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

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 presence of a short-lived anionic flavin semiquinone (ASQ) is not sufficient to infer 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 two orders of magnitude. Capacity for electron transfer among redox cofactors vs. charge recombination with nearby donors can explain the range of ASQ lifetimes we observe. In conclusion, our results support a modelmore » 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.« less

Authors:
 [1]; ORCiD logo [2];  [2];  [3];  [3];  [1];  [3]; ORCiD logo [2]; ORCiD logo [1]
  1. Univ. of Kentucky, Lexington, KY (United States)
  2. National Renewable Energy Lab. (NREL), Golden, CO (United States)
  3. Univ. of Georgia, Athens, GA (United States)
Publication Date:
Research Org.:
National Renewable Energy Lab. (NREL), Golden, CO (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI Identifier:
1395083
Report Number(s):
NREL/JA-2700-68454
Journal ID: ISSN 0021-9258
Grant/Contract Number:
AC36-08GO28308
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Journal of Biological Chemistry
Additional Journal Information:
Journal Volume: 292; Journal Issue: 34; Journal ID: ISSN 0021-9258
Publisher:
American Society for Biochemistry and Molecular Biology
Country of Publication:
United States
Language:
English
Subject:
09 BIOMASS FUELS; electron transfer; energetics; flavin; flavoprotein; fluorescence; electron bifurcation; transient absorption spectrocopy

Citation Formats

Hoben, John P., Lubner, Carolyn E., Ratzloff, Michael W., Schut, Gerrit J., Nguyen, Diep M. N., Hempel, Karl W., Adams, Michael W. W., King, Paul W., and Miller, Anne -Frances. Equilibrium and ultrafast kinetic studies manipulating electron transfer: A short-lived flavin semiquinone is not sufficient for electron bifurcation. United States: N. p., 2017. Web. doi:10.1074/jbc.M117.794214.
Hoben, John P., Lubner, Carolyn E., Ratzloff, Michael W., Schut, Gerrit J., Nguyen, Diep M. N., Hempel, Karl W., Adams, Michael W. W., King, Paul W., & Miller, Anne -Frances. Equilibrium and ultrafast kinetic studies manipulating electron transfer: A short-lived flavin semiquinone is not sufficient for electron bifurcation. United States. doi:10.1074/jbc.M117.794214.
Hoben, John P., Lubner, Carolyn E., Ratzloff, Michael W., Schut, Gerrit J., Nguyen, Diep M. N., Hempel, Karl W., Adams, Michael W. W., King, Paul W., and Miller, Anne -Frances. 2017. "Equilibrium and ultrafast kinetic studies manipulating electron transfer: A short-lived flavin semiquinone is not sufficient for electron bifurcation". United States. doi:10.1074/jbc.M117.794214.
@article{osti_1395083,
title = {Equilibrium and ultrafast kinetic studies manipulating electron transfer: A short-lived flavin semiquinone is not sufficient for electron bifurcation},
author = {Hoben, John P. and Lubner, Carolyn E. and Ratzloff, Michael W. and Schut, Gerrit J. and Nguyen, Diep M. N. and Hempel, Karl W. and Adams, Michael W. W. and King, Paul W. and Miller, Anne -Frances},
abstractNote = {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 presence of a short-lived anionic flavin semiquinone (ASQ) is not sufficient to infer 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 two orders of magnitude. Capacity for electron transfer among redox cofactors vs. charge recombination with nearby donors can explain the range of ASQ lifetimes we observe. In conclusion, 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.},
doi = {10.1074/jbc.M117.794214},
journal = {Journal of Biological Chemistry},
number = 34,
volume = 292,
place = {United States},
year = 2017,
month = 6
}

Journal Article:
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  • The recently realized biochemical phenomenon of energy conservation through electron bifurcation provides biology with an elegant means to maximize utilization of metabolic energy. The mechanism of coordinated coupling of exergonic and endergonic oxidation-reduction reactions by a single enzyme complex has been elucidated through optical and paramagnetic spectroscopic studies revealing unprecedented features. Pairs of electrons are bifurcated over more than 1 volt of electrochemical potential by generating a low-potential, highly energetic, unstable flavin semiquinone and directing electron flow to an iron-sulfur cluster with a highly negative potential to overcome the barrier of the endergonic half reaction. As a result, the unprecedentedmore » range of thermodynamic driving force that is generated by flavin-based electron bifurcation accounts for unique chemical reactions that are catalyzed by these enzymes.« less
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