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Unequal misses during the flash-induced advancement of photosystem II: effects of the S state and acceptor side cycles

Pham, Long Vo, Janna Olmos, Julian David, Chernev, Petko, Kargul, Joanna, Messinger, Johannes
Photosynthesis research 2019 v.139 no.1-3 pp. 93-106
Rhodophyta, adverse effects, electron transfer, energy, models, oxidation, oxygen, oxygen evolving complex, oxygen production, pH, quinones, redox potential, spinach, thylakoids
Photosynthetic water oxidation is catalyzed by the oxygen-evolving complex (OEC) in photosystem II (PSII). This process is energetically driven by light-induced charge separation in the reaction center of PSII, which leads to a stepwise accumulation of oxidizing equivalents in the OEC (Sᵢ states, i = 0–4) resulting in O₂ evolution after each fourth flash, and to the reduction of plastoquinone to plastoquinol on the acceptor side of PSII. However, the Sᵢ-state advancement is not perfect, which according to the Kok model is described by miss-hits (misses). These may be caused by redox equilibria or kinetic limitations on the donor (OEC) or the acceptor side. In this study, we investigate the effects of individual S state transitions and of the quinone acceptor side on the miss parameter by analyzing the flash-induced oxygen evolution patterns and the S₂, S₃ and S₀ state lifetimes in thylakoid samples of the extremophilic red alga Cyanidioschyzon merolae. The data are analyzed employing a global fit analysis and the results are compared to the data obtained previously for spinach thylakoids. These two organisms were selected, because the redox potential of QA/QA⁻ in PSII is significantly less negative in C. merolae (Eₘ = − 104 mV) than in spinach (Eₘ = − 163 mV). This significant difference in redox potential was expected to allow the disentanglement of acceptor and donor side effects on the miss parameter. Our data indicate that, at slightly acidic and neutral pH values, the Eₘ of QA⁻/QA plays only a minor role for the miss parameter. By contrast, the increased energy gap for the backward electron transfer from QA⁻ to Pheo slows down the charge recombination reaction with the S₃ and S₂ states considerably. In addition, our data support the concept that the S₂ → S₃ transition is the least efficient step during the oxidation of water to molecular oxygen in the Kok cycle of PSII.