Mechanism of the Nitric Oxide Dioxygenase Reaction of Mycobacterium tuberculosis Hemoglobin N.

Many globins convert NO to innocuous NO through their nitric oxide dioxygenase (NOD) activity. Mycobacterium tuberculosis fights the oxidative and nitrosative stress imposed by its host (the toxic effects of O and NO species and their OONO and NO derivatives) through the action of truncated hemoglobin N (trHbN), which catalyzes the NOD reaction with one of the highest rates among globins. The general NOD mechanism comprises the following steps: binding of O to the heme, diffusion of NO into the heme pocket and formation of peroxynitrite (OONO), isomerization of OONO, and release of NO. Using quantum mechanics/molecular mechanics free-energy calculations, we show that the NOD reaction in trHbN follows a mechanism in which heme-bound OONO undergoes homolytic cleavage to give Fe═O and the NO radical but that these potentially harmful intermediates are short-lived and caged by the heme pocket residues. In particular, the simulations show that Tyr33(B10) side chain is shielded from Fe═O and NO (and protected from irreversible oxidation and nitration) by forming stable hydrogen bonds with Gln58(E11) side chain and Leu54(E7) backbone. Aromatic residues Phe46(CD1), Phe32(B9), and Tyr33(B10) promote NO dissociation via C-H···O bonding and provide stabilizing interactions for the anion along its egress route.