Accessibility of tyrosine Y(.)(Z) to exogenous reductants and Mn(2+) in various Photosystem II preparations

Transmembrane electric potential difference in the protein-pigment complex of photosystem 2

The protein-pigment complex of photosystem 2 (PS2) localized in the thylakoid membranes of higher plants, algae, and cyanobacteria is the main source of oxygen on Earth. The light-induced functioning of PS2 is directly linked to electron and proton transfer across the membrane, which results in the formation of transmembrane electric potential difference (ΔΨ). The major contribution to ΔΨ of the PS2 reaction center is due to charge separation between the primary chlorophyll donor P(680) and the quinone acceptor Q(A), accompanied by re-reduction of P(680)(+) by the redox-active tyrosine residue Y(Z). The processes associated with the uptake and release of protons on the acceptor and donor sides of the enzyme, respectively, are also coupled with ΔΨ generation. The objective of this work was to describe the mechanisms of ΔΨ generation associated with the S-state transitions of the water-oxidizing complex in intact PS2 complex and in PS2 preparation depleted of Mn(4)Ca cluster in the presence of artificial electron donors. The findings elucidate the mechanisms of electrogenic reactions on the PS2 donor side and may be a basis for development of an effective solar energy conversion system.

Electron transfer between exogenous electron donors and reaction center of photosystem 2

Transfer of electrons between artificial electron donors diphenylcarbazide (DPC) and hydroxylamine (NH2OH) and reaction center of manganese-depleted photosystem 2 (PS2) complexes was studied using the direct electrometrical method. For the first time it was shown that reduction of redox-active amino acid tyrosine Y(Z)(.) by DPC is coupled with generation of transmembrane electric potential difference (DeltaPsi). The amplitude of this phase comprised ~17% of that of the DeltaPsi phase due to electron transfer between Y(Z) and the primary quinone acceptor Q(A). This phase is associated with vectorial intraprotein electron transfer between the DPC binding site on the protein-water interface and the tyrosine Y(Z)(.). The slowing of DeltaPsi decay in the presence of NH2OH indicates effective electron transfer between the artificial electron donor and reaction center of PS2. It is suggested that NH2OH is able to diffuse through channels with diameter of 2.0-3.0 A visible in PS2 structure and leading from the protein-water interface to the Mn(4)Ca cluster binding site with the concomitant electron donation to Y(Z)(.). Because the dielectrically-weighted distance between the NH2OH binding site and Y(Z)(.) is not determined, the transfer of electrons from NH2OH to Y(Z)(.) could be either electrically silent or contribute negligibly to the observed electrogenicity in comparison with hydrophobic donors.