Doyle D A, Wallace B A
Department of Crystallography, Birkbeck College, University of London, UK.
J Mol Biol. 1997 Mar 14;266(5):963-77. doi: 10.1006/jmbi.1996.0837.
The hydrophobic channel-forming polypeptide gramicidin adopts a left-handed antiparallel double helix conformation with 6.4 residues per turn when in complex with monovalent cation salts in a methanol environment. The crystal structure of the gramicidin/potassium thiocyanate complex (a = 32.06 A, b = 51.80 A, and c = 31.04 A; space group P2(1)2(1)2(1)) has been solved to 2.5 A with an R-factor of 0.193. In the structure, binding sites for the cations are formed by the polypeptide backbone carbonyl groups tilting away from the helix axis toward the ions located in the central lumen. The polypeptide backbone conformations and the side-chain orientations in this potassium complex are significantly different from those in the previously solved gramicidin/caesium chloride crystal complex, due to the requirements for interactions with the smaller sized potassium cation. The locations and numbers of potassium binding sites also differ considerably from the locations and numbers of caesium binding sites in the other structure. Combining information from all the cation binding sites in the two gramicidin/ion complexes produces different views of the three-dimensional structures of a cation as it is transported along a transmembrane pore, and provides an experimental structural basis for modeling the dynamics of peptide-ion binding and ion transport.
疏水性通道形成多肽短杆菌肽在甲醇环境中与单价阳离子盐形成复合物时，会呈现每圈含6.4个残基的左手反平行双螺旋构象。短杆菌肽/硫氰酸钾复合物的晶体结构（a = 32.06 Å，b = 51.80 Å，c = 31.04 Å；空间群P2(1)2(1)2(1)）已解析到2.5 Å的分辨率，R因子为0.193。在该结构中，阳离子的结合位点由多肽主链羰基形成，这些羰基从螺旋轴倾斜向位于中央腔的离子。由于与较小尺寸的钾阳离子相互作用的需求，该钾复合物中的多肽主链构象和侧链取向与先前解析的短杆菌肽/氯化铯晶体复合物中的显著不同。钾结合位点的位置和数量与另一种结构中铯结合位点的位置和数量也有很大差异。结合来自两种短杆菌肽/离子复合物中所有阳离子结合位点的信息，产生了阳离子沿跨膜孔运输时三维结构的不同视图，并为模拟肽 - 离子结合和离子运输的动力学提供了实验结构基础。
