Voltage changes involving photosystem II quinone-iron complex turnover

An electrometrical technique was used to investigate proton-coupled electron transfer between the primary plastoquinone acceptor Q(A)(-) and the oxidized non-heme iron Fe3+ on the acceptor side of photosystem II core particles incorporated into phospholipid vesicles. The sign of the transmembrane electric potential difference Delta psi (negative charging of the proteoliposome interior) indicates that the iron-quinone complex faces the interior surface of the proteoliposome membrane. Preoxidation of the non-heme iron was achieved by addition of potassium ferricyanide entrapped into proteoliposomes. Besides the fast unresolvable kinetic phase (tau similar to 0.1 mu s) of Delta psi generation related to electron transfer between the redox-active tyrosine Y-Z and Q(A), an additional phase in the submillisecond time domain (tau similar to 0.1 ms at 23 degrees C, pH 7.0) and relative amplitude similar to 20% of the amplitude of the fast phase was observed under exposure to the first flash. This phase was absent under the second laser flash, as well as upon the first flash in the presence of DCMU, an inhibitor of electron transfer between Q(A) and the secondary quinone Q(B). The rate of the additional electrogenic phase is decreased by about one-half in the presence of D2O and is reduced with the temperature decrease. On the basis of the above observations we suggest that the submillisecond electrogenic reaction induced by the first flash is due to the vectorial transfer of a proton from external aqueous phase to an amino acid residue(s) in the vicinity of the non-heme iron. The possible role of the non-heme iron in cyclic electron transfer in photosystem II complex is discussed.