Essential Mycobacterium Tuberculosis Research Articles

Synthesis, Activity, Toxicity, and In Silico Studies of New Antimycobacterial N-Alkyl Nitrobenzamides.

Tuberculosis (TB) is a disease that plagues the frailest members of society. We have developed a family of N-alkyl nitrobenzamides that exhibit promising antitubercular activities and can be considered a structural simplification of known inhibitors of decaprenylphosphoryl-β-D-ribofuranose 2'-oxidase (DprE1), an essential Mycobacterium tuberculosis (Mtb) enzyme and an emergent antitubercular target. Hereby, we report the development of these compounds via a simple synthetic methodology as well as their stability, cytotoxicity, and antitubercular activity. Studying their in vitro activity revealed that the 3,5-dinitro and the 3-nitro-5-trifluoromethyl derivatives were the most active, and within these, the derivatives with intermediate lipophilicities presented the best activities (MIC of 16 ng/mL). Additionally, in an ex vivo macrophage model of infection, the derivatives with chain lengths of six and twelve carbon atoms presented the best results, exhibiting activity profiles comparable to isoniazid. Although the proof is not definite, the assessment of susceptibility over multiple mycobacterial species, together with the structure similarities with known inhibitors of this enzyme, support DprE1 as a likely target of action for the compounds. This idea is also reinforced by the docking studies, where the fit of our more active compounds to the DprE1 binding pocket is very similar to what was observed for known inhibitors like DNB1.

Structure-Function Analysis of the Essential Mycobacterium tuberculosis P450 Drug Target, CYP121A1.

Cytochrome P450 CYP121A1 is a well-known drug target against Mycobacterium tuberculosis, the human pathogen that causes the deadly disease tuberculosis (TB). CYP121A1 is a unique P450 enzyme because it uses classical and non-classical P450 catalytic processes and has distinct structural features among P450s. However, a detailed investigation of CYP121A1 protein structures in terms of active site cavity dynamics and key amino acids interacting with bound ligands has yet to be undertaken. To address this research knowledge gap, 53 CYP121A1 crystal structures were investigated in this study. Critical amino acids required for CYP121A1's overall activity were identified and highlighted this enzyme's rigid architecture and substrate selectivity. The CYP121A1-fluconazole crystal structure revealed a novel azole drug-P450 binding mode in which azole heme coordination was facilitated by a water molecule. Fragment-based inhibitor approaches revealed that CYP121A1 can be inhibited by molecules that block the substrate channel or by directly interacting with the P450 heme. This study serves as a reference for the precise understanding of CYP121A1 interactions with different ligands and the structure-function analysis of P450 enzymes in general. Our findings provide critical information for the synthesis of more specific CYP121A1 inhibitors and their development as novel anti-TB drugs.

Inhibition of Indigoidine Synthesis as a High-Throughput Colourimetric Screen for Antibiotics Targeting the Essential Mycobacterium tuberculosis Phosphopantetheinyl Transferase PptT.

A recently-validated and underexplored drug target in Mycobacterium tuberculosis is PptT, an essential phosphopantetheinyl transferase (PPTase) that plays a critical role in activating enzymes for both primary and secondary metabolism. PptT possesses a deep binding pocket that does not readily accept labelled coenzyme A analogues that have previously been used to screen for PPTase inhibitors. Here we report on the development of a high throughput, colourimetric screen that monitors the PptT-mediated activation of the non-ribosomal peptide synthetase BpsA to a blue pigment (indigoidine) synthesising form in vitro. This screen uses unadulterated coenzyme A, avoiding analogues that may interfere with inhibitor binding, and requires only a single-endpoint measurement. We benchmark the screen using the well-characterised Library of Pharmaceutically Active Compounds (LOPAC1280) collection and show that it is both sensitive and able to distinguish weak from strong inhibitors. We further show that the BpsA assay can be applied to quantify the level of inhibition and generate consistent EC50 data. We anticipate these tools will facilitate both the screening of established chemical collections to identify new anti-mycobacterial drug leads and to guide the exploration of structure-activity landscapes to improve existing PPTase inhibitors.

Single START-domain protein Mtsp17 is involved in transcriptional regulation in Mycobacterium smegmatis.

