Svensson B, Etchebest C, Tuffery P, van Kan P, Smith J, Styring S
Department of Biochemistry, Arrhenius Laboratories for Natural Sciences, Stockholm University, Sweden.
Biochemistry. 1996 Nov 19;35(46):14486-502. doi: 10.1021/bi960764k.
For a detailed understanding of the function of photosystem II (PSII), a molecular structure is needed. The crystal structure has not yet been determined, but the PSII reaction center proteins D1 and D2 show homology with the L and M subunits of the photosynthetic reaction center from purple bacteria. We have modeled important parts of the D1 and D2 proteins on the basis of the crystallographic structure of the reaction center from Rhodopseudomonas viridis. The model contains the central core of the PSII reaction center, including the protein regions for the transmembrane helices B, C, D, and E and loops B-C and C-D connecting the helices. In the model, four chlorophylls, two pheophytins, and the nonheme Fe2+ ion are included. We have applied techniques from computational chemistry that incorporate statistical data on side-chain rotameric states from known protein structure and that describe interactions within the model using an empirical potential energy function. The conformation of chlorophyll pigments in the model was optimized by using exciton interaction calculations in combination with potential energy calculations to find a solution that agrees with experimentally determined exciton interaction energies. The model is analyzed and compared with experimental results for the regions of P680, the redox active pheophytin, the acceptor side Fe2+, and the tyrosyl radicals TyrD and TyrZ. P680 is proposed to be a weakly coupled chlorophyll a pair which makes three hydrogen bonds with residues on the D1 and D2 proteins. In the model the redox-active pheophytin is hydrogen bonded to D1-Glu130 and possibly also to D1-Tyr126 and D1-Tyr147. TyrD is hydrogen bonded to D2-His190 and also interacts with D2-Gln165. TyrZ is bound in a hydrophilic environment which is partially constituted by D1-Gln165, D1-Asp170, D1-Glu189, and D1-His190. These polar residues are most likely involved in proton transfer from oxidized TyrZ or in metal binding.
为了详细了解光系统II（PSII）的功能，需要其分子结构。虽然尚未确定其晶体结构，但PSII反应中心蛋白D1和D2与紫色细菌光合反应中心的L和M亚基具有同源性。我们基于绿硫红假单胞菌反应中心的晶体结构对D1和D2蛋白的重要部分进行了建模。该模型包含PSII反应中心的核心部分，包括跨膜螺旋B、C、D和E以及连接这些螺旋的B - C环和C - D环的蛋白质区域。模型中包含四个叶绿素、两个脱镁叶绿素和非血红素Fe2+离子。我们应用了计算化学技术，这些技术纳入了来自已知蛋白质结构的侧链旋转异构体状态的统计数据，并使用经验势能函数描述模型中的相互作用。通过结合激子相互作用计算和势能计算来优化模型中叶绿素色素的构象，以找到与实验测定的激子相互作用能相符的解决方案。对该模型进行了分析，并与P680区域、氧化还原活性脱镁叶绿素、受体侧Fe2+以及酪氨酸自由基TyrD和TyrZ的实验结果进行了比较。P680被认为是一对弱耦合的叶绿素a，它与D1和D2蛋白上的残基形成三个氢键。在模型中，氧化还原活性脱镁叶绿素与D1 - Glu130形成氢键，也可能与D1 - Tyr126和D1 - Tyr147形成氢键。TyrD与D2 - His190形成氢键，还与D2 - Gln165相互作用。TyrZ位于一个亲水环境中，该环境部分由D1 - Gln165、D1 - Asp170、D1 - Glu189和D1 - His190构成。这些极性残基很可能参与了来自氧化态TyrZ的质子转移或金属结合。
