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Project Progress

The light-driven steps in the biogenesis and repair of the inorganic core comprising the O₂-evolving center of oxygenic photosynthesis (photosystem II water-oxidation complex, PSII-WOC) are reviewed. These steps, known collectively as photoactivation, involve the photoassembly of the free inorganic cofactors to the cofactor-depleted PSII-(apo-WOC) driven by light and produce the active O2-evolving Mn4CaOxCly. We focus on the functional role of the inorganic components as seen through the competition with non-native cofactors (“inorganic mutants”) on water oxidation activity, the rate of the photoassembly reaction, and on structural insights gained from EPR spectroscopy of trapped intermediates formed in the initial steps of the assembly reaction. A chemical mechanism for the initial steps in photoactivation is given that is based on these data. Photoactivation experiments offer the powerful insights gained from replacement of the native cofactors, which together with the recent X-ray structural data for the resting holoenzyme provide a deeper understanding of the chemistry of water oxidation. We also review some new directions in research that photoactivation studies have inspired that look at the evolutionary history of this remarkable catalyst.

A manuscript entitled, Electron Spin Echo Envelope Modulation (ESEEM) Spectroscopy Reveals Carbonate and N-donor Protein Ligand Binding to Mn2+ in the Photo-assembly Reaction of the Mn₄Ca cluster in Photosystem II provides the first direct spectroscopic proof of binding of carbonate to manganese during the assembly of the Mn₄Ca cluster of an O₂-evolving photosynthetic organism. This evidence comes from ESEEM spectroscopy which identifies a magnetic hyperfine coupling between ¹³C-carbonate and the Mn2+ precursor to the photooxidized Mn3+ formed in the first step of the assembly process.