Does the water-oxidizing Mn₄CaO₅ cluster regulate the redox potential of the primary quinone electron acceptor Q_A in photosystem II? A study by Fourier transform infrared spectroelectrochemistry

Redox titration using fluorescence measurements of photosystem II (PSII) has long shown that impairment of the water-oxidizing Mn₄CaO₅ cluster upshifts the redox potential (E_m) of the primary quinone electron acceptor Q_A by more than 100 mV, which has been proposed as a photoprotection mechanism of PSII. However, the molecular mechanism of this long-distance interaction between the Mn₄CaO₅ cluster and Q_A in PSII remains unresolved. In this study, we reinvestigated the effect of depletion of the Mn₄CaO₅ cluster on E_m(Q_A ⁻/Q_A) using Fourier transform infrared (FTIR) spectroelectrochemistry, which can directly monitor the redox state of Q_A at an intended potential. Light-induced FTIR difference measurements at a series of electrode potentials for intact and Mn-depleted PSII preparations from spinach and Thermosynechococcus elongatus showed that depletion of the Mn₄CaO₅ cluster hardly affected the E_m(Q_A ⁻/Q_A) values. In contrast, fluorescence spectroelectrochemical measurement using the same PSII sample, electrochemical cell, and redox mediators reproduced a large upshift of apparent E_m upon Mn depletion, whereas a smaller shift was observed when weaker visible light was used for fluorescence excitation. Thus, the possibility was suggested that the measuring light for fluorescence disturbed the titration curve in Mn-depleted PSII, in contrast to no interference of infrared light with the PSII reactions in FTIR measurements. From these results, it was concluded that the Mn₄CaO₅ cluster does not directly regulate E_m(Q_A ⁻/Q_A) to control the redox reactions on the electron acceptor side of PSII.