KcsA通道对Na^+、K^+及Rb^+离子选择性的统计热力学研究

Statistic Thermodynamics of The Selectivity of KcsA Channel to Na^+、K^+ and Rb^+ Ions

钾离子的通透率至少比钠离子的通透率大10 000倍,这个问题至今没有很好地解决. 为了在分子水平阐释钾离子通道的选择性机制,以KcsA钾通道X射线衍射结构为基础,采用密度泛函理论计算了不同离子在离子通道中的位能. 计算结果表明,Rb+离子具有与K+离子相类似的位能曲线,但是其在通透过程遇到的位垒要比K+离子的位垒高,因而所对应的通透率也就小于钾离子的通透率,而钠离子的的通透率仅仅是钾离子通透率的0.0067%. 文中所涉及的系统仅仅包含269个原子,而用分子动力学虽然也可以得到相近的结果,但是它的系统大小为41 000个原子.

Potassium is at least 10 000 times more permeant than Na+, remaining to be an open question of K+ channels. To elucidate the mechanism of ion selectivity at atomic level, the density functional theory was used to calculate the potential on the basis of the X-ray structure of the KcsA K+ channel. In the viewpoint of statistic. thermodynamics, the two most readily permeant ions, K+ and Rb+ ions, can pass through the selectivity filter. In contrast, Na+ ions, which prefer staying in site 2, can not pass through the channel. The energy barrier difference between the K+ and Na+ is 27.4 kJ/mol which induced the permeant ratio of 15 000. The energy barrier difference of Rb+ ions is higher than that of K+ ions which is corresponding to the lower permeant of Rb+ ions. The results are qualitatively consistent with the experimental data. It is remarkable that the system, involved in 269 atoms, is much lower than the MD simulation which is about 41 000 atoms.