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Photosynthetic water oxidation is catalyzed by a manganese-calcium oxide cluster, which experiences five "S-states"during a light-driven reaction cycle. The unique "distorted chair"-like geometry of the Mn4CaO5(6) cluster shows structural flexibility that has been frequently proposed to involve "open"and "closed"-cubane forms from the S1 to S3 states. The isomers are interconvertible in the S1 and S2 states, while in the S3 state, the open-cubane structure is observed to dominate inThermosynechococcus elongatus (cyanobacteria) samples. In this work, using density functional theory calculations, we go beyond the S3+Yz state to the S3nYz→S4+Yz step, and report for the first time that the reversible isomerism, which is suppressed in the S3+Yz state, is fully recovered in the ensuing S3nstate due to the proton release from a manganese-bound water ligand. The altered coordination strength of the manganese-ligand facilitates formation of the closed-cubane form, in a dynamic equilibrium with the open-cubane form. This tautomerism immediately preceding dioxygen formation may constitute the rate limiting step for O2 formation, and exert a significant influence on the water oxidation mechanism in photosystem II.