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

The primary quinone electron acceptor Q(A) is a key component in the electron transfer regulation in photosystem II (PSII), and hence accurate estimation of its redox potential, E-m(Q(A)(-)/Q(A)), is crucial in understanding the regulatory mechanism. Although fluorescence detection has been extensively used for monitoring the redox state of Q(A), it was recently suggested that this method tends to provide a higher E-m(Q(A)(-)/Q(A)) 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 Q(A), to investigate the effects of stromal- and lumenal-side perturbations on E-m(Q(A)(-)/Q(A)) in PSII. It was shown that replacement of bicarbonate bound to the non-heme iron with formate upshifted E-m(Q(A)(-)/Q(A)) by similar to 55 mV, consistent with the previous fluorescence measurement. In contrast, an E-m(Q(A)(-)/Q(A)) difference between binding of 3-(3,4-dichlorophenyl)-1,1-dimethylurea and bromoxynil was found to be similar to 30 mV, which is much smaller than the previous estimate, similar to 100 mV, by the fluorescence method. This similar to 30 mV difference was verified by the decay kinetics of the S,Q A + recombination. On the lumenal side, Mn depletion hardly affected the E-m(Q(A)(-)/Q(A)), confirming the previous FTIR result. However, removal of the extrinsic proteins by NaCl or CaCl2 wash downshifted the E-m(Q(A)(-)/Q(A)) by 14-20 mV. These results suggest that electron flow through Q(A) is regulated by changes both on the stromal and lumenal sides of PSII.

Automated summary

The research applied Fourier transform infrared (FTIR) spectroelectrochemistry to investigate the effects of stromal- and lumenal-side perturbations on the estimation of the redox potential of the primary quinone electron acceptor Q(A) in photosystem II (PSII). The study showed that different perturbations had varying effects on the redox potential, highlighting the importance of considering both sides of PSII in regulating electron flow through Q(A). The results suggest that fluorescence detection may overestimate the redox potential of Q(A) depending on the sample status, and FTIR spectroelectrochemistry provides a more accurate estimation in certain cases.