Max-Planck-Institut für Bioanorganische Chemie, Stiftstr. 34-36, D-45470 Mülheim an der Ruhr, Germany.
J Am Chem Soc. 2011 Dec 14;133(49):19743-57. doi: 10.1021/ja2041805. Epub 2011 Nov 17.
Protonation states of water ligands and oxo bridges are intimately involved in tuning the electronic structures and oxidation potentials of the oxygen evolving complex (OEC) in Photosystem II, steering the mechanistic pathway, which involves at least five redox state intermediates S(n) (n = 0-4) resulting in the oxidation of water to molecular oxygen. Although protons are practically invisible in protein crystallography, their effects on the electronic structure and magnetic properties of metal active sites can be probed using spectroscopy. With the twin purpose of aiding the interpretation of the complex electron paramagnetic resonance (EPR) spectroscopic data of the OEC and of improving the view of the cluster at the atomic level, a complete set of protonation configurations for the S(2) state of the OEC were investigated, and their distinctive effects on magnetic properties of the cluster were evaluated. The most recent X-ray structure of Photosystem II at 1.9 Å resolution was used and refined to obtain the optimum structure for the Mn(4)O(5)Ca core within the protein pocket. Employing this model, a set of 26 structures was constructed that tested various protonation scenarios of the water ligands and oxo bridges. Our results suggest that one of the two water molecules that are proposed to coordinate the outer Mn ion (Mn(A)) of the cluster is deprotonated in the S(2) state, as this leads to optimal experimental agreement, reproducing the correct ground state spin multiplicity (S = 1/2), spin expectation values, and EXAFS-derived metal-metal distances. Deprotonation of Ca(2+)-bound water molecules is strongly disfavored in the S(2) state, but dissociation of one of the two water ligands appears to be facile. The computed isotropic hyperfine couplings presented here allow distinctions between models to be made and call into question the assumption that the largest coupling is always attributable to Mn(III). The present results impose limits for the total charge and the proton configuration of the OEC in the S(2) state, with implications for the cascade of events in the Kok cycle and for the water splitting mechanism.
水配体和氧桥的质子化状态与调节光合作用系统 II 中氧释放复合物 (OEC) 的电子结构和氧化电位密切相关，指导着涉及至少五个氧化还原状态中间体 S(n)（n=0-4）的机制途径，导致水氧化为分子氧。尽管在蛋白质晶体学中质子实际上是不可见的，但可以使用光谱法探测它们对金属活性位点的电子结构和磁性的影响。本研究的双重目的是帮助解释 OEC 的复杂电子顺磁共振 (EPR) 光谱数据，并改善原子水平上对簇的观察，因此研究了 OEC 的 S(2)态的完整质子化构型，并评估了它们对簇磁性的独特影响。使用最新的分辨率为 1.9Å 的光合作用系统 II 的 X 射线结构，并对其进行了优化，以获得蛋白质口袋内 Mn(4)O(5)Ca 核心的最佳结构。利用该模型，构建了一组 26 种结构，以测试水配体和氧桥的各种质子化情况。研究结果表明，在 S(2)态下，簇中两个拟配位外 Mn 离子 (Mn(A))的水分子之一可能去质子化，因为这与实验结果吻合得最好，重现了正确的基态自旋多重性 (S=1/2)、自旋期望值和 EXAFS 衍生的金属-金属距离。在 S(2)态下，Ca(2+)结合的水分子去质子化是强烈不利的，但两个水配体之一的解离似乎很容易发生。这里提出的计算各向同性超精细耦合允许对模型进行区分，并对最大耦合总是归因于 Mn(III)的假设提出质疑。目前的结果对 S(2)态下 OEC 的总电荷和质子构型施加了限制，这对 Kok 循环中的事件级联和水分解机制有影响。
