Nitric oxide does not inhibit but is metabolized by the cytochrome bcc-aa3 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-aa3 supercomplex composed of Mycobacterium tuberculosis cytochrome bcc and Mycobacterium smegmatis aa3-type terminal oxidase. Strikingly, we found that the enzyme in turnover with O2 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 O2 consumption, with 2.65 ± 0.19 molecules of NO being consumed per O2 molecule by the mycobacterial bcc-aa3. 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-aa3 supercomplexes against NO stress.