Chung Shin-Ho, Allen Toby W, Kuyucak Serdar
Department of Physics, The Faculty of Sciences, Australian National University, Canberra, ACT 0200, Australia.
Biophys J. 2002 Jul;83(1):263-77. doi: 10.1016/S0006-3495(02)75167-9.
Using the experimentally determined KcsA structure as a template, we propose a plausible explanation for the diversity of potassium channels seen in nature. A simplified model of KcsA is constructed from its atomic resolution structure by smoothing out the protein-water boundary and representing the atoms forming the channel protein as a homogeneous, low dielectric medium. The properties of the simplified and atomic-detail models, deduced from electrostatic calculations and Brownian dynamics simulations, are shown to be qualitatively similar. We then study how the current flowing across the simplified model channel changes as the shape of the intrapore region is modified. This is achieved by increasing the radius of the intracellular pore systematically from 1.5 to 5 A while leaving the dimensions of the selectivity filter and inner chamber unaltered. The strengths of the dipoles located near the entrances of the channel, the carbonyl groups lining the selectivity filter, and the helix macrodipoles are kept constant. The channel conductance increases steadily as the radius of the intracellular pore is increased. The rate-limiting step for both the outward and inward current is the time it takes for an ion to cross the residual energy barrier located in the intrapore region. The current-voltage relationship obtained with symmetrical solutions is linear when the applied potential is less than approximately 100 mV but deviates slightly from Ohm's law at higher applied potentials. The nonlinearity in the current-voltage curve becomes less pronounced as the radius of the intracellular pore is increased. When the strengths of the dipoles near the intracellular entrance are reduced, the channel shows a pronounced inward rectification. Finally, the conductance exhibits the saturation property observed experimentally. We discuss the implications of these findings on the transport of ions across the potassium channels and membrane channels in general.
以通过实验确定的KcsA结构为模板，我们对自然界中钾通道的多样性提出了一个合理的解释。通过平滑蛋白质 - 水边界，并将构成通道蛋白的原子表示为均匀的低介电介质，从其原子分辨率结构构建了一个简化的KcsA模型。从静电计算和布朗动力学模拟推导得出的简化模型和原子细节模型的性质在定性上是相似的。然后，我们研究了当孔内区域的形状被修改时，流过简化模型通道的电流如何变化。这是通过将细胞内孔的半径从1.5埃系统地增加到5埃来实现的，同时保持选择性过滤器和内腔的尺寸不变。位于通道入口附近的偶极子、排列在选择性过滤器内的羰基以及螺旋大偶极子的强度保持恒定。随着细胞内孔半径的增加，通道电导稳步增加。向外和向内电流的限速步骤是离子穿过位于孔内区域的剩余能垒所需的时间。当施加的电位小于约100 mV时，用对称溶液获得的电流 - 电压关系是线性的，但在较高的施加电位下略微偏离欧姆定律。随着细胞内孔半径的增加，电流 - 电压曲线中的非线性变得不那么明显。当细胞内入口附近的偶极子强度降低时，通道表现出明显的内向整流。最后，电导表现出实验观察到的饱和特性。我们讨论了这些发现对离子跨钾通道和一般膜通道运输的影响。
