Photosynthesis, a process catalysed by plants, algae and cyanobacteria converts sunlight to energy thus sustaining all higher life on Earth. Two large membrane protein complexes, photosystem I and II (PSI and PSII), act in series to catalyse the light-driven reactions in photosynthesis. PSII catalyses the light-driven water splitting process, which maintains the Earth’s oxygenic atmosphere1. In this process, the oxygen-evolving complex (OEC) of PSII cycles through five states, S0 to S4, in which four electrons are sequentially extracted from the OEC in four light-driven charge-separation events. Here we describe time resolved experiments on PSII nano/microcrystals from Thermosynechococcus elongatus performed with the recently developed2 technique of serial femtosecond crystallography. Structures have been determined from PSII in the dark S1 state and after double laser excitation (putative S3 state) at 5 and 5.5 Å resolution, respectively. The results provide evidence that PSII undergoes significant conformational changes at the electron acceptor side and at the Mn4CaO5 core of the OEC. These include an elongation of the metal cluster, accompanied by changes in the protein environment, which could allow for binding of the second substrate water molecule between the more distant protruding Mn (referred to as the ‘dangler’ Mn) and the Mn3CaOx cubane in the S2 to S3 transition, as predicted by spectroscopic and computational studies3,4. This work shows the great potential for time-resolved serial femtosecond crystallography for investigation of catalytic processes in biomolecules.
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Experiments were carried out at the Linac Coherent Light Source (LCLS), a national user facility operated by Stanford University on behalf of the US Department of Energy (DOE), Office of Basic Energy Sciences (OBES). This work was supported by the following agencies: the Center for Bio-Inspired Solar Fuel Production, an Energy Frontier Research Center funded by the DOE, Office of Basic Energy Sciences (award DE-SC0001016), the National Institutes of Health (award 1R01GM095583), the US National Science Foundation (award MCB-1021557 and MCB-1120997), the DFG Clusters of Excellence ‘Inflammation at Interfaces’ (EXC 306) and the ‘Center for Ultrafast Imaging’; the Deutsche Forschungsgemeinschaft (DFG); the Max Planck Society, the Atomic, Molecular and Optical Sciences Program; Chemical Sciences Geosciences and Biosciences Division, DOE OBES (M.J.B.) and the SLAC LDRD program (M.J.B., H.L.); the US DOE through Lawrence Livermore National Laboratory under the contract DE-AC52-07NA27344 and supported by the UCOP Lab Fee Program (award no. 118036) and the LLNL LDRD program (12-ERD-031); the Hamburg Ministry of Science and Research and Joachim Herz Stiftung as part of the Hamburg Initiative for Excellence in Research. The research at Purdue University was supported by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences DE-FG02-12ER16340 (Y.P.) and the National Science Foundation Graduate Research Fellowship under Grant 0833366 (K.M.D.). We also want to thank the National Science Foundation for providing funding for the publication of this work through the BioFEL Science Technology Center (award 1231306). We thank H. Isobe, M. Shoji, S. Yamanaka, Y. Umena, K. Kawakami, N. Kamiya, J. R. Shen and K. Yamaguchi for permission to show a section of Fig. 6 of their publication ref. 4 in Fig. 3d of this publication. We thank R. Neutze and his team for support and discussions during joint beamtime for the PSII project and his projects on time-resolved wide-angle scattering studies. We thank A. T. Brunger for discussions concerning data analysis. We thank T. Terwilliger for support with parameter setting of phenix.autobuild program for the SA-omit maps. We also wish to thank R. Burnap for discussions concerning interpretation of results of ligand mutagenesis. We thank J. D. Zook for his contributions concerning plastoquinone quantification. We thank M. Zhu for helping to create high resolution figures for this publication. We thank Raytheon for support of our studies by providing night-vision devices.
Graphic representation of the structure factor amplitudes for the dark data set. The video shows the structure factor amplitudes from photosystem II nanocrystal SFX data collected in the dark at 5.0 Å , representing the dark S1 state of the oxygen evolving complex. The graphic representation was generated using the CrystFEL suite17.
The video shows the structure factor amplitudes from photosystem II nanocrystal SFX data collected at 5.5 Å from the double flash state, representing the putative S3 state of the oxygen evolving complex. The graphic representation was generated using the CrystFEL suite17.