Dong Hao, Yi Myunggi, Cross Timothy A, Zhou Huan-Xiang
Department of Physics and Institute of Molecular Biophysics, Florida State University, Tallahassee, FL 32306, USA.
Chem Sci. 2013 Jul 1;4(7):2776-2787. doi: 10.1039/C3SC50293G.
The M2 protein of Influenza A virus forms a homotetrameric proton channel activated by low pH. The His37-Trp41 quartet is the heart of acid activation and proton conductance, but the functional mechanism is still controversial. We carried out calculations to model the pH-dependent structures of the His37-Trp41 quartet. In our model at neutral pH, the four His37 residues are configured into a pair of dimers; in each dimer, a proton is shared between Nδ1 on one residue and Nε2 on the other, and, under the restraint of the backbone, the two imidazole rings are nearly parallel, in contrast to a perpendicular arrangement for a free imidazole-imidazolium dimer. Within each dimer the +1 charge is highly delocalized, contributing to its stabilization in a low dielectric environment. The Nδ1-H-Nε2 strong hydrogen bonds result in significantly downfield shifted Nδ1 and Nε2 chemical shifts (at 169.7 and 167.6 ppm, respectively), in good agreement with experiments. In our model at acidic pH (where the channel becomes activated), a third proton binds to an imidazole-imidazolium dimer; the imidazole rings rotate away (each by ~55°) from each other, destroying the dimer structure. The two imidazoliums are stabilized by hydrogen bonds with water molecules and a cation-π interaction with Trp41. The Raman spectra calculated for the His37-Trp41 quartet at neutral and acidic pH are in agreement with experiments. Our calculations support an activation and conductance mechanism in which a hydronium ion from the N-terminal side passes a proton to an imidazole-imidazolium dimer; when the Trp41 gate is open, relaying of a proton onto a water molecule from the C-terminal side then allows the imidazole-imidazolium dimer to reform and be ready for the next round of proton conductance.
甲型流感病毒的M2蛋白形成一种由低pH激活的同源四聚体质子通道。His37-Trp41四重奏是酸激活和质子传导的核心，但功能机制仍存在争议。我们进行了计算，以模拟His37-Trp41四重奏的pH依赖性结构。在我们的中性pH模型中，四个His37残基构成一对二聚体；在每个二聚体中，一个质子在一个残基的Nδ1和另一个残基的Nε2之间共享，并且在主链的约束下，两个咪唑环几乎平行，这与游离咪唑-咪唑鎓二聚体的垂直排列形成对比。在每个二聚体内，+1电荷高度离域，有助于其在低介电环境中的稳定。Nδ1-H-Nε2强氢键导致Nδ1和Nε2化学位移显著向低场移动（分别为169.7和167.6 ppm），与实验结果吻合良好。在我们的酸性pH模型中（此时通道被激活），第三个质子与一个咪唑-咪唑鎓二聚体结合；咪唑环彼此旋转分开（每个旋转约55°），破坏了二聚体结构。两个咪唑鎓通过与水分子的氢键和与Trp41的阳离子-π相互作用而稳定。在中性和酸性pH下为His37-Trp41四重奏计算的拉曼光谱与实验结果一致。我们的计算支持一种激活和传导机制，即来自N端侧的水合氢离子将一个质子传递给一个咪唑-咪唑鎓二聚体；当Trp41门打开时，质子从C端侧传递到一个水分子上，然后允许咪唑-咪唑鎓二聚体重组并为下一轮质子传导做好准备。