Tuberculosis caused by the pathogen Mycobacterium tuberculosis (MTB), remains a significant threat to global health. Elucidating the mechanisms of essential MTB genes provides an important theoretical basis for drug exploitation. Gene mtsp17 is essential and is conserved in the Mycobacterium genus. Although Mtsp17 has a structure closely resembling typical steroidogenic acute regulatory protein-related lipid transfer (START) family proteins, its biological function is different. This study characterizes the transcriptomes of Mycobacterium smegmatis to explore the consequences of mtsp17 downregulation on gene expression. Suppression of the mtsp17 gene resulted in significant down-regulation of 3% and upregulation of 1% of all protein-coding genes. Expression of desA1, an essential gene involved in mycolic acid synthesis, and the anti-SigF antagonist MSMEG_0586 were down-regulated in the conditional Mtsp17 knockout mutant and up-regulated in the Mtsp17 over-expression strain. Trends in the changes of 70 of the 79 differentially expressed genes (Log2 fold change > 1.5) in the conditional Mtsp17 knockout strain were the same as in the SigF knockout strain. Our data suggest that Mtsp17 is likely an activator of desA1 and Mtsp17 regulates the SigF regulon by SigF regulatory pathways through the anti-SigF antagonist MSMEG_0586. Our findings indicate the role of Mtsp17 may be in transcriptional regulation, provide new insights into the molecular mechanisms of START family proteins, and uncover a new node in the regulatory network of mycobacteria.

Structure-based virtual screening of essential Mycobacterium tuberculosis enzymes AspS and KatG for potential inhibitors.

Mycobacterium tuberculosis (TB) is a leading global cause of disease-related death. Recent works have studied metabolic pathways of the mycobacterium, highlighting essential enzymes to target via competitive inhibition through computational molecular modeling to suppress the organism's life cycle. We used the Protein Databank (PDB), the UniProt Knowledgebase and the iDock server in this study. In vitro toxicity screening and pharmacokinetic properties were assessed to determine potential ligand safety and drug properties. Our results have revealed five and nine potential ligands for the enzymes AspS and KatG respectively. The KatG active site has displayed binding affinities of -13.443 to -12.895 kcal/mol, while AspS ligands range from -6.580 to -6.490kcal/mol. The intermolecular forces responsible for the differing binding affinities of each enzyme are primarily Coulombic interactions for AspS, versus Coulombic and extensive hydrogen bonding interactions in KatG.

Lack of Specificity of Phenotypic Screens for Inhibitors of the Mycobacterium tuberculosis FAS-II System.

Phenotypic screening of inhibitors of the essential Mycobacterium tuberculosis FAS-II dehydratase HadAB led to the identification of GSK3011724A, a compound previously reported to inhibit the condensation step of FAS-II. Whole-cell-based and cell-free assays confirmed the lack of activity of GSK3011724A against the dehydratase despite evidence of cross-resistance between GSK3011724A and HadAB inhibitors. The nature of the resistance mechanisms is suggestive of alterations in the FAS-II interactome reducing access of GSK3011724A to KasA.

Protein kinases PknA and PknB independently and coordinately regulate essential Mycobacterium tuberculosis physiologies and antimicrobial susceptibility.

The Mycobacterium tuberculosis Ser/Thr protein kinases PknA and PknB are essential for growth and have been proposed as possible drug targets. We used a titratable conditional depletion system to investigate the functions of these kinases. Depletion of PknA or PknB or both kinases resulted in growth arrest, shortening of cells, and time-dependent loss of acid-fast staining with a concomitant decrease in mycolate synthesis and accumulation of trehalose monomycolate. Depletion of PknA and/or PknB resulted in markedly increased susceptibility to β-lactam antibiotics, and to the key tuberculosis drug rifampin. Phosphoproteomic analysis showed extensive changes in protein phosphorylation in response to PknA depletion and comparatively fewer changes with PknB depletion. These results identify candidate substrates of each kinase and suggest specific and coordinate roles for PknA and PknB in regulating multiple essential physiologies. These findings support these kinases as targets for new antituberculosis drugs and provide a valuable resource for targeted investigation of mechanisms by which protein phosphorylation regulates pathways required for growth and virulence in M. tuberculosis.

Characterization of Tetrahydrolipstatin and Stereoderivatives on the Inhibition of Essential Mycobacterium tuberculosis Lipid Esterases.

