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-aa supercomplex composed of Mycobacterium tuberculosis cytochrome bcc and Mycobacterium smegmatis aa-type terminal oxidase. Strikingly, we found that the enzyme in turnover with O and reductants is resistant to inhibition by the ligand, being able to metabolize NO at 25C with an apparent turnover number as high as ?303 mol NO (mol enzyme) min at 30 µM NO. The rate of NO consumption proved to be proportional to that of O consumption, with 2.65 ± 0.19 molecules of NO being consumed per O molecule by the mycobacterial bcc-aa. 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) min at 25C and 8.4 µM NO. These results suggest a protective role of mycobacterial bcc-aa supercomplexes against NO stress.