An additional water is introduced into the manganese cluster during the formation of the S3 state of photosystem II

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Abstract
During photosynthetic water oxidation, the Mn4Ca cluster in Photosystem II progresses through five intermediate Si (i = 0–4) states. X-ray crystallography studies have reported the insertion of one new O ligand during the formation of the S3 state, but recent studies question the presence of this additional ligand based on cryo-EM and earlier room-temperature crystallography data. There is also controversy about whether the O-O bond interaction already occurs in the S3 state or in the subsequent S3 to S0 transition. Here we report conventional high-resolution data for the S1, S2, and S3 states to a resolution of ~1.9 Å, and anomalous diffraction data at two energies (9.5 keV and 7 keV), that was used to model the Mn positions, followed by determination of oxygen positions using the high-resolution maps. We show that the new oxygen atom, OX (or O6), in the S3 state is observable as a distinct peak without any restraints, confirming its ligation to Mn1 and Ca. The OX-O5 distance is ~2.1 Å, supporting no strong interaction between them in the S3 state, suggesting that if this is the O-O bond formation site, it is formed during the S3 to S0 transition initiated by the final oxidation of the cluster.
Funding Information
  • U.S. Department of Energy (DE-AC02-05CH11231)
  • National Institute of General Medical Sciences (GM110501)
  • National Institute of General Medical Sciences (GM126289)
  • National Institute of General Medical Sciences (R35GM151988)
  • National Institute of General Medical Sciences (R24GM154040)
  • National Institute of General Medical Sciences (P41GM139687)
  • Deutsche Forschungsgemeinschaft (SFB1078)

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