Electron transfer from Cyt b 559 and tyrosine-D to the S2 and S3 states of the water oxidizing complex in photosystem II at cryogenic temperatures

The Mn₄CaO₅ cluster of photosystem II (PSII) catalyzes the oxidation of water to molecular oxygen through the light-driven redox S-cycle. The water oxidizing complex (WOC) forms a triad with Tyrosine_Z and P₆₈₀, which mediates electrons from water towards the acceptor side of PSII. Under certain conditions two other redox-active components, Tyrosine_D (Y_D) and Cytochrome b ₅₅₉ (Cyt b ₅₅₉) can also interact with the S-states. In the present work we investigate the electron transfer from Cyt b ₅₅₉ and Y_D to the S₂ and S₃ states at 195 K. First, Y_D ^? and Cyt b ₅₅₉ were chemically reduced. The S₂ and S₃ states were then achieved by application of one or two laser flashes, respectively, on samples stabilized in the S₁ state. EPR signals of the WOC (the S₂-state multiline signal, ML-S₂), Y_D ^? and oxidized Cyt b ₅₅₉ were simultaneously detected during a prolonged dark incubation at 195 K. During 163 days of incubation a large fraction of the S₂ population decayed to S₁ in the S₂ samples by following a single exponential decay. Differently, S₃ samples showed an initial increase in the ML-S₂ intensity (due to S₃ to S₂ conversion) and a subsequent slow decay due to S₂ to S₁ conversion. In both cases, only a minor oxidation of Y_D was observed. In contrast, the signal intensity of the oxidized Cyt b ₅₅₉ showed a two-fold increase in both the S₂ and S₃ samples. The electron donation from Cyt b ₅₅₉ was much more efficient to the S₂ state than to the S₃ state.