Structural determinants of proton blockage in aquaporins
Structural determinants of proton blockage in aquaporins
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DOI：
10.1016/j.jmb.2004.08.036
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发表时间：
2004-10-15
影响因子：
5.6
通讯作者：
Pomès, R
中科院分区：
文献类型：
--
作者：
Chakrabarti, N;Roux, B;Pomès, R
水通道蛋白是一类重要的膜通道，对水和线性多元醇具有选择性，但对包括质子在内的离子不能通透。最近的计算研究表明，质子通过水通道的水传导途径的传递受到在签名NPA基序处达到峰值的大量自由能垒的反对。在这里，自由能模拟和连续静电计算相结合，考察了特定结构元素对细菌甘油摄取促进剂中质子阻塞的贡献的性质和大小，对于GlpF通道的人工变体，获得了窄孔中结构扩散的跳步和转弯步骤的平均力分布的GlpE势，其中孔内容物与NPA签名基序上的保守残基Asn68和Asn203之间的库仑相互作用，选择性过滤器上的Arg206，以及在NPA基序周围以头对头的方式排列的两个半螺旋M3和M7的肽骨架被选择性地关闭。将这些结果与探针阳离子在水渗透过程中转移的静电能量分布进行了比较，结果表明，质子在窄孔内运动的自由能分布主要由通道中带电基团和极性基团的分布引起的静态效应主导，而介电效应主要有助于阻止H+进入孔口(去溶惩罚)。消除与H+在NPA基序周围移动相反的自由能梯度的唯一最有效的方法是关闭螺旋M3和M7的偶极矩。两个NPA ASN残基中的任何一个突变为Asp，分别补偿了与结构扩散的跳步和转弯步骤相反的电荷偶极和偶极-偶极效应，并显著降低了质子转运的自由能垒，表明这些单个突变体可能会泄漏质子。(C)2004爱思唯尔有限公司。保留所有权利。
Aquaporins are an important class of membrane channels selective for water and linear polyols but impermeable to ions, including protons. Recent computational studies have revealed that the relay of protons through the water-conduction pathway of aquaporin channels is opposed by a substantial free energy barrier peaking at the signature NPA motifs. Here, free-energy simulations and continuum electrostatic calculations are combined to examine the nature and the magnitude of the contribution of specific structural elements to proton blockage in the bacterial glycerol uptake facilitator, GlpE Potential of mean-force profiles for both hop and turn steps of structural diffusion in the narrow pore are obtained for artificial variants of the GlpF channel in which coulombic interactions between the pore contents and conserved residues Asn68 and Asn203 at the NPA signature motifs, Arg206 at the selectivity filter, and the peptidic backbone of the two half-helices M3 and M7, which are arranged in head-to-head fashion around the NPA motifs, are turned off selectively. A comparison of these results with electrostatic energy profiles for the translocation of a probe cation throughout the water permeation pathway indicates that the free-energy profile for proton movement inside the narrow pore is dominated by static effects arising from the distribution of charged and polar groups of the channel, whereas dielectric effects contribute primarily to opposing the access of H+ to the pore mouths (desolvation penalty). The single most effective way to abolish the free-energy gradients opposing the movement of H+ around the NPA motif is to turn off the dipole moments of helices M3 and M7. Mutation of either of the two NPA Asn residues to Asp compensates for charge-dipole and dipole-dipole effects opposing the hop and turn steps of structural diffusion, respectively, and dramatically reduces the free energy barrier of proton translocation, suggesting that these single mutants could leak protons. (C) 2004 Elsevier Ltd. All rights reserved.