The crystal structure of isoniazid-bound KatG catalase-peroxidase from Synechococcus elongatus PCC7942

Computational analyses of drug resistance mutations in katG and emb complexes in Mycobacterium tuberculosis

The number of antibiotic resistant pathogens is increasing rapidly, and with this comes a substantial socioeconomic cost that threatens much of the world. To alleviate this problem, we must use antibiotics in a more responsible and informed way, further our understanding of the molecular basis of drug resistance, and design new antibiotics. Here, we focus on a key drug-resistant pathogen, Mycobacterium tuberculosis, and computationally analyze trends in drug-resistant mutations in genes of the proteins embA, embB, embC, and katG, which play essential roles in the action of the first-line drugs ethambutol and isoniazid. We use docking to predict binding modes of isoniazid to katG that agree with suggested binding sites found in our laboratory using cryo-EM. Using mutant stability predictions, we recapitulate the idea that resistance occurs when katG's heme cofactor is destabilized rather than due to a decrease in affinity to isoniazid. Conversely, we have identified resistance mutations that affect the affinity of ethambutol more drastically than the affinity of the natural substrate of embB. With this, we illustrate that we can distinguish between the two types of drug resistance-cofactor destabilization and drug affinity reduction-suggesting potential uses in the prediction of novel drug-resistant mutations.

Novel enzymes for biodegradation of polycyclic aromatic hydrocarbons identified by metagenomics and functional analysis in short-term soil microcosm experiments

Polycyclic aromatic hydrocarbons (PAHs) are highly toxic, carcinogenic substances. On soils contaminated with PAHs, crop cultivation, animal husbandry and even the survival of microflora in the soil are greatly perturbed, depending on the degree of contamination. Most microorganisms cannot tolerate PAH-contaminated soils, however, some microbial strains can adapt to these harsh conditions and survive on contaminated soils. Analysis of the metagenomes of contaminated environmental samples may lead to discovery of PAH-degrading enzymes suitable for green biotechnology methodologies ranging from biocatalysis to pollution control. In the present study, our goal was to apply a metagenomic data search to identify efficient novel enzymes in remediation of PAH-contaminated soils. The metagenomic hits were further analyzed using a set of bioinformatics tools to select protein sequences predicted to encode well-folded soluble enzymes. Three novel enzymes (two dioxygenases and one peroxidase) were cloned and used in soil remediation microcosms experiments. The experimental design of the present study aimed at evaluating the effectiveness of the novel enzymes on short-term PAH degradation in the soil microcosmos model. The novel enzymes were found to be efficient for degradation of naphthalene and phenanthrene. Adding the inorganic oxidant CaO2 further increased the degrading potential of the novel enzymes for anthracene and pyrene. We conclude that metagenome mining paired with bioinformatic predictions, structural modelling and functional assays constitutes a powerful approach towards novel enzymes for soil remediation.

Genetic mechanisms of co-emergence of INH-resistant Mycobacterium tuberculosis strains during the standard course of antituberculosis therapy

