Vyas N K, Vyas M N, Quiocho F A
Nature. 1987;327(6123):635-8. doi: 10.1038/327635a0.
The refined 1.9-A resolution structure of the periplasmic D-galactose-binding protein (GBP) reveals a calcium ion surrounded by seven ligands, all protein oxygen atoms. A nine-residue loop (amino-acid positions 134-142), which is preceded by a beta-turn and followed by a beta-strand, provides five ligands from every second residue. The last two ligands are supplied by the carboxylate group of Glu 205. The entire GBP Ca2+-binding site adopts a conformation very similar to the site in the 'helix-loop-helix' or 'EF-hand' unit commonly found in intracellular calcium-binding proteins, but without the two helices. Structural analyses have also uncovered the sugar-binding site some 30 A from the calcium and a site for interacting with the membrane-bound trg chemotactic signal transducer approximately 45 A from the calcium. Our results show that a common tight calcium binding site of ancient origin can be tethered to different secondary structures. They also provide the first demonstration of a metal-binding site in a protein which is involved in bacterial active transport and chemotaxis.
周质D-半乳糖结合蛋白（GBP）分辨率为1.9埃的精细结构显示，一个钙离子被七个配体环绕，这些配体均为蛋白质中的氧原子。一个九残基环（氨基酸位置134 - 142），其前面是一个β-转角，后面是一个β-链，每隔一个残基提供五个配体。最后两个配体由Glu 205的羧基提供。整个GBP钙离子结合位点的构象与细胞内钙结合蛋白中常见的“螺旋-环-螺旋”或“EF手”单元中的位点非常相似，但没有那两个螺旋。结构分析还发现了距离钙离子约30埃的糖结合位点以及距离钙离子约45埃的与膜结合的trg趋化信号转导器相互作用的位点。我们的结果表明，一个古老起源的常见紧密钙结合位点可以与不同的二级结构相连。它们还首次证明了蛋白质中一个参与细菌主动运输和趋化作用的金属结合位点。
