Mechanism of Initial Mn 2+ Oxidation during Photoassembly of the Water-Oxidizing Mn 4 CaO 5 Cluster in Photosystem II
Year: 2026
Type: article
Abstract: The Mn 4 CaO 5 cluster, the catalytic center of water oxidation in photosystem II (PSII), is assembled from free Mn 2+ and Ca 2+ 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 Mn 2+ 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 Mn 2+ to Y Z •, rather than proton release, is the rate-limiting step. DFT calculations of the Mn 3+ /Mn 2+ redox potential, together with Δ G° estimates derived from electron transfer rates using semiempirical equations, reveal that Mn 2+ oxidation proceeds via a slow, endergonic electron transfer from Mn 2+ to Y Z •, 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 Δ G° resulting from detachment of CP43-R357 in the “open” CP43 conformation, suggesting that thermal fluctuations of the CP43 lumenal domain facilitate initial Mn 2+ oxidation during photoactivation.
Authors: Shunya Watanabe, Takumi Noguchi
Institution: Nagoya University
Language: English
Subfield: Molecular Biology
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W7140174420