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

The midpoint potential (E-m) of Q(A)/Q(A)(-center dot), 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 similar to 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 similar to-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)(-center dot) is present. Light-induced Q(A)(-center dot) 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 O-1(2) formation. This finding is consistent with a model in which the increase in the E-m of Q(A)/Q(A)(-center dot) promotes safe, direct P(+center dot)Q(A)(-center dot) charge recombination at the expense of the damaging back-reaction route that involves chlorophyll triplet-mediated O-1(2) 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.