Identification of the dehydratase component of the mycobacterial mycolic acid-synthesizing fatty acid synthase-II complex

Exploring the Antitubercular Activity of Anthranilic Acid Derivatives: From MabA (FabG1) Inhibition to Intrabacterial Acidification

Mycobacterium tuberculosis, the pathogen that causes tuberculosis, is responsible for the death of 1.5 million people each year and the number of bacteria resistant to the standard regimen is constantly increasing. This highlights the need to discover molecules that act on new M. tuberculosis targets. Mycolic acids, which are very long-chain fatty acids essential for M. tuberculosis viability, are synthesized by two types of fatty acid synthase (FAS) systems. MabA (FabG1) is an essential enzyme belonging to the FAS-II cycle. We have recently reported the discovery of anthranilic acids as MabA inhibitors. Here, the structure–activity relationships around the anthranilic acid core, the binding of a fluorinated analog to MabA by NMR experiments, the physico-chemical properties and the antimycobacterial activity of these inhibitors were explored. Further investigation of the mechanism of action in bacterio showed that these compounds affect other targets than MabA in mycobacterial cells and that their antituberculous activity is due to the carboxylic acid moiety which induces intrabacterial acidification.

Interconnection of the mycobacterial heparin-binding hemagglutinin with cholesterol degradation and heme/iron pathways identified by proximity-dependent biotin identification in Mycobacterium smegmatis

Deciphering protein-protein interactions is a critical step in the identification and the understanding of biological mechanisms deployed by pathogenic bacteria. The development of in vivo technologies to characterize these interactions is still in its infancy, especially for bacteria whose subcellular organization is particularly complex, such as mycobacteria. In this work, we used the proximity-dependent biotin identification (BioID) to define the mycobacterial heparin-binding hemagglutinin (HbhA) interactome in the saprophytic bacterium Mycobacterium smegmatis. M. smegmatis is a commonly used model to study and characterize the physiology of pathogenic mycobacteria, such as Mycobacterium tuberculosis. Here, we adapted the BioID technology to in vivo protein-protein interactions studies in M. smegmatis, which presents several advantages, such as maintaining the complex organization of the mycomembrane, offering the possibility to study membrane or cell wall-associated proteins, including HbhA, in the presence of cofactors and post-translational modifications, such as the complex methylation pattern of HbhA. Using this technology, we found that HbhA is interconnected with cholesterol degradation and heme/iron pathways. These results are in line with previous studies showing the dual localization of HbhA, associated with the cell wall and intracytoplasmic lipid inclusions, and its induction under high iron growth conditions.

The thick waxy coat of mycobacteria, a protective layer against antibiotics and the host's immune system

Tuberculosis, caused by the pathogenic bacterium Mycobacterium tuberculosis (Mtb), is the leading cause of death from an infectious disease, with a mortality rate of over a million people per year. This pathogen's remarkable resilience and infectivity is largely due to its unique waxy cell envelope, 40% of which comprises complex lipids. Therefore, an understanding of the structure and function of the cell wall lipids is of huge indirect clinical significance. This review provides a synopsis of the cell envelope and the major lipids contained within, including structure, biosynthesis and roles in pathogenesis.

Antibiotics and resistance: the two-sided coin of the mycobacterial cell wall

Mycobacterium tuberculosis, the bacterium responsible for tuberculosis, is the global leading cause of mortality from an infectious agent. Part of this success relies on the unique cell wall, which consists of a thick waxy coat with tightly packed layers of complexed sugars, lipids and peptides. This coat provides a protective hydrophobic barrier to antibiotics and the host's defences, while enabling the bacterium to spread efficiently through sputum to infect and survive within the macrophages of new hosts. However, part of this success comes at a cost, with many of the current first- and second-line drugs targeting the enzymes involved in cell wall biosynthesis. The flip side of this coin is that resistance to these drugs develops either in the target enzymes or the activation pathways of the drugs, paving the way for new resistant clinical strains. This review provides a synopsis of the structure and synthesis of the cell wall and the major current drugs and targets, along with any mechanisms of resistance.

