Five-coordinate MnIV intermediate in the activation of nature's water splitting cofactor
Five-coordinate MnIV intermediate in the activation of nature's water splitting cofactor
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DOI：
10.1073/pnas.1817526116
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发表时间：
2019-08-20
影响因子：
11.1
通讯作者：
Cox, Nicholas
中科院分区：
文献类型：
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作者：
Chrysina, Maria;Heyno, Eiri;Cox, Nicholas
自然界的水分解辅因子在O-O键形成和O-2释放之前要经过一系列催化中间体（S-0-S-4）。在最后的第二个转变（S-2到S-3）中，辅因子氧化与水分子结合到Mn1。正是这种活化的、富含水的Mn-IV形式的辅因子，在下一次光诱导氧化成S-4后，继续形成O-O键。辅因子如何激活仍然是一个悬而未决的问题。在这里，我们报告了一个到目前为止尚未描述的中间产物（S-3'），其中在没有水插入的情况下发生了辅助因子氧化。这种中间体可以被捕获在(i) Ca2+与Sr2+交换的化学修饰的辅因子中相当一部分中心（bbb50 %）；Mn4O5Sr辅因子保持活性，但S-2-S-3和S-3-S-0的转变比Mn4O5Ca辅因子慢；（二）添加体积比为3%的甲醇；甲醇被认为是底物水的类似物。S-3’电子顺磁共振（EPR）信号明显宽于未处理的S-3信号（2.5 T vs. 1.5 T），表明辅因子仍含有5位Mn离子，如前面的S-2状态所示。磁双共振数据扩展了这些发现，揭示了S-3'辅因子的电子连接类似于前面S-2状态的高自旋形式，其中包含一个立方体Mn3O4Ca单元，连接到外部的5坐标Mn离子（Mn-4）。这些结果表明，辅因子氧化通过与Mn-4结合来调节水分子的插入。还讨论了氨与辅因子的相互作用。
Nature's water splitting cofactor passes through a series of catalytic intermediates (S-0-S-4) before O-O bond formation and O-2 release. In the second last transition (S-2 to S-3) cofactor oxidation is coupled to water molecule binding to Mn1. It is this activated, water-enriched all Mn-IV form of the cofactor that goes on to form the O-O bond, after the next light-induced oxidation to S-4. How cofactor activation proceeds remains an open question. Here, we report a so far not described intermediate (S-3') in which cofactor oxidation has occurred without water insertion. This intermediate can be trapped in a significant fraction of centers (> 50%) in (i) chemical-modified cofactors in which Ca2+ is exchanged with Sr2+; the Mn4O5Sr cofactor remains active, but the S-2-S-3 and S-3-S-0 transitions are slower than for the Mn4O5Ca cofactor; and (ii) upon addition of 3% vol/vol methanol; methanol is thought to act as a substrate water analog. The S-3' electron paramagnetic resonance (EPR) signal is significantly broader than the untreated S-3 signal (2.5 T vs. 1.5 T), indicating the cofactor still contains a 5-coordinate Mn ion, as seen in the preceding S-2 state. Magnetic double resonance data extend these findings revealing the electronic connectivity of the S-3' cofactor is similar to the high spin form of the preceding S-2 state, which contains a cuboidal Mn3O4Ca unit tethered to an external, 5-coordinate Mn ion (Mn-4). These results demonstrate that cofactor oxidation regulates water molecule insertion via binding to Mn-4. The interaction of ammonia with the cofactor is also discussed.