Equilibrium and ultrafast kinetic studies manipulating electron transfer: A short-lived flavin semiquinone is not sufficient for electron bifurcation
Equilibrium and ultrafast kinetic studies manipulating electron transfer: A short-lived flavin semiquinone is not sufficient for electron bifurcation
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DOI：
10.1074/jbc.m117.794214
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发表时间：
2017-08-25
影响因子：
4.8
通讯作者：
Miller, Anne-Frances
中科院分区：
文献类型：
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作者：
Hoben, John P.;Lubner, Carolyn E.;Miller, Anne-Frances
基于黄素的电子转移分岔是酶系统中电化学能量守恒和部署的一种基本而强大的机制。在这个过程中，在中间还原电位（即中间还原功率）下获得一对电子，每个电子被传递到不同的受体，一个具有较低的还原功率，另一个具有较高的还原功率，导致分叉。据信，在这一过程中，一种强还原的半醌类物质是必不可少的，并且预计这种物质在动力学上应该是短暂的。我们现在证明，短命阴离子黄素半醌（ASQ）的存在不足以推断分叉活性的存在，尽管这种物种可能是该过程所必需的。我们使用瞬态吸收光谱比较了在分叉NADH依赖性铁氧化还蛋白- nadp（+）氧化还原酶和非分叉黄酮蛋白硝基还原酶、NADH氧化酶和黄酮还蛋白中光化学产生ASQ的速率和衰变机制。我们发现，在不同的蛋白质环境中，不同的机制主导着ASQ的衰变，产生的寿命范围超过2个数量级。氧化还原辅助因子之间的电子转移能力与与附近供体的电荷重组可以解释我们观察到的ASQ寿命范围。我们的研究结果支持一个模型，其中有效的电子传播可以解释分叉nadh依赖性铁氧化还原酶- nadp（+）氧化还原酶I的ASQ寿命短，并且可以作为电子分叉能力的指示。
Flavin-based electron transfer bifurcation is emerging as a fundamental and powerful mechanism for conservation and deployment of electrochemical energy in enzymatic systems. In this process, a pair of electrons is acquired at intermediate reduction potential (i.e. intermediate reducing power), and each electron is passed to a different acceptor, one with lower and the other with higher reducing power, leading to bifurcation. It is believed that a strongly reducing semiquinone species is essential for this process, and it is expected that this species should be kinetically short-lived. We now demonstrate that the presence of a short-lived anionic flavin semiquinone (ASQ) is not sufficient to infer the existence of bifurcating activity, although such a species may be necessary for the process. We have used transient absorption spectroscopy to compare the rates and mechanisms of decay of ASQ generated photochemically in bifurcating NADH-dependent ferredoxin-NADP(+) oxidoreductase and the non-bifurcating flavoproteins nitroreductase, NADH oxidase, and flavodoxin. We found that different mechanisms dominate ASQ decay in the different protein environments, producing lifetimes ranging over 2 orders of magnitude. Capacity for electron transfer among redox cofactors versus charge recombination with nearby donors can explain the range of ASQ lifetimes that we observe. Our results support a model wherein efficient electron propagation can explain the short lifetime of the ASQ of bifurcating NADH-dependent ferredoxin-NADP(+) oxidoreductase I and can be an indication of capacity for electron bifurcation.