Significance of catalase-peroxidase (KatG) mutations in mediating isoniazid resistance in clinical strains of Mycobacterium tuberculosis.

OBJECTIVES: Isoniazid (INH) is still the most important first-line antitubercular drug. INH resistance is regarded as a major impediment to the tuberculosis (TB) control programme and contributes to the emergence of multidrug-resistant strains. Mutation at position 315 in the katG gene, encoding the catalase-peroxidase (KatG) enzyme, is the major cause of INH resistance in Mycobacterium tuberculosis. Therefore, investigation of the molecular mechanisms of INH resistance is the need of the hour. METHODS: To understand the clinical importance of KatG mutants (MTs) leading to INH resistance, in this study five MTs (S315T, S315I, S315R, S315N and S315G) were modelled, docked and interacted with INH in dynamic state. RESULTS: The binding affinity based on docking was found to be higher for MTs than for wild-type (WT) isolates, except for MT-S315R, indicating rigid binding of INH with MT proteins compared with the flexible binding seen in the WT. Analysis of molecular dynamics (MD) experiments suggested that fluctuations and deviations were higher at the INH binding residues for MTs than for the WT. Reduction in the hydrogen bond network after MD in all KatG enzymes implies an increase in the flexibility and stability of protein structures. Superimposition of MTs upon the WT structure showed a significant deviation that varies for the different MTs. CONCLUSIONS: It can be inferred that the five KatG MTs affect enzyme activity in different ways, which could be attributed to conformational changes in MT KatG that result in altered binding affinity to INH and eventually to INH resistance.

The Role of N-Acetyl Transferases on Isoniazid Resistance from Mycobacterium tuberculosis and Human: An In Silico Approach.

BACKGROUND: N-acetyl transferase (NAT) inactivates the pro-drug isoniazid (INH) to N-acetyl INH through a process of acetylation, and confers low-level resistance to INH in Mycobacterium tuberculosis (MTB). Similar to NAT of MTB, NAT2 in humans performs the same function of acetylation. Rapid acetylators, may not respond to INH treatment efficiently, and could be a potential risk factor, for the development of INH resistance in humans. METHODS: To understand the contribution of NAT of MTB and NAT2 of humans in developing INH resistance using in silico approaches, in this study, the wild type (WT) and mutant (MT)-NATs of MTB, and humans, were modeled and docked, with substrates and product (acetyl CoA, INH, and acetyl INH). The MT models were built, using templates 4BGF of MTB, and 2PFR of humans. RESULTS: On the basis of docking results of MTB-NAT, it can be suggested that in comparison to the WT, binding affinity of MT-G207R, was found to be lower with acetyl CoA, and higher with acetyl-INH and INH. In case of MT-NAT2 from humans, the pattern of score with respect to acetyl CoA and acetyl-INH, was similar to MT-NAT of MTB, but revealed a decrease in INH score. CONCLUSION: In MTB, MT-NAT revealed high affinity towards acetyl-INH, which can be interpreted as increased formation of acetyl-INH, and therefore, may lead to INH resistance through inactivation of INH. Similarly, in MT-NAT2 (rapid acetylators), acetylation occurs rapidly, serving as a possible risk factor for developing INH resistance in humans.

Analysis of interactions of clinical mutants of catalase-peroxidase (KatG) responsible for isoniazid resistance in Mycobacterium tuberculosis with derivatives of isoniazid.

OBJECTIVES: Isoniazid (INH) resistance is a major contributor to the emergence of multidrug resistance in Mycobacterium tuberculosis (MTB), hampering the success of tuberculosis treatment. This study aimed to identify good leads based on INH derivatives against INH-resistant MTB strains. Mutations at codon 315 in the katG gene encoding catalase-peroxidase (KatG) are the major cause of INH resistance in MTB. The most prevalent substitution is S315T; other substitutions include S315I, S315R, S315N and S315G. METHODS: In this study, all five naturally occurring mutants (S315T, S315I, S315R, S315N and S315G) of KatG were docked and simulated with 50 INH derivatives in comparison with the wild-type (WT) KatG. RESULTS: The docking results suggested that compounds C30, C45 and C50 gave the highest scores when bound to the mutants of KatG. Of note, C50 produced a high score with the WT as well as with three mutants (S315T, S315I and S315R). Simulation studies indicated that C50 exhibited minimal deviation and fluctuation between WT and three mutants compared with C30 and C45, which displayed significant changes with WT and the S315N and S315G mutants, respectively. CONCLUSIONS: C50 can be considered as a better lead for INH-resistant strains. These models demonstrate the binding interaction of all naturally occurring KatG mutants of MTB at position 315 with derivatives of INH. This information will be helpful for lead compound-based identification of derivatives that may be used against INH-resistant MTB strains and may provide a useful structural framework for designing new antitubercular agents that can circumvent INH resistance.

Overview on mechanisms of isoniazid action and resistance in Mycobacterium tuberculosis.

Isoniazid (INH) is one of the most active compounds used to treat tuberculosis (TB) worldwide. In addition, INH has been used as a prophylactic drug for individuals with latent Mycobacterium tuberculosis (MTB) infection to prevent reactivation of disease. Importantly, the definition of multidrug resistance (MDR) in TB is based on the resistance of MTB strains to INH and rifampicin (RIF). Despite its simple chemical structure, the mechanism of action of INH is very complex and involves several different concepts. Many pathways pertaining to macromolecular synthesis are affected, notably mycolic acid synthesis. The pro-drug INH is activated by catalase-peroxidase (KatG), and the active INH products are targeted by enzymes namely, enoyl acyl carrier protein (ACP) reductase (InhA) and beta-ketoacyl ACP synthase (KasA). In contrast, INH is inactivated by arylamine N-acetyltransferases (NATs). Consequently, the molecular mechanisms of INH resistance involve several genes in multiple biosynthetic networks and pathways. Mutation in the katG gene is the major cause for INH resistance, followed by inhA, ahpC, kasA, ndh, iniABC,fadE, furA, Rv1592c and Rv1772. The recent association of efflux genes with INH resistance has also gained considerable attention. Interestingly, substitutions have also been observed in nat, fabD, and accD recently in resistant isolates. Understanding the mechanisms operating behind INH action and resistance would enable better detection of INH resistance. This information would aid novel drug design strategies. Herein we review all mechanisms known to potentially contribute to the complexity of INH action and mechanisms of resistance in MTB, with insights into methods for detection of INH resistance as well as their limitations.

Binding of activated isoniazid with acetyl-CoA carboxylase from Mycobacterium tuberculosis.

AccD6 (acetyl coenzyme A (CoA) carboxylase), plays an important role in mycolic acid synthesis of Mycobacterium tuberculosis (Mtb). Induced gene expression by isoniazid (isonicotinylhydrazine - INH), anti-tuberculosis drug) shows the expression of accD6. It is our interest to study the binding of activated INH with the AccD6 model using molecular docking procedures. The study predicts a primary binding site for activated INH (isonicotinyl acyl radical) in AccD6 as a potential target.