Photosysem II: Where does the light-induced voltage come from?

Photosystem II (PS II) is a biological energy transducer. The enzyme catalyses the light-driven oxidation of water and reduction of plastoquinone. The aim of this work was to review the mechanisms of electrical events in PS II. The major contribution to the total photoelectric response is due to the charge-separation between the primary chlorophyll donor P₆₈₀ and quinone acceptor Q_A accompanied by re-reduction of P₆₈₀ ⁺ by tyrosine residue Y_Z. The remaining part of the membrane potential is believed to be associated mainly with electron and proton transfer events due to the S-state transitions of the oxygenevolving complex and proton uptake associated with protonation of the doubly reduced secondary quinone acceptor QB. Under certain non-physiological conditions, some other electrogenic reactions are observed, namely: proton-coupled electron transfer between Q_A and non- heme Fe³⁺ and electron transfer from the protein-water interface to the Y_Z radical in the presence of artificial electron donors. These data may provide a good platform for further development of artificial photosynthetic constructs and bio-inspired catalysts.