Discovery of the first Mycobacterium tuberculosis MabA (FabG1) inhibitors through a fragment-based screening

Mycobacterium tuberculosis (M.tb), the etiologic agent of tuberculosis, remains the leading cause of death from a single infectious agent worldwide. The emergence of drug-resistant M.tb strains stresses the need for drugs acting on new targets. Mycolic acids are very long chain fatty acids playing an essential role in the architecture and permeability of the mycobacterial cell wall. Their biosynthesis involves two fatty acid synthase (FAS) systems. Among the four enzymes (MabA, HadAB/BC, InhA and KasA/B) of the FAS-II cycle, MabA (FabG1) remains the only one for which specific inhibitors have not been reported yet. The development of a new LC-MS/MS based enzymatic assay allowed the screening of a 1280 fragment-library and led to the discovery of the first small molecules that inhibit MabA activity. A fragment from the anthranilic acid series was optimized into more potent inhibitors and their binding to MabA was confirmed by ¹⁹F ligand-observed NMR experiments.

Transport mechanism of Mycobacterium tuberculosis MmpL/S family proteins and implications in pharmaceutical targeting

Tuberculosis caused by Mycobacterium tuberculosis remains a serious threat to public health. The M. tuberculosis cell envelope is closely related to its virulence and drug resistance. Mycobacterial membrane large proteins (MmpL) are lipid-transporting proteins of the efflux pump resistance nodulation cell division (RND) superfamily with lipid substrate specificity and non-transport lipid function. Mycobacterial membrane small proteins (MmpS) are small regulatory proteins, and they are also responsible for some virulence-related effects as accessory proteins of MmpL. The MmpL transporters are the candidate targets for the development of anti-tuberculosis drugs. This article summarizes the structure, function, phylogenetics of M. tuberculosis MmpL/S proteins and their roles in host immune response, inhibitors and regulatory system.

Mechanisms of Resistance Associated with the Inhibition of the Dehydration Step of Type II Fatty Acid Synthase in Mycobacterium tuberculosis

Isoxyl (ISO) and thiacetazone (TAC) are two antitubercular prodrugs that abolish mycolic acid biosynthesis and kill Mycobacterium tuberculosis (Mtb) through the inhibition of the essential type II fatty acid synthase (FAS-II) dehydratase HadAB. While mutations preventing ISO and TAC either from being converted to their active form or from covalently modifying their target are the most frequent spontaneous mutations associated with high-level resistance to both drugs, the molecular mechanisms underlying the high-level ISO and TAC resistance of Mtb strains harboring missense mutations in the second, nonessential, FAS-II dehydratase HadBC have remained unexplained. Using a combination of genetic, biochemical, and biophysical approaches and molecular dynamics simulation, we here show that all four reported resistance mutations in the HadC subunit of HadBC alter the stability and/or specific activity of the enzyme, allowing it in two cases (HadBC^V85I and HadBC^K157R) to compensate for a deficiency in HadAB in whole Mtb bacilli. The analysis of the mycolic acid profiles of Mtb strains expressing the mutated forms of HadC further points to alterations in the activity of the mycolic acid biosynthetic complex and suggests an additional contributing resistance mechanism whereby HadC mutations may reduce the accessibility of HadAB to ISO and TAC. Collectively, our results highlight the importance of developing optimized inhibitors of the dehydration step of FAS-II capable of inhibiting both dehydratases simultaneously, a goal that may be achievable given the structural resemblance of the two enzymes and their reliance on the same catalytic subunit HadB.