The incidence of isoniazid (INH) resistant Mycobacterium tuberculosis is increasing globally. This study aimed to identify the molecular mechanisms behind the development of INH resistance in M. tuberculosis strains collected from the same patients during the standard course of treatment. Three M. tuberculosis strains were collected from a patient before and during antituberculosis (anti-TB) therapy. The strains were characterized using phenotypic drug susceptibility tests, Mycobacterial Interspersed Repeated Unit-Variable-Number Tandem Repeats (MIRU-VNTR), and whole-genome sequencing (WGS) to identify mutations associated with INH resistance. To validate the role of the novel mutations in INH resistance, the mutated katG genes were electroporated into a KatG-deleted M. tuberculosis strain (GA03). Three-dimensional structures of mutated KatG were modeled to predict their impact on INH binding. The pre-treatment strain was susceptible to INH. However, two INH-resistant strains were isolated from the patient after anti-TB therapy. MIRU-VNTR and WGS revealed that the three strains were clonally identical. A missense mutation (P232L) and a nonsense mutation (Q461Stop) were identified in the katG of the two post-treatment strains, respectively. Transformation experiments showed that katG of the pre-treatment strain restored INH susceptibility in GA03, whereas the mutated katG genes from the post-treatment strains rendered negative catalase activity and INH resistance. The protein model indicated that P232L reduced INH-KatG binding affinity while Q461Stop truncated gene transcription. Our results showed that the two katG mutations, P232L and Q461Stop, accounted for the co-emergence of INH-resistant clones during anti-TB therapy. The inclusion of these mutations in the design of molecular assays could increase the diagnostic performance.IMPORTANCEThe evolution of drug-resistant strains of Mycobacterium tuberculosis within the lung lesions of a patient has a detrimental impact on treatment outcomes. This is particularly concerning for isoniazid (INH), which is the most potent first-line antimycobacterial drug. However, the precise genetic factors responsible for drug resistance in patients have not been fully elucidated, with approximately 15% of INH-resistant strains harboring unknown genetic factors. This raises concerns about the emergence of drug-resistant clones within patients, further contributing to the global epidemic of resistance. In this study, we revealed the presence of two novel katG mutations, which emerged independently due to the stress exerted by antituberculosis (anti-TB) treatment on a parental strain. Importantly, we experimentally demonstrated the functional significance of both mutations in conferring resistance to INH. Overall, this research sheds light on the genetic mechanisms underlying the evolution of INH resistance within patients and provides valuable insights for improving diagnostic performance by targeting specific mutations.

Characterization of a catalase-peroxidase variant (L333V-KatG) identified in an INH-resistant Mycobacterium tuberculosis clinical isolate

Mycobacterium tuberculosis catalase-peroxidase (Mt-KatG) is a bifunctional heme-dependent enzyme that has been shown to activate isoniazid (INH), the widely used antibiotic against tuberculosis (TB). The L333V-KatG variant has been associated with INH resistance in clinical M. tuberculosis isolates from Mexico. To understand better the mechanisms of INH activation, its catalytic properties (catalase, peroxidase, and IN-NAD formation) and crystal structure were compared with those of the wild-type enzyme (WT-KatG). The rate of IN-NAD formation mediated by WT-KatG was 23% greater than L333V-KatG when INH concentration is varied. In contrast to WT-KatG, the crystal structure of the L333V-KatG variant has a perhydroxy modification of the indole nitrogen of W107 from MYW adduct. L333V-KatG shows most of the active site residues in a similar position to WT-KatG; only R418 is in the R-conformation instead of the double R and Y conformation present in WT-KatG. L333V-KatG shows a small displacement respect to WT-KatG in the helix from R385 to L404 towards the mutation site, an increase in length of the coordination bond between H270 and heme Fe, and a longer H-bond between proximal D381 and W321, compared to WT-KatG; these small displacements could explain the altered redox potential of the heme, and result in a less active and stable enzyme.

Aromatic hydrazides: A potential solution for Acinetobacter baumannii infections

The emergence of multidrug-resistant bacteria and the poor efficacy of available antibiotics against these infections have led to the urgent need for novel antibiotics. Acinetobacter baumannii is one of high-priority pathogens due to its ability to mount resistance to different classes of antibiotics. In an effort to provide novel agents in the fight against infections caused by A. baumannii, we synthesized a series of 46 aromatic hydrazides as potential treatments. In this series, 34 compounds were found to be low- to sub-μM inhibitors of A. baumannii growth, with MIC values in the range of 8 μg/mL to ≤0.125 μg/mL against a broad set of multidrug-resistant clinical isolates. These compounds were not highly active against other bacteria. We showed that one of the most potent compounds, 3e, was bacteriostatic and inhibitory to biofilm formation, although it did not disrupt the preformed biofilm. Additionally, we found that these compounds lacked mammalian cytotoxicity. The high antibacterial potency and the lack of mammalian cytotoxicity make these compounds a promising lead series for development of a novel selective anti-A. baumannii antibiotic.

