Proton-mediated photoprotection mechanism in photosystem II
Yu Sugo, Hiroshi Ishikita
Abstract
Photo-induced charge separation, which is terminated by electron transfer from the primary quinone Q A to the secondary quinone Q B , provides the driving force for O 2 evolution in photosystem II (PSII). However, the backward charge recombination using the same electron-transfer pathway leads to the triplet chlorophyll formation, generating harmful singlet-oxygen species. Here, we investigated the molecular mechanism of proton-mediated Q A ⋅– stabilization. Quantum mechanical/molecular mechanical (QM/MM) calculations show that in response to the loss of the bicarbonate ligand, a low-barrier H-bond forms between D2-His214 and Q A ⋅– . The migration of the proton from D2-His214 toward Q A ⋅– stabilizes Q A ⋅– . The release of the bicarbonate ligand from the binding Fe 2+ site is an energetically uphill process, whereas the bidentate-to-monodentate reorientation is almost isoenergetic. These suggest that the bicarbonate protonation and decomposition may be a basis of the mechanism of photoprotection via Q A ⋅– /Q A H ⋅ stabilization, increasing the Q A redox potential and activating a charge-recombination pathway that does not generate the harmful singlet oxygen.