Cation Effects on the Electron-Acceptor Side of Photosystem II

The normal pathway of electron transfer on the electron-acceptor side of photosystem II (PSII) involves electron transfer from quinone A, Q_A, to quinone B, Q_B. It is possible to redirect electrons from Q_A^- to water-soluble Co^III complexes, which opens a new avenue for harvesting electrons from water oxidation by immobilization of PSII on electrode surfaces. Herein, the kinetics of electron transfer from Q_A^- to [Co(III)(terpy)₂]³⁺ (terpy = 2,2′;6′,2″-terpyridine) are investigated with a spectrophotometric assay revealing that the reaction follows Michaelis-Menten saturation kinetics, is inhibited by cations, and is not affected by variation of the Q_A reduction potential. A negatively charged site on the stromal surface of the PSII protein complex, composed of glutamic acid residues near Q_A, is hypothesized to bind cations, especially divalent cations. The cations are proposed to tune the redox properties of Q_A through electrostatic interactions. These observations may thus explain the molecular basis of the effect of divalent cations like Ca²⁺, Sr²⁺, Mg²⁺, and Zn²⁺ on the redox properties of the quinones in PSII, which has previously been attributed to long-range conformational changes propagated from divalent cations binding to the Ca(II)-binding site in the oxygen-evolving complex on the lumenal side of the PSII complex.