Tetrahydrolipstatin (THL) is a covalent inhibitor of many serine esterases. In mycobacteria, THL has been found to covalently react with 261 lipid esterases upon treatment of Mycobacterium bovis cell lysate. However, the covalent adduct is considered unstable in some cases because of the hydrolysis of the enzyme-linked THL adduct resulting in catalytic turnover. In this study, a library of THL stereoderivatives was tested against three essential Mycobacterium tuberculosis lipid esterases of interest for drug development to assess how the stereochemistry of THL affects respective enzyme inhibition and allows for cross enzyme inhibition. The mycolyltransferase Antigen 85C (Ag85C) was found to be stereospecific with regard to THL; covalent inhibition occurs within minutes and was previously shown to be irreversible. Conversely, the Rv3802 phospholipase A/thioesterase was more accepting of a variety of THL configurations and uses these compounds as alternative substrates. The reaction of the THL stereoderivatives with the thioesterase domain of polyketide synthase 13 (Pks13-TE) also leads to hydrolytic turnover and is nonstereospecific but occurs on a slower, multihour time scale. Our findings suggest the stereochemistry of the β-lactone ring of THL is important for cross enzyme reactivity, while the two stereocenters of the peptidyl arm can affect enzyme specificity and the catalytic hydrolysis of the β-lactone ring. The observed kinetic data for all three target enzymes are supported by recently published X-ray crystal structures of Ag85C, Rv3802, and Pks13-TE. Insights from this study provide a molecular basis for the kinetic modulation of three essential M. tuberculosis lipid esterases by THL and can be applied to increase potency and enzyme residence times and enhance the specificity of the THL scaffold.

Multisystem Analysis of Mycobacterium tuberculosis Reveals Kinase-Dependent Remodeling of the Pathogen-Environment Interface.

ABSTRACTTuberculosis is the leading killer among infectious diseases worldwide. Increasing multidrug resistance has prompted new approaches for tuberculosis drug development, including targeted inhibition of virulence determinants and of signaling cascades that control many downstream pathways. We used a multisystem approach to determine the effects of a potent small-molecule inhibitor of the essential Mycobacterium tuberculosis Ser/Thr protein kinases PknA and PknB. We observed differential levels of phosphorylation of many proteins and extensive changes in levels of gene expression, protein abundance, cell wall lipids, and intracellular metabolites. The patterns of these changes indicate regulation by PknA and PknB of several pathways required for cell growth, including ATP synthesis, DNA synthesis, and translation. These data also highlight effects on pathways for remodeling of the mycobacterial cell envelope via control of peptidoglycan turnover, lipid content, a SigE-mediated envelope stress response, transmembrane transport systems, and protein secretion systems. Integrated analysis of phosphoproteins, transcripts, proteins, and lipids identified an unexpected pathway whereby threonine phosphorylation of the essential response regulator MtrA decreases its DNA binding activity. Inhibition of this phosphorylation is linked to decreased expression of genes for peptidoglycan turnover, and of genes for mycolyl transferases, with concomitant changes in mycolates and glycolipids in the cell envelope. These findings reveal novel roles for PknA and PknB in regulating multiple essential cell functions and confirm that these kinases are potentially valuable targets for new antituberculosis drugs. In addition, the data from these linked multisystems provide a valuable resource for future targeted investigations into the pathways regulated by these kinases in the M. tuberculosis cell.

Design, synthesis and evaluation against Mycobacterium tuberculosis of azole piperazine derivatives as dicyclotyrosine (cYY) mimics