Deciphering Isoniazid Drug Resistance Mechanisms on Dimeric Mycobacterium tuberculosis KatG via Post-molecular Dynamics Analyses Including Combined Dynamic Residue Network Metrics

Resistance mutations in Mycobacterium tuberculosis (Mtb) catalase peroxidase protein (KatG), an essential enzyme in isoniazid (INH) activation, reduce the sensitivity of Mtb to first-line drugs, hence presenting challenges in tuberculosis (TB) management. Thus, understanding the mutational imposed resistance mechanisms remains of utmost importance in the quest to reduce the TB burden. Herein, effects of 11 high confidence mutations in the KatG structure and residue network communication patterns were determined using extensive computational approaches. Combined traditional post-molecular dynamics analysis and comparative essential dynamics revealed that the mutant proteins have significant loop flexibility around the heme binding pocket and enhanced asymmetric protomer behavior with respect to wild-type (WT) protein. Heme contact analysis between WT and mutant proteins identified a reduction to no contact between heme and residue His270, a covalent bond vital for the heme-enabled KatG catalytic activity. Betweenness centrality calculations showed large hub ensembles with new hubs especially around the binding cavity and expanded to the dimerization domain via interface in the mutant systems, providing possible compensatory allosteric communication paths for the active site as a result of the mutations which may destabilize the heme binding pocket and the loops in its vicinity. Additionally, an interesting observation came from Eigencentrality hubs, most of which are located in the C-terminal domain, indicating relevance of the domain in the protease functionality. Overall, our results provide insight toward the mechanisms involved in KatG-INH resistance in addition to identifying key regions in the enzyme functionality, which can be used for future drug design.

Synthesis, antitubercular, antimicrobial activities and molecular docking study of quinoline bearing dihydropyrimidines

In order to develop the antimicrobial and antitubercular agents, we have derived quinoline bearing dihydropyrimidine analogues 5a-o and structures of these compounds were determined by spectroscopic techniques. Further, we have calculated the molecular properties prediction and drug-likeness by Molinspiration property calculation toolkit and MolSoft software, respectively. The most active compound against Mycobacterium tuberculosis (5m, MIC = 0.20 µg/mL) also possessed a maximum drug-likeness model score (0.42). Compounds 5m, 5g and 5k were possessed promising antibacterial activity against tested bacterial species. Compound 5k was the only compound to have eye-catcher antifungal activity. Furthermore, the MTT cytotoxicity results on HeLa cells suggested lower cytotoxicity of biologically active compounds. Supramolecular interactions of the synthesized compounds has been assessed my means of molecular docking studies. Although all the synthesized compounds are showing preferably good interactions with their respective proteins, their binding free energies values suggest that these molecules are preferred for antitubercular activity rather than antimicrobial activity.

Using cryo-EM to understand antimycobacterial resistance in the catalase-peroxidase (KatG) from Mycobacterium tuberculosis

Resolution advances in cryoelectron microscopy (cryo-EM) now offer the possibility to visualize structural effects of naturally occurring resistance mutations in proteins and also of understanding the binding mechanisms of small drug molecules. In Mycobacterium tuberculosis the multifunctional heme enzyme KatG is indispensable for activation of isoniazid (INH), a first-line pro-drug for treatment of tuberculosis. We present a cryo-EM methodology for structural and functional characterization of KatG and INH resistance variants. The cryo-EM structure of the 161 kDa KatG dimer in the presence of INH is reported to 2.7 Å resolution allowing the observation of potential INH binding sites. In addition, cryo-EM structures of two INH resistance variants, identified from clinical isolates, W107R and T275P, are reported. In combination with electronic absorbance spectroscopy our cryo-EM approach reveals how these resistance variants cause disorder in the heme environment preventing heme uptake and retention, providing insight into INH resistance.

