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(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 degrees C with an apparent turnover number as high as approximate to 303 mol NO (mol enzyme)(-1) min(-1) at 30 mu M 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 degrees C and 8.4 mu M NO. These results suggest a protective role of mycobacterial bcc-aa(3) supercomplexes against NO stress.