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Structures of the intermediates of Kok's photosynthetic water oxidation clock

机译：Kok光合水氧化钟的中间体结构

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Inspired by the period-four oscillation in flash-induced oxygen evolution of photo system II discovered by Joliot in 1969, Kok performed additional experiments and proposed a five-state kinetic model for photosynthetic oxygen evolution, known as Kok's S-state clock or cycle(1,2). The model comprises four (meta)stable intermediates (S-0, S-1, S-2 and S-3) and one transient S-4 state, which precedes dioxygen formation occurring in a concerted reaction from two water-derived oxygens bound at an oxo-bridged tetra manganese calcium (Mn4CaO5) cluster in the oxygen-evolving complex(3-7). This reaction is coupled to the two-step reduction and protonation of the mobile plastoquinone Q(B) at the acceptor side of PSII. Here, using serial femto second X-ray crystallography and simultaneous X-ray emission spectroscopy with multi-flash visible laser excitation at room temperature, we visualize all (meta) stable states of Kok's cycle as high-resolution structures (2.04-2.08 angstrom). In addition, we report structures of two transient states at 150 and 400 mu s, revealing notable structural changes including the binding of one additional 'water', Ox, during the S-2 - S-3 state transition. Our results suggest that one water ligand to calcium (W3) is directly involved in substrate delivery. The binding of the additional oxygen Ox in the S-3 state between Ca and Mnl supports O-O bond formation mechanisms involving O5 as one substrate, where Ox is either the other substrate oxygen or is perfectly positioned to refill the O5 position during O-2 release. Thus, our results exclude peroxo-bond formation in the S-3 state, and the nucleophilic attack of W3 onto W2 is unlikely.
机译：受到Joliot于1969年发现的光致系统II的闪光诱导的氧气逸出的四周期振荡的启发，Kok进行了额外的实验并提出了光合氧气逸出的五态动力学模型，称为Kok的S态时钟或周期（ 1,2）。该模型包含四个（亚）稳定的中间体（S-0，S-1，S-2和S-3）和一个瞬态S-4状态，该状态先于由两个水结合的氧结合产生的协同反应中形成双氧析氧复合物中的氧桥联四锰钙（Mn4CaO5）簇（3-7）。此反应与PSII受体一侧的可移动质体醌Q（B）的两步还原和质子化反应相关。在这里，使用系列毫微微秒X射线晶体学和同时X射线发射光谱以及室温下多闪光可见激光激发，我们将Kok周期的所有（元）稳定状态可视化为高分辨率结构（2.04-2.08埃） 。此外，我们报道了在150和400μs处的两个瞬态状态的结构，揭示了显着的结构变化，包括在S-2-> S-3状态转换过程中结合了另一种“水” Ox。我们的结果表明，钙（W3）的一种水配体直接参与底物的传递。 Ca和Mnl之间以S-3状态存在的其他氧氧的结合支持了以O5为一个底物的OO键形成机理，其中Ox是另一种底氧或在O-2释放过程中处于最佳位置以重新填充O5位置。因此，我们的结果排除了S-3状态下的过氧键形成，并且W3对W2的亲核攻击是不可能的。

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