2Semenov Institute of Chemical Physics, Russian Academy of Sciences, 119334 Moscow, Russia
3Moscow Institute of Physics and Technology, 141701 Dolgoprudny, Moscow Region, Russia
*To whom correspondence should be addressed.
Received April 10, 2019; Revised June 5, 2019; Accepted June 5, 2019
After removal of manganese ions responsible for light-driven water oxidation, redox-active tyrosine YZ (tyrosine 161 of the D1 subunit) still remains the dominant electron donor to the photooxidized chlorophyll P680 (P+680) in the reaction center of photosystem 2 (PS2). Here, we investigated P+680 reduction by YZ under single-turnover flashes in Mn-depleted PS2 core complexes in the presence of weak acids and NH4Cl. Analysis of changes in the light-induced absorption at 830 nm (reflecting P680 redox transitions) at pH 6.0 showed that P+680 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 NH4Cl increased the rate of electron transfer between YZ and P+680 approx. by a factor of 5. The suggestion that direct electron transfer from YZ to P+680 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 YZ and P+680 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 YZ and exogenous substances (including synthetic manganese-containing compounds) capable of photooxidation of water molecule in the manganese-depleted PS2 complexes.
KEY WORDS: photosystem 2, reaction center, apo-PS2, absorption changes, photoelectric response, acetate, ammonia