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

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L. A. Vitukhnovskaya^1,2, E. V. Fedorenko³, and M. D. Mamedov^1,a*

¹Belozersky Institute of Physico-Chemical Biology, Lomonosov Moscow State University, 119992 Moscow, Russia

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

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After removal of manganese ions responsible for light-driven water oxidation, redox-active tyrosine Y_Z (tyrosine 161 of the D1 subunit) still remains the dominant electron donor to the photooxidized chlorophyll P₆₈₀ (P₆₈₀⁺) in the reaction center of photosystem 2 (PS2). Here, we investigated P₆₈₀⁺ reduction by Y_Z 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 P₆₈₀⁺ 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 Y_Z and P₆₈₀⁺ approx. by a factor of 5. The suggestion that direct electron transfer from Y_Z to P₆₈₀⁺ 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_Z and P₆₈₀⁺ 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_Z and exogenous substances (including synthetic manganese-containing compounds) capable of photooxidation of water molecule in the manganese-depleted PS2 complexes.

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KEY WORDS: photosystem 2, reaction center, apo-PS2, absorption changes, photoelectric response, acetate, ammonia

DOI: 10.1134/S0006297919090086

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