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Light, Water, Oxygen! The water oxidation reaction in Photosystem II visualized by time resolved X-ray studies

Abstract

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Photosystem II (PSII) is present in all oxygenic photosynthetic organisms (plants, algea and cyanobacteria) and catalyzes the light driven oxidation of water to dioxygen and is hence responsible for the generation of all atmospheric oxygen. Its active site contains a metal cluster composed of four Mn and one Ca ion bridged by several oxygen atoms and cycles through four stable intermediate states (called S₀, S₁, S₂, S₃) during the reaction. How PSII can catalyze this very challenging reaction with minimum overpotential and without the formation of any deleterious side products is one of the big questions in photosynthesis research and answers will lilkely also have important implications for future synthetic catalysis strategies for sun light driven fuel generation. While X-rays allow to obtain high resolution information about the geometric and electronic structure of metalloenzymes, traditional synchrotron approaches usually need to be conducted at cryogenic temperatures due to damage of the sample by the measurement. But when utilizing femtosecond short X-ray pulses from X-ray free electron lasers (XFELs) it is possible to record “undamaged” snapshots of metalloenzymes at room temperature. Given adequate reaction triggering options these can be collated to a “movie” that shows the sequence of events at the catalytic site necessary for the reaction to take place. We used this approach to record data from crystals of PS II, at different time points during the light driven water oxidation reaction leading to structures at around 2 Å resolution for the four stable states (S₀ to S₃) [1] as well as for time points in the S₂-S₃ transition [2,3] and the S₃-S₀ transition, where the oxygen-oxygen bond is formed [4]. We also obtained X-ray emission (XES) data collected in parallel to the diffraction data that provide kinetic information about the Mn redox changes which can be correlated with structural observations at different time points in the reaction cycle. In this talk I will present details from these studies regarding the steps involved in the incorporation of an additional oxygen atom into the Mn₄CaO₅ cluster during the S₂-S₃ transition as well as the resetting of the cluster in the S₃-S₀ transition, including indications for the presence of an observable reaction intermediate at around 500 to 1200 µs into the S₃-S₀ transitions. I will also discuss the importance of the network of channels connecting the Mn cluster with the lumen and their potential role in proton and water transport during catalysis, based on our structural observations.

References:

[1] J. Kern et al. (2018). Nature 563, 421–425.

[2] M. Ibrahim, et al. (2020). Proc. Natl. Acad. Sci. USA. 70, 3554.

[3] R. Hussein, et al. (2021). Nat. Comm. 12, 6531.

[4] A. Bhowmick et al. (2023), Nature 617, 629-636