Manganese-depleted/reconstituted photosystem II core complexes in solution and liposomes

The Main Structural and Functional Characteristics of Photosystem-II-Enriched Membranes Isolated from Wild Type and cia3 Mutant Chlamydomonas reinhardtii

Photosystem II (PSII)-enriched membranes retain the original PSII architecture in contrast to PSII cores or PSII supercomplexes, which are usually isolated from Chlamydomonas reinhardtii. Here, we present data that fully characterize the structural and functional properties of PSII complexes in isolated PSII-enriched membranes from C. reinhardtii. The preparations were isolated from wild-type (WT) and CAH3-deficient mutant cia3 as the influence of CAH3 on the PSII function was previously proposed. Based on the equal chlorophyll content, the PSII-enriched membranes from WT and cia3 have the same amount of reaction centers (RCs), cytochrome b559, subunits of the water-oxidizing complex, Mn ions, and carotenes. They differ in the ratio of other carotenoids, the parts of low/intermediate redox forms of cytochrome b559, and the composition of outer light-harvesting complexes. The preparations had 40% more chlorophyll molecules per RC compared to higher plants. Functionally, PSII-enriched membranes from WT and cia3 show the same photosynthetic activity at optimal pH 6.5. However, the preparations from cia3 contained more closed RCs even at pH 6.5 and showed more pronounced suppression of PSII photosynthetic activity at shift pH up to 7.0, established in the lumen of dark-adapted cells. Nevertheless, the PSII photosynthetic capacities remained the same.

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_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.

Vectorial charge transfer reactions on the donor side of manganese-depleted and reconstituted photosystem 2 core complexes

The light-induced functioning of photosystem 2 (PS 2) is directly linked to the translocation of both electrons and protons across the membrane, which results in the formation of transmembrane electric potential difference (ΔΨ). Generation of ΔΨ due to S-state transitions of the water oxidation complex was demonstrated for the first time in Mn-depleted and reconstituted PS 2 core complexes incorporated into liposomes. The kinetics and relative amplitudes of the electrogenic reactions in dark-adapted samples during S1→S2, S2→S3, and S4→S0 transitions in response to the first, second and third laser flashes were comparable to those obtained in the intact PS 2 core particles. These results expand current understanding of the nature and mechanisms of electrogenic (vectorial) reactions due to a charge transfer on the donor side of PS 2.