Kinase Targets for Mycolic Acid Biosynthesis in Mycobacterium tuberculosis

BACKGROUND

Mycolic acids (MAs) are the characteristic, integral building blocks for the mycomembrane belonging to the insidious bacterial pathogen Mycobacterium tuberculosis (M.tb). These C60-C90 long α-alkyl-β-hydroxylated fatty acids provide protection to the tubercle bacilli against the outside threats, thus allowing its survival, virulence and resistance to the current antibacterial agents. In the post-genomic era, progress has been made towards understanding the crucial enzymatic machineries involved in the biosynthesis of MAs in M.tb. However, gaps still remain in the exact role of the phosphorylation and dephosphorylation of regulatory mechanisms within these systems. To date, a total of 11 serine-threonine protein kinases (STPKs) are found in M.tb. Most enzymes implicated in the MAs synthesis were found to be phosphorylated in vitro and/or in vivo. For instance, phosphorylation of KasA, KasB, mtFabH, InhA, MabA, and FadD32 downregulated their enzymatic activity, while phosphorylation of VirS increased its enzymatic activity. These observations suggest that the kinases and phosphatases system could play a role in M.tb adaptive responses and survival mechanisms in the human host. As the mycobacterial STPKs do not share a high sequence homology to the human's, there have been some early drug discovery efforts towards developing potent and selective inhibitors.

OBJECTIVE

Recent updates to the kinases and phosphatases involved in the regulation of MAs biosynthesis will be presented in this mini-review, including their known small molecule inhibitors.

CONCLUSION

Mycobacterial kinases and phosphatases involved in the MAs regulation may serve as a useful avenue for antitubercular therapy.

Roles of Alanine Dehydrogenase and Induction of Its Gene in Mycobacterium smegmatis under Respiration-Inhibitory Conditions

Here we demonstrated that the inhibition of electron flux through the respiratory electron transport chain (ETC) by either the disruption of the gene for the major terminal oxidase (aa₃ cytochrome c oxidase) or treatment with KCN resulted in the induction of ald encoding alanine dehydrogenase in Mycobacterium smegmatis A decrease in functionality of the ETC shifts the redox state of the NADH/NAD⁺ pool toward a more reduced state, which in turn leads to an increase in cellular levels of alanine by Ald catalyzing the conversion of pyruvate to alanine with the concomitant oxidation of NADH to NAD⁺ The induction of ald expression under respiration-inhibitory conditions in M. smegmatis is mediated by the alanine-responsive AldR transcriptional regulator. The growth defect of M. smegmatis by respiration inhibition was exacerbated by inactivation of the ald gene, suggesting that Ald is beneficial to M. smegmatis in its adaptation and survival under respiration-inhibitory conditions by maintaining NADH/NAD⁺ homeostasis. The low susceptibility of M. smegmatis to bcc₁ complex inhibitors appears to be, at least in part, attributable to the high expression level of the bd quinol oxidase in M. smegmatis when the bcc₁-aa₃ branch of the ETC is inactivated.IMPORTANCE We demonstrated that the functionality of the respiratory electron transport chain is inversely related to the expression level of the ald gene encoding alanine dehydrogenase in Mycobacterium smegmatis Furthermore, the importance of Ald in NADH/NAD⁺ homeostasis during the adaptation of M. smegmatis to severe respiration-inhibitory conditions was demonstrated in this study. On the basis of these results, we propose that combinatory regimens including both an Ald-specific inhibitor and respiration-inhibitory antitubercular drugs such as Q203 and bedaquiline are likely to enable a more efficient therapy for tuberculosis.

Phylogeny Trumps Chemotaxonomy: A Case Study Involving Turicella otitidis

The genus Turicella was proposed to harbor clinical strains isolated from middle-ear fluids of patients with otitis media. 16S rRNA phylogeny showed that it belonged to the mycolic acid-containing actinobacteria, currently classified in the order Corynebacteriales, and was closely related to the genus Corynebacterium. A new genus was proposed for the organisms as unlike corynebacteria they lacked mycolic acids and had different menaquinones. Here, we carried out large-scale comparative genomics on representative strains of the genera Corynebacterium and Turicella to check if this chemotaxonomic classification is justified. Three genes that are known to play an essential role in mycolic acid biosynthesis were absent in Turicella and two other mycolate-less Corynebacterium spp., explaining the lack of mycolic acids resulted from the deletion of genes and does not confer any phylogenetic context. Polyphasic phylogenetic analyses using 16S rRNA, bacterial core genes and genes responsible for synthesizing menaquinones unequivocally indicate that Turicella is a true member of the genus Corynebacterium. Here, we demonstrate that menaquinone and mycolic acid that have been used as critical taxonomic markers should be interpreted carefully, particularly when genome-based taxonomy is readily available. Based on the phylogenetic analysis, we propose to reclassify Turicella otitidis as Corynebacterium otitidis comb. nov.