Three series of azole piperazine derivatives that mimic dicyclotyrosine (cYY), the natural substrate of the essential Mycobacterium tuberculosis cytochrome P450 CYP121A1, were prepared and evaluated for binding affinity and inhibitory activity (MIC) against M. tuberculosis. Series A replaces one phenol group of cYY with a C3-imidazole moiety, series B includes a keto group on the hydrocarbon chain preceding the series A imidazole, whilst series C explores replacing the keto group of the piperidone ring of cYY with a CH2-imidazole or CH2-triazole moiety to enhance binding interaction with the heme of CYP121A1. The series displayed moderate to weak type II binding affinity for CYP121A1, with the exception of series B 10a, which displayed mixed type I binding. Of the three series, series C imidazole derivatives showed the best, although modest, inhibitory activity against M. tuberculosis (17d MIC = 12.5 μg/mL, 17a 50 μg/mL). Crystal structures were determined for CYP121A1 bound to series A compounds 6a and 6b that show the imidazole groups positioned directly above the haem iron with binding between the haem iron and imidazole nitrogen of both compounds at a distance of 2.2 Å. A model generated from a 1.5 Å crystal structure of CYP121A1 in complex with compound 10a showed different binding modes in agreement with the heterogeneous binding observed. Although the crystal structures of 6a and 6b would indicate binding with CYP121A1, the binding assays themselves did not allow confirmation of CYP121A1 as the target.

Identification of KasA as the cellular target of an anti-tubercular scaffold.

Phenotypic screens for bactericidal compounds are starting to yield promising hits against tuberculosis. In this regard, whole-genome sequencing of spontaneous resistant mutants generated against an indazole sulfonamide (GSK3011724A) identifies several specific single-nucleotide polymorphisms in the essential Mycobacterium tuberculosis β-ketoacyl synthase (kas) A gene. Here, this genomic-based target assignment is confirmed by biochemical assays, chemical proteomics and structural resolution of a KasA-GSK3011724A complex by X-ray crystallography. Finally, M. tuberculosis GSK3011724A-resistant mutants increase the in vitro minimum inhibitory concentration and the in vivo 99% effective dose in mice, establishing in vitro and in vivo target engagement. Surprisingly, the lack of target engagement of the related β-ketoacyl synthases (FabH and KasB) suggests a different mode of inhibition when compared with other Kas inhibitors of fatty acid biosynthesis in bacteria. These results clearly identify KasA as the biological target of GSK3011724A and validate this enzyme for further drug discovery efforts against tuberculosis.

Increasing the structural coverage of tuberculosis drug targets

High-resolution three-dimensional structures of essential Mycobacterium tuberculosis (Mtb) proteins provide templates for TB drug design, but are available for only a small fraction of the Mtb proteome. Here we evaluate an intra-genus "homolog-rescue" strategy to increase the structural information available for TB drug discovery by using mycobacterial homologs with conserved active sites. Of 179 potential TB drug targets selected for x-ray structure determination, only 16 yielded a crystal structure. By adding 1675 homologs from nine other mycobacterial species to the pipeline, structures representing an additional 52 otherwise intractable targets were solved. To determine whether these homolog structures would be useful surrogates in TB drug design, we compared the active sites of 106 pairs of Mtb and non-TB mycobacterial (NTM) enzyme homologs with experimentally determined structures, using three metrics of active site similarity, including superposition of continuous pharmacophoric property distributions. Pair-wise structural comparisons revealed that 19/22 pairs with >55% overall sequence identity had active site Cα RMSD <1Å, >85% side chain identity, and ≥80% PSAPF (similarity based on pharmacophoric properties) indicating highly conserved active site shape and chemistry. Applying these results to the 52 NTM structures described above, 41 shared >55% sequence identity with the Mtb target, thus increasing the effective structural coverage of the 179 Mtb targets over three-fold (from 9% to 32%). The utility of these structures in TB drug design can be tested by designing inhibitors using the homolog structure and assaying the cognate Mtb enzyme; a promising test case, Mtb cytidylate kinase, is described. The homolog-rescue strategy evaluated here for TB is also generalizable to drug targets for other diseases.

Crystal structure of the essential Mycobacterium tuberculosis phosphopantetheinyl transferase PptT, solved as a fusion protein with maltose binding protein.

Phosphopantetheinyl transferases (PPTases) are key enzymes in the assembly-line production of complex molecules such as fatty acids, polyketides and polypeptides, where they activate acyl or peptidyl carrier proteins, transferring a 4'-phosphopantetheinyl moiety from coenzyme A (CoA) to a reactive serine residue on the carrier protein. The human pathogen Mycobacterium tuberculosis encodes two PPTases, both essential and therefore attractive drug targets. We report the structure of the type-II PPTase PptT, obtained from crystals of a fusion protein with maltose binding protein. The structure, at 1.75Å resolution (R=0.156, Rfree=0.191), reveals an α/β fold broadly similar to other type-II PPTases, but with differences in peripheral structural elements. A bound CoA is clearly defined with its pantetheinyl arm tucked into a hydrophobic pocket. Interactions involving the CoA diphosphate, bound Mg(2+) and three active site acidic side chains suggest a plausible pathway for proton transfer during catalysis.

