Electrogenicity accompanies photoreduction of the iron-sulfur clusters FA and FB in photosystem I

AbstractPhotovoltage responses accompanying electron transfer on the acceptor side of photosystem I (PS I) were investigated in proteoliposomes containing PS I complexes from the cyanobacterium Synechococcus sp. PCC 6301 using a direct electrometrical technique. The relative contributions of the FX→FB and the FX→FA electron transfer reactions to the overall electrogenicity were elucidated by comparing the sodium dithionite-induced decrease in the magnitude of the total photoelectric responses in control and in FB-less (HgCl2-treated) PS I complexes. The results obtained suggest that the electrogenesis on the acceptor side of PS I is related to electron transfers between both FX and FA and FA and FB. Based on the electrogenic nature of the latter reaction in PS I complexes, we conclude that FA rather than FB is the acceptor proximal to FX

Electrogenic reduction of the primary electron donor P700+ in photosystem I by redox dyes

AbstractThe kinetics of reduction of the photo-oxidized primary electron donor P700+ by redox dyes N,N,N′,N′-tetramethyl-p-phenylendiamine, 2,6-dichlorophenol-indophenol and phenazine methosulfate was studied in proteoliposomes containing Photosystem I complexes from cyanobacteria Synechocystis sp. PCC 6803 using direct electrometrical technique. In the presence of high concentrations of redox dyes, the fast generation of a membrane potential related to electron transfer between P700 and the terminal iron-sulfur clusters FA/FB was followed by a new electrogenic phase in the millisecond time domain, which contributes approximately 20% to the overall photoelectric response. This phase is ascribed to the vectorial transfer of an electron from the redox dye to the protein-embedded chlorophyll of P700+. Since the contribution of this electrogenic phase in the presence of artificial redox dyes is approximately equal to that of the phase observed earlier in the presence of cytochrome c6, it is likely that electrogenic reduction of P700+ in vivo occurs due to vectorial electron transfer within RC molecule rather than within the cytochrome c6-P700 complex