Nonner W, Catacuzzeno L, Eisenberg B
Department of Physiology and Biophysics, University of Miami School of Medicine, Miami, Florida 33101-4819, USA.
Biophys J. 2000 Oct;79(4):1976-92. doi: 10.1016/S0006-3495(00)76446-0.
L-type calcium channels are Ca(2+) binding proteins of great biological importance. They generate an essential intracellular signal of living cells by allowing Ca(2+) ions to move across the lipid membrane into the cell, thereby selecting an ion that is in low extracellular abundance. Their mechanism of selection involves four carboxylate groups, containing eight oxygen ions, that belong to the side chains of the "EEEE" locus of the channel protein, a setting similar to that found in many Ca(2+)-chelating molecules. This study examines the hypothesis that selectivity in this locus is determined by mutual electrostatic screening and volume exclusion between ions and carboxylate oxygens of finite diameters. In this model, the eight half-charged oxygens of the tethered carboxylate groups of the protein are confined to a subvolume of the pore (the "filter"), but interact spontaneously with their mobile counterions as ions interact in concentrated bulk solutions. The mean spherical approximation (MSA) is used to predict ion-specific excess chemical potentials in the filter and baths. The theory is calibrated using a single experimental observation, concerning the apparent dissociation constant of Ca(2+) in the presence of a physiological concentration of NaCl. When ions are assigned their independently known crystal diameters and the carboxylate oxygens are constrained, e.g., to a volume of 0.375 nm(3) in an environment with an effective dielectric coefficient of 63.5, the hypothesized selectivity filter produces the shape of the calcium binding curves observed in experiment, and it predicts Ba(2+)/Ca(2+) and Na(+)/Li(+) competition, and Cl(-) exclusion as observed. The selectivities for Na(+), Ca(2+), Ba(2+), other alkali metal ions, and Cl(-) thus can be predicted by volume exclusion and electrostatic screening alone. Spontaneous coordination of ions and carboxylates can produce a wide range of Ca(2+) selectivities, depending on the volume density of carboxylate groups and the permittivity in the locus. A specific three-dimensional structure of atoms at the binding site is not needed to explain Ca(2+) selectivity.
L型钙通道是具有重要生物学意义的Ca(2+)结合蛋白。它们通过允许Ca(2+)离子穿过脂质膜进入细胞，从而产生活细胞必需的细胞内信号，进而选择细胞外丰度较低的离子。其选择机制涉及四个羧基基团，包含八个氧离子，这些羧基基团属于通道蛋白“EEEE”位点的侧链，这种结构与许多Ca(2+)螯合分子中的结构相似。本研究检验了一种假说，即该位点的选择性由离子与有限直径的羧基氧之间的相互静电筛选和体积排斥作用所决定。在该模型中，蛋白质连接的羧基基团的八个半带电氧被限制在孔的一个子体积（“过滤器”）中，但会像离子在浓溶液中相互作用那样，与它们的可移动反离子自发相互作用。平均球近似（MSA）用于预测过滤器和浴槽中离子特异性的过量化学势。该理论通过一个关于在生理浓度的NaCl存在下Ca(2+)的表观解离常数的单一实验观察进行校准。当为离子指定其独立已知的晶体直径，并将羧基氧限制在例如有效介电系数为63.5的环境中的0.375 nm(3)体积内时，所假设的选择性过滤器产生了实验中观察到的钙结合曲线的形状，并且预测了观察到的Ba(2+)/Ca(2+)和Na(+)/Li(+)竞争以及Cl(-)排斥。因此，仅通过体积排斥和静电筛选就可以预测Na(+)、Ca(2+)、Ba(2+)、其他碱金属离子和Cl(-)的选择性。离子与羧酸盐的自发配位可以产生广泛的Ca(2+)选择性，这取决于羧基基团的体积密度和该位点的电容率。解释Ca(2+)选择性不需要结合位点处原子的特定三维结构。
