The photosynthetic water-splitting reaction catalysed by the oxygen-evolving center (OEC) in the photosystem II of plants releases protons, electrons and dioxygen, which is one of the most important processes for energy and material conversions on Earth. OEC is a Mn4Ca-cluster and cycles through five different states (Sn, n = 0–4), wherein S0 is the most reduced state, S1 is the dark stable state and S2, S3 and S4 are intermediate states. OEC rapidly releases O2 in the S4 state, then returns to the S0 state. The detailed structure of the OEC in the S1 state was reported by Umena et al. in 2011  and improved by Suga et al. in 2015 . These reports from Shen's group have revealed that OEC is composed of an asymmetric Mn4CaO5-cluster coordinated by four water molecules, one imidazole and six carboxylate groups from protein side chains (Fig. 1). Determination of the accurate structure of the OEC in the S1 state [1,2] is a breakthrough in the field of photosynthesis. However, because of the complexities of the large protein environment and the dynamic changes of the OEC during the water-splitting reaction, it is a great challenge to reveal its detailed reaction mechanism. Figure 1. View largeDownload slide Structures of OEC in the S1 and S3 states. Figure 1. View largeDownload slide Structures of OEC in the S1 and S3 states. Recently, Shen's group reached a new milestone . They reported the structure of the S3 intermediate state OEC by using time-resolved serial femtosecond X-ray crystallography with an X-ray free electron laser , which, for the first time, identified the formation of the O–O bond between the ‘famous’ μ4-O5 and a newly inserted oxygen atom (O6) with a length of 1.5 Å (Fig. 1). The O=O bond formation is key to understanding the mechanism of the photosynthetic water-splitting reaction, which has attracted the extensive attention of various spectroscopic and theoretical studies during the last several decades . A recent breakthrough from Shen's group has provided unambiguous evidence for the crucial O–O bond formation ; meanwhile, new questions arise for future studies. For example, how is the O6 atom inserted into the OEC? What are the functional roles of the four water molecules coordinated to the OEC, and what is the real function of the μ2-O4 atom? Finally, what is the structure of the OEC in the S4 state? According to the binding mode of the O–O bond in the S3 state, the release of O2 in the S4 state would produce four reactive sites, namely three 5-coordinated manganese (i.e. Mn1, Mn3, Mn4) and one 6-coordinated calcium. Based on our knowledge from an artificial Mn4CaO4-cluster , significant structural rearrangements would occur to accommodate the newly formed Mn4CaO4-cluster. In summary, the previous outstanding achievements by Shen's group revealed the accurate structures of the OEC in the S1 state [1,2], which was already considered to be a breakthrough in photosynthesis. The recent finding on the mechanism of O–O bond formation  is a new milestone for photosynthetic research. These achievements have contributed to enhancing our knowledge on photosynthetic oxygen evolution tremendously, and will significantly promote the development of the new generation of man-made OEC for water-splitting reactions in artificial photosynthesis, aiming to produce clean and renewable fuels from sunlight and water. REFERENCES 1. Umena Y , Kawakami K , Shen JR et al. Nature 2011 ; 473 : 55 – 60 . 2. Suga M , Akita F , Hirata K et al. Nature 2015 ; 517 : 99 – 103 . 3. Suga M , Akita F , Sugahara M et al. Nature 2017 ; 543 : 131 – 5 . 4. Perez-Navarro M , Neese F , Lubitz W et al. Curr Opin Chem Biol 2016 ; 31 : 113 – 9 . 5. Zhang C , Chen C , Dong H et al. Science 2015 ; 348 : 690 – 3 . © The Author(s) 2017. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)
National Science Review – Oxford University Press
Published: Jul 1, 2018
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