Generation of Photoelectric Responses by Photosystem II Core Complexes in the Presence of Externally Added Cytochrome c.

The effect of exogenous cytochrome c (cyt c) on kinetics of photoelectric responses (Δψ) of two types of photosystem II (PSII) core complexes (intact - PSII with active water-oxidizing complex and Mn-depleted complex) reconstituted into liposomes has been investigated by direct electrometric technique. PSII complexes were localized in the proteoliposome membranes with their donor side outward. An additional electrogenic phase was observed in the kinetics of Δψ generation in response to a laser flash besides the main fast (<0.3 µs) electrogenic component due to electron transfer from the redox-active tyrosine Y to the primary quinone acceptor Q in the presence of oxidized cyt c (cyt c) entrapped in the internal space of proteoliposomes with intact PSII complexes. This component with characteristic time τ ≈ 40 µs and relative amplitude of ~10% of the total Δψ was attributed to the vectorial electron transfer from Q to cyt c serving as an external acceptor. An additional electrogenic component with τ ~ 70 µs and a relative amplitude of ~20% of the total Δψ also appeared in the kinetics of Δψ formation, when cyt c was added to the suspension of proteoliposomes containing Mn-depleted PSII core complexes. This component was attributed to the electrogenic transfer of an electron from cyt c to photooxidized tyrosine Y. These data imply that cyt c serves as a very effective exogenous electron acceptor for Q in the case of intact PSII core complexes, and cyt c is an extremely efficient artificial electron donor for Y in the Mn-depleted PSII. The obtained data on the roles of cyt c and cyt c as an electron donor and acceptor for PSII, respectively, can be used to develop hybrid photoelectrochemical solar energy-converting systems based on photosynthetic pigment-protein complexes.