Lockless Steve W, Zhou Ming, MacKinnon Roderick
Laboratory of Molecular Neurobiology and Biophysics, The Rockefeller University, New York, New York, United States of America.
PLoS Biol. 2007 May;5(5):e121. doi: 10.1371/journal.pbio.0050121.
Thermodynamic measurements of ion binding to the Streptomyces lividans K(+) channel were carried out using isothermal titration calorimetry, whereas atomic structures of ion-bound and ion-free conformations of the channel were characterized by x-ray crystallography. Here we use these assays to show that the ion radius dependence of selectivity stems from the channel's recognition of ion size (i.e., volume) rather than charge density. Ion size recognition is a function of the channel's ability to adopt a very specific conductive structure with larger ions (K(+), Rb(+), Cs(+), and Ba(2+)) bound and not with smaller ions (Na(+), Mg(2+), and Ca(2+)). The formation of the conductive structure involves selectivity filter atoms that are in direct contact with bound ions as well as protein atoms surrounding the selectivity filter up to a distance of 15 A from the ions. We conclude that ion selectivity in a K(+) channel is a property of size-matched ion binding sites created by the protein structure.
利用等温滴定量热法对离子与变铅青链霉菌钾通道的结合进行了热力学测量，而通道的离子结合和离子自由构象的原子结构则通过X射线晶体学进行了表征。在此，我们运用这些分析方法来表明，选择性的离子半径依赖性源于通道对离子大小（即体积）而非电荷密度的识别。离子大小识别是通道采用一种非常特殊的传导结构的能力的函数，这种结构在结合较大离子（K⁺、Rb⁺、Cs⁺和Ba²⁺）时存在，而结合较小离子（Na⁺、Mg²⁺和Ca²⁺）时不存在。传导结构的形成涉及与结合离子直接接触的选择性过滤器原子以及距离离子达15埃的围绕选择性过滤器的蛋白质原子。我们得出结论，钾通道中的离子选择性是由蛋白质结构产生的大小匹配离子结合位点的一种特性。
