Mammalian Heme Peroxidases and Mycobacterium tuberculosis

Abstract

Tuberculosis (TB) is a lethal infectious disease which is caused by Mycobacterium tuberculosis. The alarming rate at which the incidence of bacterial resistance to known antibiotics has been rising is a serious cause of concern. At present, the two well known anti-tuberculosis drugs, isonicotinic acid hydrazide (INH, isoniazid) and pyrazinamide (PZA, pyrazin-2carboxamide) which are important components of the current course of the first-line TB chemotherapy suffer from increasing bacterial resistance. The other drugs of the combination therapy include rifampicin and ethambutol. It may be noted that both INH and PZA are prodrugs and require specific enzymes to convert them into drugs. INH is activated by a bacterial heme enzyme catalase peroxidase (MtCP) into a free radical form (Scheme I) (Zhang et al., 1992). The structure of unliganded MtCP is known (Bertrand et al., 2004) and detailed information is available about the substrate-binding site and the residues that might be involved in the binding and conversion of INH into a beneficial product. However, a precise mode of binding and the mechanism of action are not yet clearly understood because the structure of INH bound MtCP is not yet determined. On the other hand, PZA is metabolized into its active form pyrazinoic acid (POA) by amidase activity of the Mycobacterium tuberculosis nicotinamidase/pyrazinamidase (PncA) (Scheme II) (Konno et al., 1967). Although the crystal structure of pyrazinamidase in complex with POA is known but the structure of the complex with the original compound PZA is not yet determined. Therefore, the mode of binding of PZA with PncA has not so far been revealed. As shown by the crystal structure of the complex of LPO with INH, the binding of INH to lactoperoxidase (LPO) occurs through the distal heme cavity where INH interacts with a conserved water molecule W1 which is hydrogen bonded to ferric iron (Singh et al., 2010). Similarly, as revealed by the structure determination of the complex formed between LPO and PZA, PZA has been located in the substrate-binding site and interacts with substrate recognition residues of LPO (PDB ID: 3R4X) indicating a possible role of LPO in the conversion of PZA into an active form. Although the crystal structure of the PZA bound PncA is not known but a piece of information is available on the possible mode of ligand binding based on the molecular modeling data (Petrella et al., 2011). Therefore, it is of great interest that both prodrugs, INH and PZA bind to LPO specifically at the substrate-binding site on the distal heme side as the substrates bind to LPO (Singh et al., 2009) so that these