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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.

Author information

1
From the Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506.
2
the National Renewable Energy Laboratory, Golden, Colorado 80401, and.
3
the Department of Biochemistry and Molecular Biology, University of Georgia, Athens, Georgia 30602.
4
From the Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506, afmill3r2@gmail.com.

Abstract

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.

KEYWORDS:

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

PMID:
28615449
PMCID:
PMC5572931
DOI:
10.1074/jbc.M117.794214
[Indexed for MEDLINE]
Free PMC Article

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