Salting out the ionic selectivity of a wide channel: The asymmetry of OmpF
Salting out the ionic selectivity of a wide channel: The asymmetry of OmpF
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DOI：
10.1529/biophysj.104/043414
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发表时间：
2004-08-01
影响因子：
3.4
通讯作者：
Bezrukov, SM
中科院分区：
文献类型：
--
作者：
Alcaraz, A;Nestorovich, EM;Bezrukov, SM
尽管细菌孔蛋白OMPF的晶体学结构已知十年，但其离子选择性的物理机制仍在研究中。我们在一系列具有不同pH，盐浓度，倒盐梯度以及带电和无电的脂质的实验中解决了这个问题。测量逆转电位，我们表明OMPF选择性（传统上被视为阳性略有阳离子）在很大程度上取决于pH和盐浓度，并且有条件地不对称，即计算出的选择性对盐浓度梯度的方向敏感。在中性pH和亚二摩尔盐浓度下，通道表现出几乎理想的阳离子选择性（t（g）（ +）= 0.98 +/- 0.01）。用DPHP将中性DPHPC取代，我们证明了宿主脂质的固定电荷对通道反转电位的影响很小但可测量。可用的结构信息允许对我们的实验发现进行定性解释。这些发现现在使我们重新检查了OMPF通道102个可滴定位点的电离状态。使用针对我们特定目的量身定制的连续静电静电的标准方法，我们发现通道中的电荷分布是溶液酸度的函数，并将通道选择性中的pH依赖性不对称性与电荷分布中的pH依赖性不对称性相关联。为了找到对我们结果的简单现象学描述，我们还通过大通道讨论了电辐射的不同宏观模型。
Although the crystallographic structure of the bacterial porin OmpF has been known for a decade, the physical mechanisms of its ionic selectivity are still under investigation. We address this issue in a series of experiments with varied pH, salt concentrations, inverted salt gradient, and charged and uncharged lipids. Measuring reversal potential, we show that OmpF selectivity ( traditionally regarded as slightly cationic) depends strongly on pH and salt concentration and is conditionally asymmetric, that is, the calculated selectivity is sensitive to the direction of salt concentration gradient. At neutral pH and subdecimolar salt concentrations the channel exhibits nearly ideal cation selectivity (t(G)(+) = 0.98 +/- 0.01). Substituting neutral DPhPC with DPhPS, we demonstrate that the fixed charge of the host lipid has a small but measurable effect on the channel reversal potential. The available structural information allows for a qualitative explanation of our experimental findings. These findings now lead us to re-examine the ionization state of 102 titratable sites of the OmpF channel. Using standard methods of continuum electrostatics tailored to our particular purpose, we find the charge distribution in the channel as a function of solution acidity and relate the pH-dependent asymmetry in channel selectivity to the pH-dependent asymmetry in charge distribution. In an attempt to find a simple phenomenological description of our results, we also discuss different macroscopic models of electrodiffusion through large channels.