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Energetics of the exchangeable quinone, Q B , in Photosystem II

Abstract : Photosystem II (PSII), the light-driven water/plastoquinone photo-oxidoreductase, is of central importance in the planetary energy cycle. The product of the reaction, plastohydroquinone (PQH2), is released into the membrane from the QB-site, where it is formed. A plastoquinone (PQ) from the membrane pool then binds into the QB-site. The thermodynamic properties of the PQ in the QB-site, QB, in its different redox forms have received relatively little attention despite their functional importance. Here we report the midpoint potentials (Em) of QB in PSII from Thermosynechococcus elongatus using EPR spectroscopy: Em(QB/QB •−) ≈ 90 mV and Em(QB •− /QBH2) ≈ 40 mV. These data allow the following conclusions: 1) the semiquinone, QB •− , is thermodynamically stable under physiological conditions; 2) release of QBH2 (PQH2) into the pool has a driving force of ~50 meV; 3) PQ is more tighly bound than PQH2; 4) the difference between the Em values for QB/QB •− and QA/QA •− is ~234 meV and represents the driving force for electron transfer from QA •− to QB. We also used the pH-dependence of the thermoluminescence associated with QB •− to provide a functional estimate for this energy gap and obtained a similar value (~230 meV). This estimate is larger than the generally accepted value (~80 meV). The energetics of QB are compared to those in homologous purple bacterial reaction centers. A recent contradictory report on the redox properties of QB (Kato Y, Nagao R, Noguchi T 2016 Proc Natl Acad Sci 113(3):620-625) is rationalised in terms of specific technical difficulties associated with titrating an exchangeable cofactor.
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Sven de Causmaecker, Jeffrey Douglass, Andrea Fantuzzi, Wolfgang Nitschke, A. William Rutherford. Energetics of the exchangeable quinone, Q B , in Photosystem II. Proceedings of the National Academy of Sciences of the United States of America , National Academy of Sciences, 2019, 116 (39), pp.19458-19463. ⟨10.1073/pnas.1910675116⟩. ⟨hal-02407784⟩

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