Nitric oxide activation by caa₃ oxidoreductase from Thermus thermophilus

Visible and UV-resonance Raman spectroscopy was employed to investigate the reaction of NO with cytochrome caa₃ from Thermus thermophilus. We show the formation of the hyponitrite (HO-N=N-O)^- bound to the heme a₃ species (ν_N=N = 1330 cm^-1) forming a high spin complex in the oxidized heme a₃ Fe/Cu_B binuclear center of caa₃-oxidoreductase. In the absence of heme a₃ Fe²⁺-NO formation, the electron required for the formation of the N=N bond originates from the autoreduction of Cu_B by NO, producing nitrite. With the identification of the hyponitrite intermediate the hypothesis of a common phylogeny of aerobic respiration and bacterial denitrification is fully supported and the mechanism for the 2e^-/2H⁺ reduction of NO to N₂O can be described with more certainty.