Max-Planck-Institut für Chemische Energiekonversion, Mülheim/Ruhr, Germany.
Research School of Chemistry, The Australian National University, Canberra, Australia.
Photosynth Res. 2019 Oct;142(1):105-125. doi: 10.1007/s11120-019-00648-3. Epub 2019 Jun 11.
Biological water oxidation, performed by a single enzyme, photosystem II, is a central research topic not only in understanding the photosynthetic apparatus but also for the development of water splitting catalysts for technological applications. Great progress has been made in this endeavor following the report of a high-resolution X-ray crystallographic structure in 2011 resolving the cofactor site (Umena et al. in Nature 473:55-60, 2011), a tetra-manganese calcium complex. The electronic properties of the protein-bound water oxidizing MnOCa complex are crucial to understand its catalytic activity. These properties include: its redox state(s) which are tuned by the protein matrix, the distribution of the manganese valence and spin states and the complex interactions that exist between the four manganese ions. In this short review we describe how magnetic resonance techniques, particularly EPR, complemented by quantum chemical calculations, have played an important role in understanding the electronic structure of the cofactor. Together with isotope labeling, these techniques have also been instrumental in deciphering the binding of the two substrate water molecules to the cluster. These results are briefly described in the context of the history of biological water oxidation with special emphasis on recent work using time resolved X-ray diffraction with free electron lasers. It is shown that these data are instrumental for developing a model of the biological water oxidation cycle.
生物水氧化是由单一酶——光系统 II 完成的，这不仅是理解光合作用装置的核心研究课题，也是开发用于技术应用的水分解催化剂的核心研究课题。自 2011 年报道分辨率为高分辨率 X 射线晶体结构以来，在这方面取得了巨大进展，该结构解析了辅因子位点（Umena 等人，《自然》473:55-60, 2011），即一个四锰钙配合物。了解其催化活性的关键是了解与蛋白质结合的水氧化 MnOCa 配合物的电子性质。这些性质包括：其氧化还原状态（由蛋白质基质调节）、锰价态和自旋态的分布以及四个锰离子之间存在的复杂相互作用。在这篇简短的综述中，我们描述了磁共振技术（特别是电子顺磁共振）如何与量子化学计算相结合，在理解辅因子的电子结构方面发挥了重要作用。结合同位素标记，这些技术在解析两个底物水分子与簇的结合方面也发挥了重要作用。这些结果简要地描述了生物水氧化的历史背景，特别强调了最近使用自由电子激光的时间分辨 X 射线衍射的工作。结果表明，这些数据对于开发生物水氧化循环模型非常重要。
