Department of Chemistry, Princeton University, Princeton, New Jersey, USA.
Biophys J. 2012 Jul 18;103(2):313-22. doi: 10.1016/j.bpj.2012.05.031. Epub 2012 Jul 17.
Photosynthetic O(2) production from water is catalyzed by a cluster of four manganese ions and a tyrosine residue that comprise the redox-active components of the water-oxidizing complex (WOC) of photosystem II (PSII) in all known oxygenic phototrophs. Knowledge of the oxidation states is indispensable for understanding the fundamental principles of catalysis by PSII and the catalytic mechanism of the WOC. Previous spectroscopic studies and redox titrations predicted the net oxidation state of the S(0) state to be (Mn(III))(3)Mn(IV). We have refined a previously developed photoassembly procedure that directly determines the number of oxidizing equivalents needed to assemble the Mn(4)Ca core of WOC during photoassembly, starting from free Mn(II) and the Mn-depleted apo-WOC complex. This experiment entails counting the number of light flashes required to produce the first O(2) molecules during photoassembly. Unlike spectroscopic methods, this process does not require reference to synthetic model complexes. We find the number of photoassembly intermediates required to reach the lowest oxidation state of the WOC, S(0), to be three, indicating a net oxidation state three equivalents above four Mn(II), formally (Mn(III))(3)Mn(II), whereas the O(2) releasing state, S(4), corresponds formally to (Mn(IV))(3)Mn(III). The results from this study have major implications for proposed mechanisms of photosynthetic water oxidation.
光合作用中，水的氧合反应由一个锰离子簇和一个酪氨酸残基共同催化，这个锰离子簇和酪氨酸残基构成了所有已知放氧光合作用生物中光系统 II（PSII）水氧化复合物（WOC）的氧化还原活性组件。了解氧化态对于理解 PSII 的催化基本原理和 WOC 的催化机制是不可或缺的。先前的光谱研究和氧化还原滴定预测，S(0)态的总氧化态为（Mn(III))(3)Mn(IV)。我们改进了之前开发的光组装程序，该程序直接确定了在光组装过程中组装 WOC 的 Mn(4)Ca 核心所需的氧化当量数，起始物为游离的 Mn(II)和 Mn 耗尽的 apo-WOC 复合物。该实验需要计算在光组装过程中产生第一个 O(2)分子所需的闪光次数。与光谱方法不同，该过程不需要参考合成模型配合物。我们发现，达到 WOC 的最低氧化态 S(0)所需的光组装中间体数量为三个，表明 WOC 的总氧化态比四个 Mn(II)高出三个当量，形式上为（Mn(III))(3)Mn(II)，而释放 O(2)的状态 S(4)形式上对应于（Mn(IV))(3)Mn(III)。这项研究的结果对光合作用中水氧化的拟议机制有重大影响。
