New water oxidation mechanism in Photosystem II resolves major experimental controversies
Light driven oxygen formation in Photosystem II protein is a fundamental process that sustains our biosphere and serves as a blue print to future clean energy solutions due to its high energy conversion efficiency. Last decade of intense research by advanced physical techniques delivered new insights on the structure and function of the Mn4CaO5 cluster a center of the oxygen evolving complex (OEC). However, discrepancies in experimental observations and computational models persist impeding the understanding of the O-O bond formation and the role of the protein environment in the process. Here we show that i) assignment of the OEC unique oxygen O3 ligated by histidine (His337) via dynamic H-bond as a slow exchanging substrate and ii) its coupling with O6 oxygen generated at Mn1 in the S2 to S3 transition give the O-O bond formation mechanism most consistent with all currently available experimental data. Proposal shows how protein environment can steer the O-O bond formation by charge control via H-bond and open coordination of Mn1. Obtained O3-O6 peroxide is at lower energy than peroxides in the most studied O5-O6 bond formation pathway. His337 appears to be similar to distal His in globins used for management of the O2 and H2O2 intermediates. The new mechanism breaks the prior impasse and will undoubtedly invigorate future detailed studies uncovering its further details.