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Molecular structure and ionic mechanisms of an ion‐selective transmembrane channel: Monovalent versus divalent cation selectivity
Author(s) -
Urry Dan W.,
Trapane Tina L.,
Prasad Kari U.
Publication year - 2009
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/qua.560220706
Subject(s) - chemistry , divalent , barium , ionic bonding , ion , ionophore , ion channel , hydrogen bond , gramicidin , aqueous solution , crystallography , membrane , inorganic chemistry , molecule , organic chemistry , biochemistry , receptor
The original proposed structure and transport mechanism of the Gramicidin A transmembrane channel are reviewed, as are the structural issues to be resolved in proving the structure in the membrane. The proposed structural elements of (1) the single‐stranded β 6.3 3,3 helix, (2) the association of amino end to amino end by hydrogen bonding, and (3) the left‐handed helix sense have been confirmed. Two binding sites related by twofold symmetry and separated by just over 20 Å have been observed, and a free‐energy profile for sodium ion permeation through the channel has been obtained. A comparison of 1‐ 13 C‐ L ‐tryptophanyl chemical shifts induced by sodium ions and by barium ions demonstrates the barium ion binding to the mouth of the channel with a location displaced toward the aqueous solution. With the nuclear magnetic resonance observed rapid exchange between the barium ion binding site and the solution it is concluded that the central barrier, due to the positive image force of the lipid surrounding the channel, prevents divalent ion permeability.