Disruption of key NADH-binding pocket residues of the Mycobacterium tuberculosis InhA affects DD-CoA binding ability

Tuberculosis (TB) is a global health problem that affects over 10 million people. There is an urgent need to develop novel antimicrobial therapies to combat TB. To achieve this, a thorough understanding of key validated drug targets is required. The enoyl reductase InhA, responsible for synthesis of essential mycolic acids in the mycobacterial cell wall, is the target for the frontline anti-TB drug isoniazid. To better understand the activity of this protein a series of mutants, targeted to the NADH co-factor binding pocket were created. Residues P193 and W222 comprise a series of hydrophobic residues surrounding the cofactor binding site and mutation of both residues negatively affect InhA function. Construction of an M155A mutant of InhA results in increased affinity for NADH and DD-CoA turnover but with a reduction in V_max for DD-CoA, impairing overall activity. This suggests that NADH-binding geometry of InhA likely permits long-range interactions between residues in the NADH-binding pocket to facilitate substrate turnover in the DD-CoA binding region of the protein. Understanding the precise details of substrate binding and turnover in InhA and how this may affect protein-protein interactions may facilitate the development of improved inhibitors enabling the development of novel anti-TB drugs.

Metabolomic analysis based on 1H-nuclear magnetic resonance spectroscopy metabolic profiles in tuberculous, malignant and transudative pleural effusion

Pleural effusion is a common clinical manifestation with various causes. Current diagnostic and therapeutic methods have exhibited numerous limitations. By involving the analysis of dynamic changes in low molecular weight catabolites, metabolomics has been widely applied in various types of disease and have provided platforms to distinguish many novel biomarkers. However, to the best of our knowledge, there are few studies regarding the metabolic profiling for pleural effusion. In the current study, 58 pleural effusion samples were collected, among which 20 were malignant pleural effusions, 20 were tuberculous pleural effusions and 18 were transudative pleural effusions. The small molecule metabolite spectrums were obtained by adopting ¹H nuclear magnetic resonance technology, and pattern-recognition multi-variable statistical analysis was used to screen out different metabolites. One-way analysis of variance, and Student-Newman-Keuls and the Kruskal-Wallis test were adopted for statistical analysis. Over 400 metabolites were identified in the untargeted metabolomic analysis and 26 metabolites were identified as significantly different among tuberculous, malignant and transudative pleural effusions. These metabolites were predominantly involved in the metabolic pathways of amino acids metabolism, glycometabolism and lipid metabolism. Statistical analysis revealed that eight metabolites contributed to the distinction between the three groups: Tuberculous, malignant and transudative pleural effusion. In the current study, the feasibility of identifying small molecule biochemical profiles in different types of pleural effusion were investigated reveal novel biological insights into the underlying mechanisms. The results provide specific insights into the biology of tubercular, malignant and transudative pleural effusion and may offer novel strategies for the diagnosis and therapy of associated diseases, including tuberculosis, advanced lung cancer and congestive heart failure.

The Non-Essential Mycolic Acid Biosynthesis Genes hadA and hadC Contribute to the Physiology and Fitness of Mycobacterium smegmatis

Gram positive mycobacteria with a high GC content, such as the etiological agent of tuberculosis Mycobacterium tuberculosis, possess an outer membrane mainly composed of mycolic acids (MAs), the so-called mycomembrane, which is essential for the cell. About thirty genes are involved in the biosynthesis of MAs, which include the hadA, hadB and hadC genes that encode the dehydratases Fatty Acid Synthase type II (FAS-II) known to function as the heterodimers HadA-HadB and HadB-HadC. The present study shows that M. smegmatis cells remain viable in the absence of either HadA and HadC or both. Inactivation of HadC has a dramatic effect on the physiology and fitness of the mutant strains whereas that of HadA exacerbates the phenotype of a hadC deletion. The hadC mutants exhibit a novel MA profile, display a distinct colony morphology, are less aggregated, are impaired for sliding motility and biofilm development and are more resistant to detergent. Conversely, the hadC mutants are significantly more susceptible to low- and high-temperature and to selective toxic compounds, including several current anti-tubercular drugs.

