Transmembrane charge transfer in photosynthetic reaction centers: some similarities and distinctions.

This mini review presents a general comparison of structural and functional peculiarities of three types of photosynthetic reaction centers (RCs)--photosystem (PS) II, RC from purple bacteria (bRC) and PS I. The nature and mechanisms of the primary electron transfer reactions, as well as specific features of the charge transfer reactions at the donor and acceptor sides of RCs are considered. Comparison of photosynthetic RCs shows general similarity between the core central parts of all three types, between the acceptor sides of bRC and PS II, and between the donor sides of bRC and PS I. In the latter case, the similarity covers thermodynamic, kinetic and dielectric properties, which determine the resemblance of mechanisms of electrogenic reduction of the photooxidized primary donors. Significant distinctions between the donor and acceptor sides of PS I and PS II are also discussed. The results recently obtained in our laboratory indicate in favor of the following sequence of the primary and secondary electron transfer reactions: in PS II (bRC): Р(680)(Р(870)) → Chl(D1)(В(А)) → Phe(bPhe) → Q(A); and in PS I: Р(700) → А(0А)/A(0B) → Q(A)/Q(B).

Photosysem II: where does the light-induced voltage come from?

Photosystem II (PS II) is a biological energy transducer. The enzyme catalyses the light-driven oxidation of water and reduction of plastoquinone. The aim of this work was to review the mechanisms of electrical events in PS II. The major contribution to the total photoelectric response is due to the charge-separation between the primary chlorophyll donor P680 and quinone acceptor QA accompanied by re-reduction of P680+ by tyrosine residue YZ. The remaining part of the membrane potential is believed to be associated mainly with electron and proton transfer events due to the S-state transitions of the oxygen-evolving complex and proton uptake associated with protonation of the doubly reduced secondary quinone acceptor QB. Under certain non-physiological conditions, some other electrogenic reactions are observed, namely: proton-coupled electron transfer between QA and non-heme Fe3+ and electron transfer from the protein-water interface to the YZ radical in the presence of artificial electron donors. These data may provide a good platform for further development of artificial photosynthetic constructs and bio-inspired catalysts.

Electrogenic reactions on the donor side of Mn-depleted photosystem II core particles in the presence of MnCl2 and synthetic trinuclear Mn-complexes.

An electrometric technique was used to investigate the generation of a photovoltage (Deltapsi) by Mn-depleted spinach photosystem II (PS II) core particles incorporated into liposomes. In the presence of MnCl2, the fast kinetically unresolvable phase of Deltapsi generation, related to electron transfer between the redox-active tyrosine YZ and the primary plastoquinone acceptor QA was followed by an additional electrogenic phase (tau approximately 20 micros, approximately 5% of the phase attributed to YZoxQA-). The latter phase was ascribed to the transfer of an electron from the Mn, bound to the Mn-binding site of the PS II reaction center to the YZox. An additional electrogenicity observed upon addition of synthetic trinuclear Mn complex-1 has a tau approximately 50 micros (approximately 4% of the YZoxQA) and tau approximately 160 ms (approximately 25%). The fast electrogenic component could be ascribed to reduction of YZox by Mn, delivered to the Mn-binding site in Mn-depleted samples after the release of the tripod ligands from the complex-1 while the slow electrogenic phase to the electron transfer from the Mn-containing complex-1 attached to the protein-water boundary to the oxidized Mn at the protein-embedded Mn-binding site.