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Scientific result | Photosynthesis

Bicarbonate, a new redox regulator for Photosystem II

​This work, stemming from a collaboration between researchers from Imperial College London and an SB2SM Research Team, resolved a long-standing ambiguity about the redox potential (Em) value of the primary quinone, QA, of the Photosystem II (PSII). In addition, the authors discovered a new and very important phenomenon for the PSII: a reversible control effect of the redox potential of QA induced by binding of bicarbonate.

Published on 31 January 2017


The midpoint potential (Em) of QA/Q-AQA/QA-, 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 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 1O2 formation. This finding is consistent with a model in which the increase in the Em of QA/QAQA/QA−• promotes safe, direct P+QAP++•QA−• 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 QA and the binding by HCO3regulates and protects PSII. The potential for a sink (CO2) to source (PSII) feedback mechanism is discussed.

Read the French version.

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