The O2-evolving complex (OEC) of photosystem II (PSII) catalyzes the oxidation of water to dioxygen. In addition to a tetramanganese-oxo (Mn4) cluster, calcium and chloride ions, the OEC also contains Tyrosine Z (Yz), a redox intermediate in the water oxidation reaction. The redox mechanism employed by Yz is under much debate. Using a novel method to study Yz oxidation based on the kinetic competition with secondary donors, we examine the electron-donation pathways of manganese-depleted PSII over a range of temperature and pH. H/D substitution causes a shift in the onset temperature for Yz oxidation, enabling measurements of lyonium isotope effects. In deuterated samples, the onset temperature for Yz oxidation is upshifted, suggesting that proton movement is a required step. Proton inventory experiments were performed to determine the number of protons that shift during the Yz oxidation reaction. Our findings indicate the movement of a single proton during the rate-liraiting step of the oxidation process. The results presented herein demonstrate a need for proton movement in conjunction with Yz oxidation and support previous proposals that a proton-coupled electron, transfer (PCET) step is necessary for oxidation of Yz. The possible involvement of PCET in the energetics of specific steps in the mechanism of water oxidation is discussed.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry