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<p>In photosystem II (PSII) chlorophyll (Chl) fluorescence yield (<i>F</i>) rises during low-to-high light induction. Based on a critical review of the literature, the following evidence is summarized. (1) As the primary acceptor quinone Q<sub>A</sub> gets reduced, fluorescence immediately rises to <i>F</i><sub>c</sub> =1.8 <i>F</i><sub>o</sub>. (2) During microseconds of illumination following Q<sub>A</sub> reduction, <i>F</i><sub>c</sub> rises to <i>F</i><sub>f</sub> where excitation is terminated via carotenoid triplet states (<sup>3</sup>Car). (3) After many milliseconds of illumination, as the secondary acceptor quinone Q<sub>B</sub> is reduced, fluorescence rises to a maximum <i>F</i><sub>m</sub>, wherein excitation is still terminated via <sup>3</sup>Car. This dual phase fluorescence rise is driven by protein conformation changes. The two phases suggest that the C2S2M2 structure of the PSII dimer (Caffarri et al. 2009) may be rewritten as 2SCM such that for a monomer the arrangement is S – CP43 – D1D2 – CP47 – M. The D1- and D2- antenna branches are excitonically separated. Fluorescence of the D1 pigment-protein branch rises during microseconds after Q<sub>A</sub> reduction, that of the D2 branch rises on a time scale of milliseconds during Q<sub>B</sub> reduction. A hypothesis is proposed here based on electrostatic profiling of photosynthetic pigments (Sirohiwal et al. 2021; Saito et al. 2023). When acceptor quinones are oxidized excitation flows through the stromal Chl layer of Chl-proteins CP43 and CP47 to the photochemical Pheophytin/Chl based reaction center. As Q<sub>A</sub> gets reduced during microseconds in CP43, and Q<sub>B</sub> gets reduced during milliseconds in CP47, an associated dynamic signal induces protein conformation changes shifting excitations from the stromal to the lumenal Chl layer. With this, excitation becomes diverted from the photosynthetic reaction center and connected to the characteristically lumenal ChlZ<sub>D1</sub> and ChlZ<sub>D2</sub>. Together with their nearby Car<sub>D1</sub> and Car<sub>D2</sub> these ChlsZ form quenching centers, terminating excitation via <sup>3</sup>Car with a lifetime of about 1 ns, sufficient to induce the maximum <i>F</i><sub>m</sub> fluorescence.</p>

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QA and QB reduction states induce protein dynamics underlying the dual phase fluorescence rise in Photosystem II

  • 期刊论文

  • 发表日期：

    2026年5月27日

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摘要

In photosystem II (PSII) chlorophyll (Chl) fluorescence yield (F) rises during low-to-high light induction. Based on a critical review of the literature, the following evidence is summarized. (1) As the primary acceptor quinone QA gets reduced, fluorescence immediately rises to Fc =1.8 Fo. (2) During microseconds of illumination following QA reduction, Fc rises to Ff where excitation is terminated via carotenoid triplet states (3Car). (3) After many milliseconds of illumination, as the secondary acceptor quinone QB is reduced, fluorescence rises to a maximum Fm, wherein excitation is still terminated via 3Car. This dual phase fluorescence rise is driven by protein conformation changes. The two phases suggest that the C2S2M2 structure of the PSII dimer (Caffarri et al. 2009) may be rewritten as 2SCM such that for a monomer the arrangement is S – CP43 – D1D2 – CP47 – M. The D1- and D2- antenna branches are excitonically separated. Fluorescence of the D1 pigment-protein branch rises during microseconds after QA reduction, that of the D2 branch rises on a time scale of milliseconds during QB reduction. A hypothesis is proposed here based on electrostatic profiling of photosynthetic pigments (Sirohiwal et al. 2021; Saito et al. 2023). When acceptor quinones are oxidized excitation flows through the stromal Chl layer of Chl-proteins CP43 and CP47 to the photochemical Pheophytin/Chl based reaction center. As QA gets reduced during microseconds in CP43, and QB gets reduced during milliseconds in CP47, an associated dynamic signal induces protein conformation changes shifting excitations from the stromal to the lumenal Chl layer. With this, excitation becomes diverted from the photosynthetic reaction center and connected to the characteristically lumenal ChlZD1 and ChlZD2. Together with their nearby CarD1 and CarD2 these ChlsZ form quenching centers, terminating excitation via 3Car with a lifetime of about 1 ns, sufficient to induce the maximum Fm fluorescence.