Electron Transfer on the Donor Side of Manganese-Depleted Photosystem 2.

After removal of manganese ions responsible for light-driven water oxidation, redox-active tyrosine Y (tyrosine 161 of the D1 subunit) still remains the dominant electron donor to the photooxidized chlorophyll P (P) in the reaction center of photosystem 2 (PS2). Here, we investigated P reduction by Y under single-turnover flashes in Mn-depleted PS2 core complexes in the presence of weak acids and NHCl. Analysis of changes in the light-induced absorption at 830 nm (reflecting P redox transitions) at pH 6.0 showed that P reduction is well approximated by two kinetic components with the characteristic times (τ) of ~7 and ~31 μs and relative contributions of ~54 and ~37%, respectively. In contrast to the very small effect of sodium formate (200 mM), addition of sodium acetate and NHCl increased the rate of electron transfer between Y and P approx. by a factor of 5. The suggestion that direct electron transfer from Y to P has a biphasic kinetics and reflects the presence of two different populations of PS2 centers was confirmed by the data obtained using direct electrometrical technique. It was demonstrated that the submillisecond two-phase kinetics of the additional electrogenic phase in the kinetics of photoelectric response due to the electron transfer between Y and P is significantly accelerated in the presence of acetate or ammonia. These results contribute to the understanding of the mechanism of interaction between the oxidized tyrosine Y and exogenous substances (including synthetic manganese-containing compounds) capable of photooxidation of water molecule in the manganese-depleted PS2 complexes.