Exploring the Nitric Oxide Detoxification Mechanism of Mycobacterium tuberculosis Truncated Haemoglobin N

Mycobacterium tuberculosis, the causative agent of human tuberculosis, encodes a haemoprotein named Truncated Haemoglobin N (trHbN), which in its active site transforms nitric oxide (NO) to nitrate anion.$\left( {{\text{NO}}_3^ - } \right)$. The NO-dioxygenase activity of trHbN seems to be crucial for the bacillus, which can survive under the nitrosative stress conditions that occur upon infection of the host. As a defense mechanism against the copious amounts of NO produced by macrophages upon infection, the protein must achieve a high level of NO-dioxygenase activity to eliminate NO, but this is modulated by its efficiency in capturing O₂ and NO. Migration of small diatomic ligands through the protein matrix is related to the presence of a doubly branched tunnel system connecting the surface and the haem cavity site. In this work, we have studied the mechanism that controls ligand diffusion and product egression with state-of-the-art molecular dynamics simulations. The results support a dual path mechanism for migration of O₂ and NO through distinct branches of the tunnel, where migration of NO is facilitated upon binding of O₂ to the haem group. Finally, egression of $\left( {{\text{NO}}_3^ - } \right)$ is preceded by the entrance of water to the haem cavity and occurs through a different pathway. Overall, the results highlight the intimate relationship between structure, dynamical behavior and biological function of trHbN.