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J Biol Chem. 2017 Aug 25;292(34):14039-14049. doi: 10.1074/jbc.M117.794214. Epub 2017 Jun 14.

Equilibrium and ultrafast kinetic studies manipulating electron transfer: A short-lived flavin semiquinone is not sufficient for electron bifurcation.

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From the Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506.
the National Renewable Energy Laboratory, Golden, Colorado 80401, and.
the Department of Biochemistry and Molecular Biology, University of Georgia, Athens, Georgia 30602.
From the Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506,


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.


electron bifurcation; electron transfer; energetics; flavin; flavoprotein; fluorescence; transient absorption spectroscopy

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