The Mn4CaO5 cluster, the catalytic center of water oxidation in photosystem II (PSII), is assembled from free Mn2+ and Ca2+ ions through a light-driven process known as photoactivation. Despite its importance, the molecular mechanism of photoactivation remains poorly understood. Here, we investigate the mechanism underlying the initial oxidation of Mn2+ at its binding site in apo-PSII using time-resolved infrared spectroscopy combined with density functional theory (DFT) calculations. Two distinct kinetic phases with time constants of 100-150 μs and 1.5-2.5 ms are observed at pH 6.5-7.5. The minimal H/D isotope effect indicates that electron transfer from Mn2+ to YZ•, rather than proton release, is the rate-limiting step. DFT calculations of the Mn3+/Mn2+ redox potential, together with ΔG° estimates derived from electron transfer rates using semiempirical equations, reveal that Mn2+ oxidation proceeds via a slow, endergonic electron transfer from Mn2+ to YZ•, followed by rapid proton transfer from a water ligand to D1-H332, which provides the driving force for the reaction. The faster kinetic component is attributed to a decreased ΔG° resulting from detachment of CP43-R357 in the "open" CP43 conformation, suggesting that thermal fluctuations of the CP43 lumenal domain facilitate initial Mn2+ oxidation during photoactivation.
Watanabe et al. (Mon,) studied this question.