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

The midpoint potential (Em ) 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 Em 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 QA -• is present. Light-induced QA -• formation triggered HCO3 - loss as manifest by the slowed electron transfer and the upshift in the Em of QA HCO3 - -depleted PSII also showed diminished light-induced 1 O2 formation. This finding is consistent with a model in which the increase in the Em 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 1 O2 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 QA and the binding by HCO3 - regulates and protects PSII. The potential for a sink (CO2 ) to source (PSII) feedback mechanism is discussed.