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

AbstractAfter 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 (\({\rm{P}}_{680}^+\)) in the reaction center of photosystem 2 (PS2). Here, we investigated\({\rm{P}}_{680}^+\) 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\({\rm{P}}_{680}^+\) 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 con-trast 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\({\rm{P}}_{680}^+\) approx. by a factor of 5. The suggestion that direct electron transfer from YZ to\({\rm{P}}_{680}^+\) 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\({\rm{P}}_{680}^+\) is significantly accelerated in the presence of acetate or ammonia. These resul...
Source: Biochemistry (Moscow) - Category: Biochemistry Source Type: research