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Resolution of distinct membrane-bound enzymes from Enterobacter cloacae SLD1a-1 that are responsible for selective reduction of nitrate and selenate oxyanions

Lookup NU author(s): Dr Carys Watts, Dr Clive Butler

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Abstract

Enterobacter cloacae SLD1a-1 is capable of reductive detoxification of selenate to elemental selenium under aerobic growth conditions. The initial reductive step is the two-electron reduction of selenate to selenite and is catalyzed by a molybdenum-dependent enzyme demonstrated previously to be located in the cytoplasmic membrane, with its active site facing the periplasmic compartment (C. A. Watts, H. Ridley, K. L. Condie, J. T. Leaver, D. J. Richardson, and C. S. Butler, FEMS Microbiol. Lett. 228:273-279, 2003). This study describes the purification of two distinct membrane-bound enzymes that reduce either nitrate or selenate oxyanions. The nitrate reductase is typical of the NAR-type family, with α and β subunits of 140 kDa and 58 kDa, respectively. It is expressed predominantly under anaerobic conditions in the presence of nitrate, and while it readily reduces chlorate, it displays no selenate reductase activity in vitro. The selenate reductase is expressed under aerobic conditions and expressed poorly during anaerobic growth on nitrate. The enzyme is a heterotrimeric (αβγ) complex with an apparent molecular mass of ∼600 kDa. The individual subunit sizes are ∼100 kDa (α), ∼55 kDa (β), and ∼36 kDa (γ), with a predicted overall subunit composition of α3β3γ 3. The selenate reductase contains molybdenum, heme, and nonheme iron as prosthetic constituents. Electronic absorption spectroscopy reveals the presence of a b-type cytochrome in the active complex. The apparent K m for selenate was determined to be ∼2 mM, with an observed Vmax of 500 nmol SeO42- min-1 mg-1 (kcat, ∼5.0 s-1). The enzyme also displays activity towards chlorate and bromate but has no nitrate reductase activity. These studies report the first purification and characterization of a membrane-bound selenate reductase. Copyright © 2006, American Society for Microbiology. All Rights Reserved.


Publication metadata

Author(s): Ridley H, Watts CA, Richardson DJ, Butler CS

Publication type: Article

Publication status: Published

Journal: Applied and Environmental Microbiology

Year: 2006

Volume: 72

Issue: 8

Pages: 5173-5180

ISSN (print): 0099-2240

ISSN (electronic): 1098-5336

Publisher: American Society for Microbiology

URL: http://dx.doi.org/10.1128/AEM.00568-06

DOI: 10.1128/AEM.00568-06

PubMed id: 16885262


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