Nitric Oxide Does Not Inhibit but Is Metabolized by the Cytochrome bcc-aa(3) Supercomplex

Nitric oxide (NO) is a well-known active site ligand and inhibitor of respiratory terminal oxidases. Here, we investigated the interaction of NO with a purified chimeric bcc-aa(3) supercomplex composed of Mycobacterium tuberculosis cytochrome bcc and Mycobacterium smegmatis aa(3)-type terminal oxidase. Strikingly, we found that the enzyme in turnover with O(2) and reductants is resistant to inhibition by the ligand, being able to metabolize NO at 25 °C with an apparent turnover number as high as ≈303 mol NO (mol enzyme)(−1) min(−1) at 30 µM NO. The rate of NO consumption proved to be proportional to that of O(2) consumption, with 2.65 ± 0.19 molecules of NO being consumed per O(2) molecule by the mycobacterial bcc-aa(3). The enzyme was found to metabolize the ligand even under anaerobic reducing conditions with a turnover number of 2.8 ± 0.5 mol NO (mol enzyme)(−1) min(−1) at 25 °C and 8.4 µM NO. These results suggest a protective role of mycobacterial bcc-aa(3) supercomplexes against NO stress.