pH regulates cation selectivity of poly-(R)-3-hydroxybutyrate/polyphosphate channels from E. coli in planar lipid bilayers.
pH regulates cation selectivity of poly-(R)-3-hydroxybutyrate/polyphosphate channels from E. coli in planar lipid bilayers.
复制标题
pH 调节平面脂质双层中大肠杆菌的聚（R）-3-羟基丁酸/聚磷酸盐通道的阳离子选择性。
作者：
S. Das;R. Reusch
聚（R）-3-羟基丁酸酯/聚磷酸盐（PHB/聚P）复合物，无论是从细菌的质膜分离的还是从合成聚合物制备的，在平面脂质双层中形成离子通道，对Ca（2+）的选择性高于Na（+）这种对二价阳离子优于一价阳离子的偏好归因于沿着聚P主链的高密度负电荷沿着和聚P的较高结合能。二价阳离子。在这里，我们通过改变pH值来修改聚P的电荷密度，并观察对阳离子选择性的影响。PHB/polyP复合物，分离自E.当Ca（2+）是唯一的渗透阳离子时，电导从pH 7.4时的97 +/- 6 pS稳定下降到pH 5.5时的47 +/- 3 pS。然而，在Ca（2+）和Na（+）的不对称溶液中，电导从pH 7.4的98 +/- 4适度增加到pH 6.5的129 +/- 4 pS，并且在pH 5.6时大幅增加到178 +/- 6 pS，表明Na（+）渗透性增加或通道结构解体。在生理pH下，Ca（2+）比Na（+）更受青睐，但在磷酸盐pK（2）附近通道变得非选择性（约6.8），并且在酸性pH下对Na（+）的选择性弱于Ca（2+）。显然，PHB/polyP复合物是多功能的离子载体，其选择性可以通过局部pH的微小调节来调节。细菌中的polyP通道以及聚羟基丁酸和polyP在变铅青链霉菌钾通道中的作用。
Poly-(R)-3-hydroxybutyrate/polyphosphate (PHB/polyP) complexes, whether isolated from the plasma membranes of bacteria or prepared from the synthetic polymers, form ion channels in planar lipid bilayers that are highly selective for Ca(2+) over Na(+) at physiological pH. This preference for divalent over monovalent cations is attributed to a high density of negative charge along the polyP backbone and the higher binding energies of divalent cations. Here we modify the charge density of polyP by varying the pH, and observe the effect on cation selectivity. PHB/polyP complexes, isolated from E. coli, were incorporated into planar lipid bilayers, and unitary current-voltage relations were determined as a function of pH. When Ca(2+) was the sole permeant cation, conductance diminished steadily from 97 +/- 6 pS at pH 7.4 to 47 +/- 3 pS at pH 5.5. However, in asymmetric solutions of Ca(2+) and Na(+), there was a moderate increase in conductance from 98 +/- 4 at pH 7.4 to 129 +/- 4 pS at pH 6.5, and a substantially larger increase to 178 +/- 6 pS at pH 5.6, signifying an increase in Na(+) permeability or disorganization of channel structure. Reversal potentials point to a sharp decrease in preference for Ca(2+) over Na(+) over a relatively small decrease in pH. Ca(2+) was strongly favored over Na(+) at physiological pH, but the channels became nonselective near the pK(2) of phosphate (approximately 6.8), and displayed weak selectivity for Na(+) over Ca(2+) at acidic pH. Evidently, PHB/polyP complexes are versatile ion carriers whose selectivity may be modulated by small adjustments of the local pH. The results may be relevant to the physiological function of PHB/polyP channels in bacteria and the role of PHB and polyP in the Streptomyces lividans potassium channel.