Bicarbonate-Mediated CO Formation on Both Sides of Photosystem II.

The effect of bicarbonate (HCO) on photosystem II (PSII) activity was discovered in the 1950s, but only recently have its molecular mechanisms begun to be clarified. Two chemical mechanisms have been proposed. One is for the electron-donor side, in which mobile HCO enhances and possibly regulates water oxidation by acting as proton acceptor, after which it dissociates into CO and HO. The other is for the electron-acceptor side, in which (i) reduction of the Q quinone leads to the release of HCO from its binding site on the non-heme iron and (ii) the potential of the Q/Q couple increases when HCO dissociates. This suggested a protective/regulatory role of HCO as it is known that increasing the of Q decreases the extent of back-reaction-associated photodamage. Here we demonstrate, using plant thylakoids, that time-resolved membrane-inlet mass spectrometry together with C isotope labeling of HCO allows donor- and acceptor-side formation of CO by PSII to be demonstrated and distinguished, which opens the door for future studies of the importance of both mechanisms under conditions.