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.