Evolution of Mycolic Acid Biosynthesis Genes and Their Regulation during Starvation in Mycobacterium tuberculosis

Mycobacterium tuberculosis, the etiological agent of tuberculosis, is a Gram-positive bacterium with a unique cell envelope composed of an essential outer membrane. Mycolic acids, which are very-long-chain (up to C100) fatty acids, are the major components of this mycomembrane. The enzymatic pathways involved in the biosynthesis and transport of mycolates are fairly well documented and are the targets of the major antituberculous drugs. In contrast, only fragmented information is available on the expression and regulation of the biosynthesis genes. In this study, we report that the hadA, hadB, and hadC genes, which code for the mycolate biosynthesis dehydratase enzymes, are coexpressed with three genes that encode proteins of the translational apparatus. Consistent with the well-established control of the translation potential by nutrient availability, starvation leads to downregulation of the hadABC genes along with most of the genes required for the synthesis, modification, and transport of mycolates. The downregulation of a subset of the biosynthesis genes is partially dependent on RelMtb, the key enzyme of the stringent response. We also report the phylogenetic evolution scenario that has shaped the current genetic organization, characterized by the coregulation of the hadABC operon with genes of the translational apparatus and with genes required for the modification of the mycolates.

IMPORTANCE

Mycobacterium tuberculosis infects one-third of the human population worldwide, and despite the available therapeutic arsenal, it continues to kill millions of people each year. There is therefore an urgent need to identify new targets and develop a better understanding of how the bacterium is adapting itself to host defenses during infection. A prerequisite of this understanding is knowledge of how this adaptive skill has been implanted by evolution. Nutrient scarcity is an environmental condition the bacterium has to cope with during infection. In many bacteria, adaptation to starvation relies partly on the stringent response. M. tuberculosis's unique outer membrane layer, the mycomembrane, is crucial for its viability and virulence. Despite its being the target of the major antituberculosis drugs, only scattered information exists on how the genes required for biosynthesis of the mycomembrane are expressed and regulated during starvation. This work has addressed this issue as a step toward the identification of new targets in the fight against M. tuberculosis.

Mycolic acids: deciphering and targeting the Achilles' heel of the tubercle bacillus

Mycolic acids are unique long chain fatty acids found in the lipid-rich cell walls of mycobacteria including the tubercle bacillus Mycobacterium tuberculosis. Essential for viability and virulence, enzymes involved in the biosynthesis of mycolic acids represent novel targets for drug development. This is particularly relevant to the impact on global health given the rise of multidrug resistant and extensively drug resistant strains of M. tuberculosis. In this review, we discuss recent advances in our understanding of how mycolic acid are synthesised, especially the potential role of specialised fatty acid synthase complexes. Also, we examine the role of a recently reported mycolic acid transporter MmpL3 with reference to several reports of the targeting of this transporter by diverse compounds with anti-M. tuberculosis activity. Additionally, we consider recent findings that place mycolic acid biosynthesis in the context of the cell biology of the bacterium, viz its localisation and co-ordination with the bacterial cytoskeleton, and its role beyond maintaining cell envelope integrity.

Expression, purification and crystallization of the (3R)-hydroxyacyl-ACP dehydratase HadAB complex from Mycobacterium tuberculosis

The (3R)-hydroxyacyl-ACP dehydratase HadAB, involved in the biosynthetic pathway for mycolic acid (MA) of Mycobacterium tuberculosis, catalyzes the third step in the fatty acid (FA) elongation cycle, which is an ideal and actual target for anti-tubercular agent. Though HadAB is predicted to be a member of the hotdog superfamily, it shares no sequence identity with typical hotdog fold isoenzyme FabZ. To characterize the significance of HadAB from the perspective of structural biology, large amount of pure HadAB complex is required for biochemical characterization and crystallization. Here, we used a unique expression and purification method. HadA and HadB were cloned separately and co-expressed in Escherichia coli. After GST affinity chromatography, two steps of anion exchange chromatography and gel filtration, the purity of the protein as estimated by SDS-PAGE was >95%. Using hanging-drop vapor-diffusion method, crystals were obtained and diffracted X-rays to 1.75Å resolution. The crystal belongs to space group P41212, with unit-cell parameters a=b=82.0Å, c=139.8Å, α=β=γ=90.0°.

