Nitric oxide reacts with the ferryl-oxo catalytic intermediate of the CuB-lacking cytochrome bd terminal oxidase

Cytochrome bd is a bacterial respiratory oxidase carrying three hemes but no copper. We show that nitric oxide (NO) reacts with the intermediate F of cytochrome bd from Azotobacter vinelandii: (i) with a 1:1 stoichiometry, (ii) rapidly (k = 1.2 ± 0.1 × 105 M−1 s−1 at 20 °C), and (iii) yielding the oxidized enzyme with nitrite bound to heme d at the active site. Unexpectedly, the NO reaction mechanism of this catalytic intermediate in the CuB-lacking cytochrome bd appears similar to that of beef heart cytochrome c oxidase, where CuB was proposed to play a key role.