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From manganese oxidation to water oxidation: assembly and evolution of the water-splitting complex in photosystem IIFree, publicly-accessible full text available May 1, 2023
Photosynthetic O 2 evolution is catalyzed by the Mn 4 CaO 5 cluster of the water oxidation complex of the photosystem II (PSII) complex. The photooxidative self-assembly of the Mn 4 CaO 5 cluster, termed photoactivation, utilizes the same highly oxidizing species that drive the water oxidation in order to drive the incorporation of Mn 2+ into the high-valence Mn 4 CaO 5 cluster. This multistep process proceeds with low quantum efficiency, involves a molecular rearrangement between light-activated steps, and is prone to photoinactivation and misassembly. A sensitive polarographic technique was used to track the assembly process under flash illumination as a function of the constituent Mn 2+ and Ca 2+ ions in genetically engineered membranes of the cyanobacterium Synechocystis sp. PCC6803 to elucidate the action of Ca 2+ and peripheral proteins. We show that the protein scaffolding organizing this process is allosterically modulated by the assembly protein Psb27, which together with Ca 2+ stabilizes the intermediates of photoactivation, a feature especially evident at long intervals between photoactivating flashes. The results indicate three critical metal-binding sites: two Mn and one Ca, with occupation of the Ca site by Ca 2+ critical for the suppression of photoinactivation. The long-observed competition betweenmore »