Mechanism of chromate reduction by the Escherichia coli protein, NfsA, and the role of different chromate reductases in minimizing oxidative stress during chromate reduction

Environ Microbiol. 2004 Aug;6(8):851-60. doi: 10.1111/j.1462-2920.2004.00639.x.


Chromate [Cr(VI)] is a serious environmental pollutant, which is amenable to bacterial bioremediation. NfsA, the major oxygen-insensitive nitroreductase of Escherichia coli, is a flavoprotein that is able to reduce chromate to less soluble and less toxic Cr(III). We show that this process involves single-electron transfer, giving rise to a flavin semiquinone form of NfsA and Cr(V) as intermediates, which redox cycle, generating more reactive oxygen species (ROS) than a divalent chromate reducer, YieF. However, NfsA generates less ROS than a known one-electron chromate reducer, lipoyl dehydrogenase (LpDH), suggesting that NfsA employs a mixture of uni- and di-valent electron transfer steps. The presence of YieF, ChrR (another chromate reductase we previously characterized), or NfsA in an LpDH-catalysed chromate reduction reaction decreased ROS generation by c. 65, 40, or 20%, respectively, suggesting that these enzymes can pre-empt ROS generation by LpDH. We previously showed that ChrR protects Pseudomonas putida against chromate toxicity; here we show that NfsA or YieF overproduction can also increase the tolerance of E. coli to this compound.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Amino Acid Sequence
  • Chromates / metabolism*
  • Chromates / toxicity
  • Dihydrolipoamide Dehydrogenase / metabolism
  • Drug Resistance, Neoplasm
  • Electron Transport
  • Electron-Transferring Flavoproteins / metabolism
  • Escherichia coli / drug effects
  • Escherichia coli / metabolism*
  • Escherichia coli Proteins / genetics
  • Escherichia coli Proteins / metabolism*
  • Gene Dosage
  • Molecular Sequence Data
  • Nitroreductases / genetics
  • Nitroreductases / metabolism*
  • Oxidation-Reduction
  • Oxidative Stress
  • Oxidoreductases / physiology*
  • Pseudomonas / genetics
  • Reactive Oxygen Species / analysis
  • Sequence Homology, Amino Acid


  • Chromates
  • Electron-Transferring Flavoproteins
  • Escherichia coli Proteins
  • Reactive Oxygen Species
  • Oxidoreductases
  • NfsA protein, E coli
  • Nitroreductases
  • Dihydrolipoamide Dehydrogenase
  • chromate reductase