Laisk Agu, Oja Vello, Eichelmann Hillar
Tartu Ülikooli Tehnoloogia Instituut, University of Tartu, Nooruse st. 1, Tartu 50411, Estonia.
Tartu Ülikooli Tehnoloogia Instituut, University of Tartu, Nooruse st. 1, Tartu 50411, Estonia.
Biochim Biophys Acta. 2016 Jun;1857(6):819-30. doi: 10.1016/j.bbabio.2016.03.032. Epub 2016 Apr 4.
Electrochromic shift measurements confirmed that the Q-cycle operated in sunflower leaves. The slow temporarily increasing post-pulse phase was recorded, when ATP synthase was inactivated in the dark and plastoquinol (PQH(2)) oxidation was initiated by a short pulse of far-red light (FRL). During illumination by red light, the Q-cycle-supported proton arrival at the lumen and departure via ATP synthase were simultaneous, precluding extreme build-up of the membrane potential. To investigate the kinetics of the Q-cycle, less than one PQH(2) per cytochrome b(6)f (Cyt b(6)f) were reduced by illuminating the leaf with strong light pulses or single-turnover Xe flashes. The post-pulse rate of oxidation of these PQH2 molecules was recorded via the rate of reduction of plastocyanin (PC(+)) and P700(+), monitored at 810 and 950 nm. The PSII-reduced PQH(2) molecules were oxidized with multi-phase overall kinetics, τ(d)=1, τ(p)=5.6 and τ(s)=16 ms (22 °C). We conclude that τ(d) characterizes PSII processes and diffusion, τ(p) is the bifurcated oxidation of the primary quinol and τ(s) is the Q-cycle-involving reduction of the secondary quinol at the n-site, its transport to the p-site, and bifurcated oxidation there. The extraordinary slow kinetics of the Q-cycle may be related to the still unsolved mechanism of the "photosynthetic control."
电致变色位移测量证实了向日葵叶片中Q循环的运行。当ATP合酶在黑暗中失活且通过远红光（FRL）短脉冲引发质体醌醇（PQH₂）氧化时，记录到了缓慢的脉冲后暂时增加阶段。在红光照射期间，由Q循环支持的质子到达类囊体腔和通过ATP合酶离开是同时发生的，这避免了膜电位的过度积累。为了研究Q循环的动力学，通过强光脉冲或单周转Xe闪光照射叶片，使每个细胞色素b₆f（Cyt b₆f）还原的PQH₂少于一个。通过在810和950 nm处监测质体蓝素（PC⁺）和P700⁺的还原速率，记录这些PQH₂分子的脉冲后氧化速率。PSII还原的PQH₂分子以多相整体动力学被氧化，τ(d)=1，τ(p)=5.6和τ(s)=16 ms（22℃）。我们得出结论，τ(d)表征PSII过程和扩散，τ(p)是初级醌醇的分叉氧化，τ(s)是在n位点次级醌醇的Q循环参与还原、其向p位点的转运以及在那里的分叉氧化。Q循环异常缓慢的动力学可能与尚未解决的“光合控制”机制有关。
