Bicarbonate-induced redox tuning in Photosystem II for regulation and protection

The midpoint potential (E_m) of [Formula: see text], the one-electron acceptor quinone of Photosystem II (PSII), provides the thermodynamic reference for calibrating PSII bioenergetics. Uncertainty exists in the literature, with two values differing by ∼80 mV. Here, we have resolved this discrepancy by using spectroelectrochemistry on plant PSII-enriched membranes. Removal of bicarbonate (HCO₃^-) shifts the E_m from ∼-145 mV to -70 mV. The higher values reported earlier are attributed to the loss of HCO₃^- during the titrations (pH 6.5, stirred under argon gassing). These findings mean that HCO₃^- binds less strongly when Q_A^-• is present. Light-induced Q_A^-• formation triggered HCO₃^- loss as manifest by the slowed electron transfer and the upshift in the E_m of Q_A HCO₃^--depleted PSII also showed diminished light-induced ¹O₂ formation. This finding is consistent with a model in which the increase in the E_m of [Formula: see text] promotes safe, direct [Formula: see text] charge recombination at the expense of the damaging back-reaction route that involves chlorophyll triplet-mediated ¹O₂ formation [Johnson GN, et al. (1995) Biochim Biophys Acta 1229:202-207]. These findings provide a redox tuning mechanism, in which the interdependence of the redox state of Q_A and the binding by HCO₃^- regulates and protects PSII. The potential for a sink (CO₂) to source (PSII) feedback mechanism is discussed.