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We show that the heme-copper terminal oxidases ofThermus thermophilus (calledba 3 andcaa 3 ) are able to catalyze the reduction of nitric oxide (NO) to nitrous oxide (N2 O) under reducing anaerobic conditions. The rate of NO consumption and N2 O production were found to be linearly dependent on enzyme concentration, and activity was abolished by enzyme denaturation. Thus, contrary to the eukaryotic enzyme, bothT. thermophilus oxidases display a NO reductase activity (3.0 ± 0.7 mol NO/molba 3 × min and 32 ± 8 mol NO/molcaa 3 × min at [NO] ≈ 50 μM and 20°C) that, though considerably lower than that of bona fide NO reductases (300–4,500 mol NO/mol enzyme × min), is definitely significant. We also show that forba 3 oxidase, NO reduction is associated to oxidation of cytochromeb at a rate compatible with turnover, suggesting a mechanism consistent with the stoichiometry of the overall reaction. We propose that the NO reductase activity ofT. thermophilus oxidases may depend on a peculiar CuB + coordination, which may be revealed by the forthcoming three-dimensional structure. These findings support the hypothesis of a common phylogeny of aerobic respiration and bacterial denitrification, which was proposed on the basis of structural similarities between thePseudomonas stutzeri NO reductase and thecbb 3 terminal oxidases. Our findings represent functional evidence in support of this hypothesis.

The heme-copper oxidases of Thermus thermophilus catalyze the reduction of nitric oxide: Evolutionary implications