Nonner W, Eisenberg B
Department of Physiology and Biophysics, University of Miami School of Medicine, Florida 33101, USA.
Biophys J. 1998 Sep;75(3):1287-305. doi: 10.1016/S0006-3495(98)74048-2.
L-type Ca channels contain a cluster of four charged glutamate residues (EEEE locus), which seem essential for high Ca specificity. To understand how this highly charged structure might produce the currents and selectivity observed in this channel, a theory is needed that relates charge to current. We use an extended Poisson-Nernst-Planck (PNP2) theory to compute (mean) Coulombic interactions and thus to examine the role of the mean field electrostatic interactions in producing current and selectivity. The pore was modeled as a central cylinder with tapered atria; the cylinder (i.e., "pore proper") contained a uniform volume density of fixed charge equivalent to that of one to four carboxyl groups. The pore proper was assigned ion-specific, but spatially uniform, diffusion coefficients and excess chemical potentials. Thus electrostatic selection by valency was computed self-consistently, and selection by other features was also allowed. The five external parameters needed for a system of four ionic species (Na, Ca, Cl, and H) were determined analytically from published measurements of thre limiting conductances and two critical ion concentrations, while treating the pore as a macroscopic ion-exchange system in equilibrium with a uniform bath solution. The extended PNP equations were solved with these parameters, and the predictions were compared to currents measured in a variety of solutions over a range of transmembrane voltages. The extended PNP theory accurately predicted current-voltage relations, anomalous mole fraction effects in the observed current, saturation effects of varied Ca and Na concentrations, and block by protons. Pore geometry, dielectric permittivity, and the number of carboxyl groups had only weak effects. The successful prediction of Ca fluxes in this paper demonstrates that ad hoc electrostatic parameters, multiple discrete binding sites, and logistic assumptions of single-file movement are all unnecessary for the prediction of permeation in Ca channels over a wide range of conditions. Further work is needed, however, to understand the atomic origin of the fixed charge, excess chemical potentials, and diffusion coefficients of the channel. The Appendix uses PNP2 theory to predict ionic currents for published "barrier-and-well" energy profiles of this channel.
L型钙通道包含一簇四个带电荷的谷氨酸残基（EEEE位点），这似乎对高钙特异性至关重要。为了理解这个高电荷结构如何产生在该通道中观察到的电流和选择性，需要一种将电荷与电流联系起来的理论。我们使用扩展的泊松 - 能斯特 - 普朗克（PNP2）理论来计算（平均）库仑相互作用，从而研究平均场静电相互作用在产生电流和选择性方面的作用。将孔建模为带有锥形心房的中心圆柱体；圆柱体（即“孔本身”）包含与一到四个羧基相当的固定电荷的均匀体积密度。为孔本身指定了离子特异性但空间均匀的扩散系数和过量化学势。因此，通过价态的静电选择是自洽计算的，并且也允许通过其他特征进行选择。对于四种离子种类（Na、Ca、Cl和H）的系统所需的五个外部参数，是根据已发表的三个极限电导率和两个临界离子浓度的测量值解析确定的，同时将孔视为与均匀浴溶液处于平衡的宏观离子交换系统。用这些参数求解扩展的PNP方程，并将预测结果与在一系列跨膜电压下在各种溶液中测量的电流进行比较。扩展的PNP理论准确地预测了电流 - 电压关系、观察到的电流中的异常摩尔分数效应、不同Ca和Na浓度的饱和效应以及质子阻断。孔的几何形状、介电常数和羧基数量只有微弱影响。本文对钙通量的成功预测表明，对于在广泛条件下预测钙通道中的渗透，特设的静电参数、多个离散结合位点以及单排移动的逻辑假设都是不必要 的。然而，需要进一步开展工作来理解通道固定电荷、过量化学势和扩散系数的原子起源。附录使用PNP2理论为该通道已发表的“势垒 - 阱”能量分布预测离子电流。
