Vyas N K, Vyas M N, Quiocho F A
Howard Hughes Medical Institute, Baylor College of Medicine, Houston, TX 77030.
Science. 1988 Dec 2;242(4883):1290-5. doi: 10.1126/science.3057628.
D-galactose-binding (or chemoreceptor) protein of Escherichia coli serves as an initial component for both chemotaxis towards galactose and glucose and high-affinity active transport of the two sugars. Well-refined x-ray structures of the liganded forms of the wild-type and a mutant protein isolated from a strain defective in chemotaxis but fully competent in transport have provided a molecular view of the sugar-binding site and of a site for interacting with the Trg transmembrane signal transducer. The geometry of the sugar-binding site, located in the cleft between the two lobes of the bilobate protein, is novel in that it is designed for tight binding and sequestering of either the alpha or beta anomer of the D-stereoisomer of the 4-epimers galactose and glucose. Binding specificity and affinity are conferred primarily by polar planar side-chain residues that form intricate networks of cooperative and bidentate hydrogen bonds with the sugar substrates, and secondarily by aromatic residues that sandwich the pyranose ring. Each of the pairs of anomeric hydroxyls and epimeric hydroxyls is recognized by a distinct Asp residue. The site for interaction with the transducer is about 18 A from the sugar-binding site. Mutation of Gly74 to Asp at this site, concomitant with considerable changes in the local ordered water structures, contributes to the lack of productive interaction with the transmembrane signal transducer.
大肠杆菌的D-半乳糖结合（或化学感受器）蛋白是对半乳糖和葡萄糖进行趋化作用以及这两种糖的高亲和力主动转运的初始成分。从趋化作用缺陷但转运功能完全正常的菌株中分离出的野生型和突变型蛋白的配体形式的精细X射线结构，提供了糖结合位点以及与Trg跨膜信号转导器相互作用位点的分子视图。糖结合位点位于双叶蛋白两个叶之间的裂隙中，其几何结构新颖，专为紧密结合和隔离4-差向异构体半乳糖和葡萄糖的D-立体异构体的α或β异头物而设计。结合特异性和亲和力主要由与糖底物形成复杂的协同和双齿氢键网络的极性平面侧链残基赋予，其次由夹着吡喃糖环的芳香族残基赋予。每对异头羟基和差向异构羟基都由一个独特的天冬氨酸残基识别。与信号转导器相互作用的位点距离糖结合位点约18埃。该位点的甘氨酸74突变为天冬氨酸，伴随着局部有序水结构的显著变化，导致与跨膜信号转导器缺乏有效的相互作用。
