Bicarbonate-controlled reduction of oxygen by the Q A semiquinone in Photosystem II in membranes

Abstract

Significance In Photosystem II (PSII), O 2 reduction by Q A • − is often discussed but has not been demonstrated. Here, we show in PSII membranes that Q A • − can reduce O 2 to superoxide, but only when bicarbonate is absent from its binding site on the nonheme Fe 2+ . Bicarbonate’s role in PSII was recently shown to involve a regulatory/protective redox-tuning mechanism linking PSII function to CO 2 concentration. A key aspect is the presence of stable Q A • − causing release of bicarbonate from its site on Fe 2+ . Here, we show that under these conditions, O 2 binds to the empty site on the Fe 2+ and is reduced by Q A • − . This unexpected reaction may be a further indication of cross-talk between the regulation of PSII and CO 2 fixation.