Purification and characterization of the MQH2:NO oxidoreductase from the hyperthermophilic archaeon Pyrobaculum aerophilum

Simon De Vries, Marc J.F. Strampraad, Shen Lu, Pierre Moënne-Loccoz, Imke Schröder

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45 Scopus citations


The membrane-bound NO reductase from the hyperthermophilic denitrifying archaeon Pyrobaculum aerophilum was purified to homogeneity. The enzyme displays MQH2:NO oxidoreductase (qNOR) activity, consists of a single subunit, and contains heme and nonheme iron in a 2:1 ratio. The combined results of EPR, resonance Raman, and UV-visible spectroscopy show that one of the hemes is bis-His-coordinated low spin (gz = 3.015; gy = 2.226; gx = 1.45), whereas the other heme adopts a high spin configuration. The enzyme also contains one nonheme iron center, which in the oxidized enzyme is antiferromagnetically coupled to the high spin heme. This binuclear high spin heme/nonheme iron center is EPR-silent and the site of NO reduction. The reduced high spin heme is bound to a neutral histidine and can bind CO to form of a low spin complex. The oxidized high spin heme binds NO, yielding a ferric nitrosyl complex, the intermediate causing the commonly found substrate inhibition in NO reductases (Ki(NO) = 7 μM). The qNOR as present in the membrane is, in contrast to the purified enzyme, quite thermostable, incubation at 100 °C for 86 min leading to 50% inhibition. The pure enzyme lacks heme b and instead contains stoichiometric amounts of hemes Op1 and Op2, ethenylgeranylgeranyl and hydroxyethylgeranylgeranyl derivatives of heme b, respectively. The archaeal qNOR is the first example of a NO reductase, which contains modified hemes reminiscent of cytochrome bo3 and aa3 oxidases. This report is the first describing the purification and structural and spectroscopic properties of a thermostable NO reductase.

Original languageEnglish (US)
Pages (from-to)35861-35868
Number of pages8
JournalJournal of Biological Chemistry
Issue number38
StatePublished - Sep 19 2003

ASJC Scopus subject areas

  • Biochemistry
  • Molecular Biology
  • Cell Biology


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