Molecular basis for the inhibition of β-hydroxyacyl-ACP dehydratase HadAB complex from Mycobacterium tuberculosis by flavonoid inhibitors

Dehydration is one of the key steps in the biosynthesis of mycolic acids and is vital to the growth of Mycobacterium tuberculosis (Mtb). Consequently, stalling dehydration cures tuberculosis (TB). Clinically used anti-TB drugs like thiacetazone (TAC) and isoxyl (ISO) as well as flavonoids inhibit the enzyme activity of the β-hydroxyacyl-ACP dehydratase HadAB complex. How this inhibition is exerted, has remained an enigma for years. Here, we describe the first crystal structures of the MtbHadAB complex bound with flavonoid inhibitor butein, 2',4,4'-trihydroxychalcone or fisetin. Despite sharing no sequence identity from Blast, HadA and HadB adopt a very similar hotdog fold. HadA forms a tight dimer with HadB in which the proteins are sitting side-by-side, but are oriented anti-parallel. While HadB contributes the catalytically critical His-Asp dyad, HadA binds the fatty acid substrate in a long channel. The atypical double hotdog fold with a single active site formed by MtbHadAB gives rise to a long, narrow cavity that vertically traverses the fatty acid binding channel. At the base of this cavity lies Cys61, which upon mutation to Ser confers drug-resistance in TB patients. We show that inhibitors bind in this cavity and protrude into the substrate binding channel. Thus, inhibitors of MtbHadAB exert their effect by occluding substrate from the active site. The unveiling of this mechanism of inhibition paves the way for accelerating development of next generation of anti-TB drugs.

A new dehydratase conferring innate resistance to thiacetazone and intra-amoebal survival of Mycobacterium smegmatis

Nontuberculous mycobacteria are innately resistant to most antibiotics, although the mechanisms responsible for their drug resistance remain poorly understood. They are particularly refractory to thiacetazone (TAC), a second-line antitubercular drug. Herein, we identified MSMEG_6754 as essential for the innate resistance of Mycobacterium smegmatis to TAC. Transposon-mediated and targeted disruption of MSMEG_6754 resulted in hypersusceptibility to TAC. Conversely, introduction of MSMEG_6754 into Mycobacterium tuberculosis increased resistance 100-fold. Resolution of the crystal structure of MSMEG_6754 revealed a homodimer in which each monomer comprises two hot-dog domains characteristic of dehydratase-like proteins and very similar to the HadAB complex involved in mycolic acid biosynthesis. Gene inactivation of the essential hadB dehydratase could be achieved in M. smegmatis and M. tuberculosis only when the strains carried an integrated copy of MSMEG_6754, supporting the idea that MSMEG_6754 and HadB share redundant dehydratase activity. Using M. smegmatis-Acanthamoeba co-cultures, we found that intra-amoebal growth of the MSMEG_6754 deleted strain was significantly reduced compared with the parental strain. This in vivo growth defect was fully restored upon complementation with catalytically active MSMEG_6754 or HadABC, indicating that MSMEG_6754 plays a critical role in the survival of M. smegmatis within the environmental host.

Regulation of the ahpC gene encoding alkyl hydroperoxide reductase in Mycobacterium smegmatis

The ahpC (MSMEG_4891) gene encodes alkyl hydroperoxide reductase C in Mycobacterium smegmatis mc²155 and its expression is induced under oxidative stress conditions. Two well-defined inverted repeat sequences (IR1 and IR2) were identified in the upstream region of ahpC. Using a crp (cAMP receptor protein: MSMEG_6189) mutant and in vitro DNA-binding assay, it was demonstrated that the IR1 sequence serves as a Crp-binding site and that Crp functions as an activator in the regulation of ahpC expression. The expression level of ahpC was shown to be proportional to intracellular cAMP levels. Intracellular levels of cAMP were increased in M. smegmatis, when it was treated with oxidative stress inducers. The IR2 sequence is very similar to the known consensus sequence of FurA-binding sites and involved in the negative regulation of ahpC expression. Taken together, these results suggest that the induction of ahpC expression under oxidative stress conditions probably results from a combinatory effect of both inactivation of FurA by oxidative stress and activation of Crp in response to increased levels of cAMP.

