title: Mechanism of chromate reduction by the Escherichia coli protein, NfsA, and the role of different chromate reductases in minimizing oxidative stress during chromate reduction.
abstract: 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.
authors.affiliation: Department of Microbiology and Immunology, Sherman Fairchild Science Building, Stanford University School of Medicine, 299 Campus Drive, Stanford, CA 94305, USA.
authors.forename: D F
authors.initials: DF
authors.lastname: Ackerley
authors.forename: C F
authors.initials: CF
authors.lastname: Gonzalez
authors.forename: M
authors.initials: M
authors.lastname: Keyhan
authors.forename: R
authors.initials: R
authors.lastname: Blake
authors.forename: A
authors.initials: A
authors.lastname: Matin
mesh_terms: D000595:Amino Acid Sequence; D002840:Chromates / Q000378:metabolism* / Q000633:toxicity; D008058:Dihydrolipoamide Dehydrogenase / Q000378:metabolism; D019008:Drug Resistance, Neoplasm; D004579:Electron Transport; D044927:Electron-Transferring Flavoproteins / Q000378:metabolism; D004926:Escherichia coli / Q000187:drug effects / Q000378:metabolism*; D029968:Escherichia coli Proteins / Q000235:genetics / Q000378:metabolism*; D018628:Gene Dosage; D008969:Molecular Sequence Data; D009601:Nitroreductases / Q000235:genetics / Q000378:metabolism*; D010084:Oxidation-Reduction; D018384:Oxidative Stress; D010088:Oxidoreductases / Q000502:physiology*; D011549:Pseudomonas / Q000235:genetics; D017382:Reactive Oxygen Species / Q000032:analysis; D017386:Sequence Homology, Amino Acid
chemical_list: D002840:Chromates; D044927:Electron-Transferring Flavoproteins; D029968:Escherichia coli Proteins; D017382:Reactive Oxygen Species; D010088:Oxidoreductases; C116053:NfsA protein, E coli; D009601:Nitroreductases; D008058:Dihydrolipoamide Dehydrogenase; C063243:chromate reductase