen bond between N-acetamide group of the sugar and the side chain of Asp88 as suggested from X-ray crystal structure studies. Our modeling also suggest that the variation in the length of the loop D observed among galactose binding legume lectins may not have any effect on the binding of sugar at the monosaccharide specific site of the lectins. Soybean agglutinin (SBA) is a member of the leguminous family of lectins. They generally possess a single carbohydrate binding site, besides the tightly bound Ca2+ and Mn2+ ions which are required for their carbohydrate binding activity. They possess a high degree of sequence homology and about 50% of the amino acid residues are invariant. Some of these invariant amino acid residues are involved in the binding of sugar moieties and in metal ion coordination. X-ray crystallographic studies showed that their three-dimensional structures are very similar, though they differ in their carbohydrate binding specificity (1-6). Three of the invariant residues Asp, Gly, and Asn, besides an aromatic residue (Phe or Tyr), are involved in carbohydrate binding. Independent of their sugar specificity, these four residues in legume lectins provide the basic frame for the sugar to bind.
基于大豆凝集素（SBA）与刺桐属（EcorL）凝集素的同源性，已构建了其与半乳糖（Gal）和N - 乙酰半乳糖胺（GalNAc）的复合物模型。用同源技术构建的SBA - Gal三维结构在β - 折叠区域以及Ca2 +和Mn2 +离子结合区域与通过X射线晶体学技术确定的SBA -（β - LacNAc）2Gal - R复合物吻合良好。然而，在模型结构和X射线结构之间观察到显著差异，特别是在X射线结构中多肽链无法明确追踪的环区域。从同源模型预测的氢键模式表明，在所有其他豆科凝集素中发现的不变残基，即天冬氨酸、甘氨酸、天冬酰胺和芳香族残基（苯丙氨酸），与半乳糖结合，其方式与SBA - 五糖复合物的X射线结构报道的略有不同。GalNAc对SBA的结合亲和力高于Gal，这是由于与Tyr107的额外疏水相互作用，而不是如X射线晶体结构研究所暗示的糖的N - 乙酰胺基团与Asp88侧链之间的氢键。我们的模型还表明，在半乳糖结合豆科凝集素中观察到的环D长度变化可能对凝集素单糖特异性位点的糖结合没有任何影响。大豆凝集素（SBA）是豆科凝集素家族的成员。除了紧密结合的Ca2 +和Mn2 +离子是其碳水化合物结合活性所必需的之外，它们通常具有单个碳水化合物结合位点。它们具有高度的序列同源性，约50％的氨基酸残基是不变的。其中一些不变的氨基酸残基参与糖部分的结合和金属离子配位。X射线晶体学研究表明，它们的三维结构非常相似，尽管它们的碳水化合物结合特异性不同（1 - 6）。除了芳香族残基（苯丙氨酸或酪氨酸）外，三个不变残基天冬氨酸、甘氨酸和天冬酰胺参与碳水化合物结合。独立于它们的糖特异性，豆科凝集素中的这四个残基为糖结合提供了基本框架。