The Molecular Genetics of Mycolic Acid Biosynthesis

Mycolic acids are major and specific long-chain fatty acids that represent essential components of the Mycobacterium tuberculosis cell envelope. They play a crucial role in the cell wall architecture and impermeability, hence the natural resistance of mycobacteria to most antibiotics, and represent key factors in mycobacterial virulence. Biosynthesis of mycolic acid precursors requires two types of fatty acid synthases (FASs), the eukaryotic-like multifunctional enzyme FAS I and the acyl carrier protein (ACP)–dependent FAS II systems, which consists of a series of discrete mono-functional proteins, each catalyzing one reaction in the pathway. Unlike FAS II synthases of other bacteria, the mycobacterial FAS II is incapable of de novo fatty acid synthesis from acetyl-coenzyme A, but instead elongates medium-chain-length fatty acids previously synthesized by FAS I, leading to meromycolic acids. In addition, mycolic acid subspecies with defined biological properties can be distinguished according to the chemical modifications decorating the meromycolate. Nearly all the genetic components involved in both elongation and functionalization of the meromycolic acid have been identified and are generally clustered in distinct transcriptional units. A large body of information has been generated on the enzymology of the mycolic acid biosynthetic pathway and on their genetic and biochemical/structural characterization as targets of several antitubercular drugs. This chapter is a comprehensive overview of mycolic acid structure, function, and biosynthesis. Special emphasis is given to recent work addressing the regulation of mycolic acid biosynthesis, adding new insights to our understanding of how pathogenic mycobacteria adapt their cell wall composition in response to environmental changes.

Mycobacterium tuberculosis Lsr2 is a global transcriptional regulator required for adaptation to changing oxygen levels and virulence

To survive a dynamic host environment, Mycobacterium tuberculosis must endure a series of challenges, from reactive oxygen and nitrogen stress to drastic shifts in oxygen availability. The mycobacterial Lsr2 protein has been implicated in reactive oxygen defense via direct protection of DNA. To examine the role of Lsr2 in pathogenesis and physiology of M. tuberculosis, we generated a strain deleted for lsr2. Analysis of the M. tuberculosis Δlsr2 strain demonstrated that Lsr2 is not required for DNA protection, as this strain was equally susceptible as the wild type to DNA-damaging agents. The lsr2 mutant did display severe growth defects under normoxic and hyperoxic conditions, but it was not required for growth under low-oxygen conditions. However, it was also required for adaptation to anaerobiosis. The defect in anaerobic adaptation led to a marked decrease in viability during anaerobiosis, as well as a lag in recovery from it. Gene expression profiling of the Δlsr2 mutant under aerobic and anaerobic conditions in conjunction with published DNA binding-site data indicates that Lsr2 is a global transcriptional regulator controlling adaptation to changing oxygen levels. The Δlsr2 strain was capable of establishing an early infection in the BALB/c mouse model; however, it was severely defective in persisting in the lungs and caused no discernible lung pathology. These findings demonstrate M. tuberculosis Lsr2 is a global transcriptional regulator required for control of genes involved in adaptation to extremes in oxygen availability and is required for persistent infection.

M. tuberculosis causes nearly two million deaths per year and infects nearly one-third of the world population. The success of this aerobic pathogen is due in part to its ability to successfully adapt to constantly changing oxygen availability throughout the infectious cycle, from the high oxygen tension during aerosol transmission to anaerobiosis within necrotic lesions. An understanding of how M. tuberculosis copes with these changes in oxygen tension is critical for its eventual eradication. Using a mutation in lsr2, we demonstrate that the Lsr2 protein present in all mycobacteria is a global transcriptional regulator in control of genes required for adaptation to changes in oxygen levels. M. tuberculosis lacking lsr2 was unable to adapt to both high and very low levels of oxygen and was defective in long-term anaerobic survival. Lsr2 was also required for disease pathology and for chronic infection in a mouse model of TB.

