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Bidirectional catalysis by copper-containing nitrite reductase.

Hein J Wijma1, Gerard W Canters, Simon de Vries

  • 1Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands.

Biochemistry
|August 11, 2004
PubMed
Summary
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Copper-containing nitrite reductase catalyzes nitric oxide oxidation, the reverse of its normal function. This reaction is thermodynamically unfavorable at physiological pH, making nitrous oxide production unlikely in vivo.

Area of Science:

  • Biochemistry
  • Enzymology
  • Microbial Metabolism

Background:

  • Copper-containing nitrite reductase (NiR) from Alcaligenes faecalis S-6 typically reduces nitrite to nitric oxide (NO).
  • The physiological electron donor for this enzyme is pseudoazurin.

Purpose of the Study:

  • To investigate the reverse reaction catalyzed by NiR: the oxidation of nitric oxide to nitrite.
  • To determine the thermodynamic and kinetic parameters for both forward and reverse reactions across a pH range of 6-8.

Main Methods:

  • Monitoring nitric oxide concentration using a Clark-type electrode.
  • Measuring the redox state of pseudoazurin via optical spectroscopy.
  • Thermodynamic and kinetic analyses at varying pH levels.

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Main Results:

  • NiR was confirmed to catalyze the oxidation of nitric oxide to nitrite.
  • The reaction equilibrium favors nitrite and reduced pseudoazurin formation above pH 6.2, with an equilibrium constant of 10^3 at pH 8.
  • The catalytic efficiency for nitrite reduction decreases with increasing pH, while the reverse reaction rate increases.

Conclusions:

  • Dissimilatory nitrite reductases catalyze a thermodynamically unfavorable reaction at physiological pH (7-8).
  • Nitrous oxide production by copper-containing NiR is unlikely in vivo with native electron donors.
  • The enzyme's catalytic bias shifts towards nitrite reduction at lower pH.