Concerted hydrogen-atom abstraction in photosynthetic water oxidation

Photosystem II evolves oxygen by using water in the unlikely role of a redu ctant. The absorption of sunlight by chlorophyll produces highly oxidizing equivalents that are filled with electrons stripped from water. This proton -coupled redox chemistry occurs at the oxygen-evolving complex, which conta ins a tetramanganese cluster, a redox-active tyrosine amino acid hydrogen-b onded to a histidine amino acid, a calcium ion and chloride. Hydrogen-atom abstraction by the tyrosyl radical from water bound to the manganese cluste r is now widely held to occur in this process, at least for some of the ste ps in the catalytic cycle. We discuss kinetic and energetic constraints on the hydrogen-atom abstraction process.