Ion pair binding of Ca2+ and Cl- ions in micellar-packaged gramicidin A.
The two independent NMR experiments were performed to investigate the interaction between CaCl2 and the gramicidin A (GA) ion transport channel, using 13C-enriched GA and GA molecules incorporated into dodecylphosphocholine (DPC) micelles. The chemical shifts of C-13 labeled carbonyl carbons vs. CaCl2 concentration demonstrate that Ca2+ and Cl- ions interact as an ion pair within the GA structure with the Cl- ion located near the position of the carbonyl group of the Trp11 residue some 5.5 A from the mouth of the GA helix, and the Ca2+ ion bound at the position of the carbonyl group of the Trp15 residue some 2.5 A from the entrance to the helical pore. The measurements of the 35Cl line-widths and transverse relaxation times illustrate that the interaction occurs between Cl- ions and GA in DPC when in CaCl2 solution, that no interaction is detected between Cl- ions and GA in DPC when in NaCl solution, and that the interaction between Cl- ions and GA in DPC when in MgCl2 solution is much weaker than in CaCl2 solution. In short, a Cl- ion can enter the GA when it is paired with a divalent Ca2+ ion; and Ca2+ and Cl- ions as a pair exchange rapidly with sites of the GA dimer.
Jing
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Laboratory of Molecular Biophysics, University of Alabama at Birmingham 35294-0019, USA.
Urry
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DW
D000595:Amino Acid Sequence; D002118:Calcium / Q000378:metabolism*; D002712:Chlorides / Q000378:metabolism*; D002942:Circular Dichroism; D006096:Gramicidin / Q000378:metabolism*; D009682:Magnetic Resonance Spectroscopy; D008823:Micelles; D008969:Molecular Sequence Data; D010767:Phosphorylcholine / Q000031:analogs & derivatives / Q000378:metabolism
D002712:Chlorides; D008823:Micelles; D010767:Phosphorylcholine; D006096:Gramicidin; C028810:dodecylphosphocholine; D002118:Calcium