Effects of Stromal and Lumenal Side Perturbations on the Redox Potential of the Primary Quinone Electron Acceptor QA in Photosystem II

• Published in: Biochemistry (Easton) Vol. 60; no. 48; pp. 3697 - 3706
• Main Authors: Kato, Yuki, Noguchi, Takumi
• Format: Journal Article
• Language: English
• Published: American Chemical Society 07.12.2021
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/acs.biochem.1c00624

The primary quinone electron acceptor QA is a key component in the electron transfer regulation in photosystem II (PSII), and hence accurate estimation of its redox potential, E m(QA –/QA), is crucial in understanding the regulatory mechanism. Although fluorescence detection has been extensively used for monitoring the redox state of QA, it was recently suggested that this method tends to provide a higher E m(QA –/QA) estimate depending on the sample status due to the effect of measuring light [Kato et al. (2019) Biochim. Biophys. Acta 1860, 148082]. In this study, we applied the Fourier transform infrared (FTIR) spectroelectrochemistry, which uses non-reactive infrared light to monitor the redox state of QA, to investigate the effects of stromal- and lumenal-side perturbations on E m(QA –/QA) in PSII. It was shown that replacement of bicarbonate bound to the non-heme iron with formate upshifted E m(QA –/QA) by ∼55 mV, consistent with the previous fluorescence measurement. In contrast, an E m(QA –/QA) difference between binding of 3-(3,4-dichlorophenyl)-1,1-dimethylurea and bromoxynil was found to be ∼30 mV, which is much smaller than the previous estimate, ∼100 mV, by the fluorescence method. This ∼30 mV difference was verified by the decay kinetics of the S2QA – recombination. On the lumenal side, Mn depletion hardly affected the E m(QA –/QA), confirming the previous FTIR result. However, removal of the extrinsic proteins by NaCl or CaCl2 wash downshifted the E m(QA –/QA) by 14–20 mV. These results suggest that electron flow through QA is regulated by changes both on the stromal and lumenal sides of PSII.