•

A cryo-EM structure to 2.7 Å resolution of M. tuberculosis KatG with isoniazid

•

Cryo-EM is able to visualize multiple dynamic binding modes of isoniazid to KatG

•

Structural disorder in isoniazid resistance mutations is observed

•

Structural disorder of the resistance mutations results in the lack of heme retention

Synthesis and evaluation of thiophene based small molecules as potent inhibitors of Mycobacterium tuberculosis

Herein, we report the synthesis and anti-tubercular studies of novel molecules based on thiophene scaffold. We identified two novel small molecules 4a and 4b, which demonstrated 2-fold higher in vitro activity (MIC₉₉: 0.195 μM) compared to first line TB drug, isoniazid (0.39 μM). The identified leads demonstrated additive effect with front line TB drugs (isoniazid, rifampicin and levofloxacin) and synergistic effect with a recently FDA-approved drug, bedaquiline. Mechanistic studies (i) negated the role of Pks13 and (ii) suggested the involvement of KatG in the anti-tubercular activity of these identified leads.

Structure, kinetics, molecular and redox properties of a cytosolic and developmentally regulated fungal catalase-peroxidase

CAT-2, a cytosolic catalase-peroxidase (CP) from Neurospora crassa, which is induced during asexual spore formation, was heterologously expressed and characterized. CAT-2 had the Met-Tyr-Trp (M-Y-W) adduct required for catalase activity. Its K_M for H₂O₂ was micromolar for peroxidase and millimolar for catalase activity. A E_m = -158 mV reduction potential value was obtained and the Soret band shift suggested a mixture of low and high spin ferric iron. CAT-2 EPR spectrum at 10 K indicated an axial and a rhombic component. With peroxyacetic acid (PAA), formation of Compound I* was observed with EPR. CAT-2 homodimer crystallographic structure contained two K⁺ ions; Glu107 residues were displaced to bind them. CAT-2 showed the essential amino acid residues for activity in similar positions to other CPs. CAT-2 Arg426 is oriented towards the M-Y-W adduct, interacting with the deprotonated Tyr238 hydroxyl group. A perhydroxy modification of the indole nitrogen of Trp90 was oriented toward the catalytic His91. In contrast to cytochrome c peroxidase and ascorbate peroxidase, the catalase-peroxidase heme propionates are not exposed to the solvent. Together with other N. crassa enzymes that utilize H₂O₂ as a substrate, CAT-2 has many tryptophan and proline residues at its surface, probably related to H₂O₂ selection in water.

Modeling the structural origins of drug resistance to isoniazid via key mutations in Mycobacterium tuberculosis catalase-peroxidase, KatG

WHO reported 10.4 million new tuberculosis (TB) cases and 1.8 million deaths in 2015, making M. tuberculosis the most successful human pathogen with highest mortality among infectious diseases [1,2]. Drug-resistant TB is a major threat to global TB control [2,3]. Recently Torres et al. [4] identified 14 novel substitutions in M. tuberculosis-KatG (the enzyme associated with resistance to isoniazid-an important first-line anti-TB drug) and demonstrated that 12 of the 14 can cause INH-resistance in M. smegmatis. This study presents an in silico structure-based analysis of these 14 amino acid substitutions using homology models and x-ray crystal structures (when available) in M. tuberculosis. Our models demonstrate that several of these mutations cluster around three openings in the KatG tertiary structure which appear to initiate channels to the heme group at the catalytic center of the enzyme. We studied the effects of these mutations on the tertiary structure of KatG, focusing on conformational changes in the three channels in the protein structure. Our results suggest that the 14 novel mutations sufficiently restrict one or more of these access channels, thus potentially preventing INH from reaching the catalytic heme. These observations provide valuable insights into the structure-based origins of INH resistance and provide testable hypotheses for future experimental studies.