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

Photosystem II (PSII), the water/plastoquinone photo-oxidoreductase, plays a key energy input role in the biosphere. [Formula: see text] , the reduced semiquinone form of the nonexchangeable quinone, is often considered capable of a side reaction with O(2), forming superoxide, but this reaction has not yet been demonstrated experimentally. Here, using chlorophyll fluorescence in plant PSII membranes, we show that O(2) does oxidize [Formula: see text] at physiological O(2) concentrations with a t(1/2) of 10 s. Superoxide is formed stoichiometrically, and the reaction kinetics are controlled by the accessibility of O(2) to a binding site near [Formula: see text] , with an apparent dissociation constant of 70 ± 20 µM. Unexpectedly, [Formula: see text] could only reduce O(2) when bicarbonate was absent from its binding site on the nonheme iron (Fe(2+)) and the addition of bicarbonate or formate blocked the O(2)-dependant decay of [Formula: see text]. These results, together with molecular dynamics simulations and hybrid quantum mechanics/molecular mechanics calculations, indicate that electron transfer from [Formula: see text] to O(2) occurs when the O(2) is bound to the empty bicarbonate site on Fe(2+). A protective role for bicarbonate in PSII was recently reported, involving long-lived [Formula: see text] triggering bicarbonate dissociation from Fe(2+) [Brinkert et al., Proc. Natl. Acad. Sci. U.S.A. 113, 12144–12149 (2016)]. The present findings extend this mechanism by showing that bicarbonate release allows O(2) to bind to Fe(2+) and to oxidize [Formula: see text]. This could be beneficial by oxidizing [Formula: see text] and by producing superoxide, a chemical signal for the overreduced state of the electron transfer chain.