Jing N, Urry D W
Laboratory of Molecular Biophysics, University of Alabama at Birmingham 35294-0019, USA.
Biochim Biophys Acta. 1995 Aug 23;1238(1):12-21. doi: 10.1016/0005-2736(95)00094-j.
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.
进行了两项独立的核磁共振实验，以研究氯化钙与短杆菌肽A（GA）离子转运通道之间的相互作用，实验使用了掺入十二烷基磷酸胆碱（DPC）胶束中的13C富集的GA和GA分子。C-13标记的羰基碳的化学位移与氯化钙浓度的关系表明，Ca2+和Cl-离子在GA结构内作为离子对相互作用，Cl-离子位于Trp11残基羰基基团位置附近，距离GA螺旋口约5.5埃，Ca2+离子结合在Trp15残基羰基基团位置，距离螺旋孔入口约2.5埃。35Cl线宽和横向弛豫时间的测量结果表明，在氯化钙溶液中时，Cl-离子与DPC中的GA之间发生相互作用；在氯化钠溶液中时，未检测到Cl-离子与DPC中的GA之间的相互作用；在氯化镁溶液中时，Cl-离子与DPC中的GA之间的相互作用比在氯化钙溶液中弱得多。简而言之，当Cl-离子与二价Ca2+离子配对时，它可以进入GA；并且Ca2+和Cl-离子作为一对会与GA二聚体的位点快速交换。
