CRYSTAL-STRUCTURE OF THE GRAMICIDIN POTASSIUM THIOCYANATE COMPLEX/

The hydrophobic channel-forming polypeptide gramicidin adopts a left-h anded antiparallel double helix conformation with 6.4 residues per tur n when in complex with monovalent cation salts in a methanol environme nt. The crystal structure of the gramicidin/potassium thiocyanate comp lex (a=32.06 Angstrom, b=51.80 Angstrom, and c=31.04 Angstrom; space g roup P2(1)2(1)2(1)) has been solved to 2.5 Angstrom with an R-factor o f 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 bac kbone conformations and the side-chain orientations in this potassium complex are significantly different from those in the previously solve d gramicidin/caesium chloride crystal complex, due to the requirements for interactions with the smaller sized potassium cation. The locatio ns and numbers of potassium binding sites also differ considerably fro m the locations and numbers of caesium binding sites in the other stru cture. Combining information from all the cation binding sites in the two gramicidin/ion complexes produces different views of the three-dim ensional structures of a cation as it is transported along a transmemb rane pore, and provides an experimental structural basis for modeling the dynamics of peptide-ion binding and ion transport. (C) 1997 Academ ic Press Limited.