Photogeneration and reactivity of flavin anionic semiquinone in a bifurcating electron transfer flavoprotein
Photogeneration and reactivity of flavin anionic semiquinone in a bifurcating electron transfer flavoprotein
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DOI：
10.1016/j.bbabio.2021.148415
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发表时间：
2021-03-26
影响因子：
4.3
通讯作者：
Miller, Anne-Frances
中科院分区：
文献类型：
--
作者：
Duan, H. Diessel;Khan, Sharique A.;Miller, Anne-Frances
电子转移分叉允许通过在一对电子之间重新分配能量，以较弱的载流子为代价产生强还原载流子。因此，来自NADH的两个弱还原性电子被消耗以产生强还原性铁氧还蛋白或黄素氧还蛋白，通过氧化受体的还原来支付。普遍的机制要求参与的强烈还原黄素半醌，这一直难以观察到的网站肯定在多黄素系统。使用蓝光（450 nm）光激发的黄素的分叉电子转移黄素蛋白（ETF），我们证明积累的阴离子黄素半醌超过所观察到的平衡滴定，并建立其能力，以减少低电位的电子受体苄基紫精。这必须发生在分叉黄素，因为电子转移（ET）黄素的中点电位不够负。我们表明，双-三丙烷缓冲液是一个有效的电子供体的黄素光还原，但如果该系统是用ET黄素化学还原，使只有分叉黄素被氧化和光化学活性，黄素阴离子半醌更迅速地形成。因此，激发的分叉黄素能够利用储存在ET黄素中的电子。黄素半醌光生在分叉点，因此必须伴随着额外的半醌形成氧化的ET黄素。与分叉黄素半醌的预期不稳定性一致，它在光照停止后立即消退。然而，在平衡滴定半醌产量的比较表明，在连续光照在pH 9的稳定状态人口的0.3当量的分叉黄素半醌积累，然后经历进一步的光还原对苯二酚。虽然短暂的，人口分叉黄素半醌解释了系统的能力，进行光驱动的电子转移从bis-tris丙烷到苄基紫精，在效果捕获能量从光。
Electron transfer bifurcation allows production of a strongly reducing carrier at the expense of a weaker one, by redistributing energy among a pair of electrons. Thus, two weakly-reducing electrons from NADH are consumed to produce a strongly reducing ferredoxin or flavodoxin, paid for by reduction of an oxidizing acceptor. The prevailing mechanism calls for participation of a strongly reducing flavin semiquinone which has been difficult to observe with site-certainly in multi-flavin systems. Using blue light (450 nm) to photoexcite the flavins of bifurcating electron transfer flavoprotein (ETF), we demonstrate accumulation of anionic flavin semiquinone in excess of what is observed in equilibrium titrations, and establish its ability to reduce the low-potential electron acceptor benzyl viologen. This must occur at the bifurcating flavin because the midpoint potentials of the electron transfer (ET) flavin are not sufficiently negative. We show that bis-tris propane buffer is an effective electron donor to the flavin photoreduction, but that if the system is prepared with the ET flavin chemically reduced, so that only the bifurcating flavin is oxidized and photochemically active, flavin anionic semiquinone is formed more rapidly. Thus, excited bifurcating flavin is able to draw on an electron stored at the ET flavin. Flavin semiquinone photogenerated at the bifurcation site must therefore be accompanied by additional semiquinone formation by oxidation of the ET flavin. Consistent with the expected instability of bifurcating flavin semiquinone, it subsides immediately upon cessation of illumination. However comparison with yields of semiquinone in equilibrium titrations suggest that during continuous illumination at pH 9 a steady state population of 0.3 equivalents of bifurcating flavin semiquinone accumulates, and then undergoes further photoreduction to the hydroquinone. Although transient, the population of bifurcating flavin semiquinone explains the system's ability to conduct light-driven electron transfer from bis-tris propane to benzyl viologen, in effect trapping energy from light.