Screening, Identification, and Characterization of Mechanistically Diverse Inhibitors of the Mycobacterium Tuberculosis Enzyme, Pantothenate Kinase (CoaA)

The authors describe the discovery of anti-mycobacterial compounds through identifying mechanistically diverse inhibitors of the essential Mycobacterium tuberculosis (Mtb) enzyme, pantothenate kinase (CoaA). Target-driven drug discovery technologies often work with purified enzymes, and inhibitors thus discovered may not optimally inhibit the form of the target enzyme predominant in the bacterial cell or may not be available at the desired concentration. Therefore, in addition to addressing entry or efflux issues, inhibitors with diverse mechanisms of inhibition (MoI) could be prioritized before hit-to-lead optimization. The authors describe a high-throughput assay based on protein thermal melting to screen large numbers of compounds for hits with diverse MoI. Following high-throughput screening for Mtb CoaA enzyme inhibitors, a concentration-dependent increase in protein thermal stability was used to identify true binders, and the degree of enhancement or reduction in thermal stability in the presence of substrate was used to classify inhibitors as competitive or non/uncompetitive. The thermal shift–based MoI assay could be adapted to screen hundreds of compounds in a single experiment as compared to traditional biochemical approaches for MoI determination. This MoI was confirmed through mechanistic studies that estimated Kie and Kies for representative compounds and through nuclear magnetic resonance–based ligand displacement assays.

MycobacteriumtuberculosismtrA merodiploid strains with point mutations in the signal-receiving domain of MtrA exhibit growth defects in nutrient broth

The genetic and biochemical aspects of the essential Mycobacterium tuberculosis MtrAB two-component regulatory signal transduction (2CRS) system have not been extensively investigated. We show by bacterial two-hybrid assay that the response regulator (RR) MtrA and the sensor kinase MtrB interact. We further demonstrate that divalent metal ions [Mg(2+), Ca(2+) or both] promote MtrB kinase autophosphorylation activity, but only Mg(2+) promotes phosphotransfer to MtrA. Replacement of the conserved aspartic acid residues at positions 13 and 56 with alanine (D13A), glutamine (D56E) or asparagine (D56N) prevented MtrA phosphorylation, indicating that these residues are important for phosphorylation. The MtrA D56E and MtrA D13A proteins bound to the promoter of fbpB, the gene encoding antigen 85B protein, efficiently in the absence of phosphorylation, whereas MtrA D56N did not. We also show that M. tuberculosis mtrA merodiploids overproducing MtrA D13A, unlike cells overproducing wild-type MtrA, grow poorly in nutrient broth and show reduced expression of fbpB. These latter findings are reminiscent of a phenotype associated with MtrA overproduction during intramacrophage growth. Our results suggest that MtrA D13A behaves like a constitutively active response regulator and that further characterization of mtrA merodiploid strains will provide valuable clues to the MtrAB system.

Allosteric Activation Mechanism of the Mycobacterium tuberculosis Receptor Ser/Thr Protein Kinase, PknB

The essential Mycobacterium tuberculosis Ser/Thr protein kinase (STPK), PknB, plays a key role in regulating growth and division, but the structural basis of activation has not been defined. Here, we provide biochemical and structural evidence that dimerization through the kinase-domain (KD) N-lobe activates PknB by an allosteric mechanism. Promoting KD pairing using a small-molecule dimerizer stimulates the unphosphorylated kinase, and substitutions that disrupt N-lobe pairing decrease phosphorylation activity in vitro and in vivo. Multiple crystal structures of two monomeric PknB KD mutants in complex with nucleotide reveal diverse inactive conformations that contain large active-site distortions that propagate > 30 Å from the mutation site. These results define flexible, inactive structures of a monomeric bacterial receptor KD and show how "back-to-back" N-lobe dimerization stabilizes the active KD conformation. This general mechanism of bacterial receptor STPK activation affords insights into the regulation of homologous eukaryotic kinases that form structurally similar dimers.