PH-Dependent Regulation of the Relaxation Rate of the Radical Anion of the Secondary Quinone Electron Acceptor QB in Photosystem II As Revealed by Fourier Transform Infrared Spectroscopy.

Abstract

Photosystem II (PSII) is a protein complex that performs water oxidation using light energy during photosynthesis. In PSII, electrons abstracted from water are eventually transferred to the secondary quinone electron acceptor, QB, and upon double reduction, QB is converted to quinol by binding two protons. Thus, excess electron transfer in PSII increases the pH of the stroma. In this study, to investigate the pH-dependent regulation of the electron flow in PSII, we have estimated the relaxation rate of the QB- radical anion in the pH region between 5 and 8 by direct monitoring of its population using light-induced Fourier transform infrared difference spectroscopy. The decay of QB- by charge recombination with the S2 state of the water oxidation center in PSII membranes was shown to be accelerated at higher pH, whereas that of QA- examined in the presence of a herbicide was virtually unaffected at pH ≤7.5 and slightly slowed at pH 8. These observations were consistent with the previous studies that included rather indirect monitoring of the QB- and QA- decays using fluorescence detection. The accelerated relaxation of QB- was explained by the shift of a redox equilibrium between QA- and QB- to the QA- side due to the decrease in the redox potential of QB at higher pH, which is induced by deprotonation of a single amino acid residue near QB. It is proposed that this pH-dependent QB- relaxation is one of the mechanisms of electron flow regulation in PSII for its photoprotection.