Electron transfer through photosystem II acceptors: Interaction with anions.

We present an overview of anionic interactions with the oxidation-reduction reactions of photosystem II (PSII) acceptors. In section 1, a framework is laid for the electron acceptor side of PSII: the overview begins with a current scheme of the electron transport pathway and of the localization of components in the thylakoid membrane, which is followed by a brief description of the electron acceptor Q or QA and the various heterogeneities associated with it. In section 2, we review briefly the nature of the active species of the bicarbonate (HCO3 (-)) effect, the location of the site of action of HCO3 (-), and its relationship to interactions with other anions. In section 3, we review data on the anion effects on the reoxidation of QA (-) and on the various reactions involved in the two-electron gate mechanism of PSII, and provide a hypothesis as to the action of HCO3 (-) on the protonation reactions. New data obtained by one of us (G) in collaboration with J.J.S. van Rensen, J.F.H Snel and W. Tonk for HCO3 (-)-depleted thylakoids, demonstrating the abolition of the binary oscillations contained within the periodicity of 4 observed for proton release, are also reviewed. In section 4, we comment on the measured binding constant of HCO3 (-) at the anion binding site. And, in section 5, we review our current concept of the mechanism of the HCO3 (-) effect on the electron acceptor side of PSII, and comment on the possible physiological roles for HCO3 (-). Measurements of HCO3 (-) reversible anionic inhibition in intact cells of a green alga Scenedesmus are also reviewed.