Mycolic acids: structures, biosynthesis, and beyond

Mycolic acids are major and specific lipid components of the mycobacterial cell envelope and are essential for the survival of members of the genus Mycobacterium that contains the causative agents of both tuberculosis and leprosy. In the alarming context of the emergence of multidrug-resistant, extremely drug-resistant, and totally drug-resistant tuberculosis, understanding the biosynthesis of these critical determinants of the mycobacterial physiology is an important goal to achieve, because it may open an avenue for the development of novel antimycobacterial agents. This review focuses on the chemistry, structures, and known inhibitors of mycolic acids and describes progress in deciphering the mycolic acid biosynthetic pathway. The functional and key biological roles of these molecules are also discussed, providing a historical perspective in this dynamic area.

MmpL11 protein transports mycolic acid-containing lipids to the mycobacterial cell wall and contributes to biofilm formation in Mycobacterium smegmatis

A growing body of evidence indicates that MmpL (mycobacterial membrane protein large) transporters are dedicated to cell wall biosynthesis and transport mycobacterial lipids. How MmpL transporters function and the identities of their substrates have not been fully elucidated. We report the characterization of Mycobacterium smegmatis MmpL11. We showed previously that M. smegmatis lacking MmpL11 has reduced membrane permeability that results in resistance to host antimicrobial peptides. We report herein the further characterization of the M. smegmatis mmpL11 mutant and identification of the MmpL11 substrates. We found that biofilm formation by the M. smegmatis mmpL11 mutant was distinct from that by wild-type M. smegmatis. Analysis of cell wall lipids revealed that the mmpL11 mutant failed to export the mycolic acid-containing lipids monomeromycolyl diacylglycerol and mycolate ester wax to the bacterial surface. In addition, analysis of total lipids indicated that the mycolic acid-containing precursor molecule mycolyl phospholipid accumulated in the mmpL11 mutant compared with wild-type mycobacteria. MmpL11 is encoded at a chromosomal locus that is conserved across pathogenic and nonpathogenic mycobacteria. Phenotypes of the M. smegmatis mmpL11 mutant are complemented by the expression of M. smegmatis or M. tuberculosis MmpL11, suggesting that MmpL11 plays a conserved role in mycobacterial cell wall biogenesis.

Point mutations within the fatty acid synthase type II dehydratase components HadA or HadC contribute to isoxyl resistance in Mycobacterium tuberculosis

The mechanism by which the antitubercular drug isoxyl (ISO) inhibits mycolic acid biosynthesis has not yet been reported. We found that point mutations in either the HadA or HadC component of the type II fatty acid synthase (FAS-II) are associated with increased levels of resistance to ISO in Mycobacterium tuberculosis. Overexpression of the HadAB, HadBC, or HadABC heterocomplex also produced high-level resistance. These results show that the FAS-II dehydratases are involved in ISO resistance.

MmpL genes are associated with mycolic acid metabolism in mycobacteria and corynebacteria

Mycolic acids are vital components of the cell wall of the tubercle bacillus Mycobacterium tuberculosis and are required for viability and virulence. While mycolic acid biosynthesis is studied extensively, components involved in mycolate transport remain unidentified. We investigated the role of large membrane proteins encoded by mmpL genes in mycolic acid transport in mycobacteria and the related corynebacteria. MmpL3 was found to be essential in mycobacteria and conditional depletion of MmpL3 in Mycobacterium smegmatis resulted in loss of cell wall mycolylation, and of the cell wall-associated glycolipid, trehalose dimycolate. In parallel, an accumulation of trehalose monomycolate (TMM) was observed, suggesting that mycolic acids were transported as TMM. In contrast to mycobacteria, we found redundancy in the role of two mmpL genes, in Corynebacterium glutamicum; a complete loss of trehalose-associated and cell wall bound corynomycolates was observed in an NCgl0228-NCgl2769 double mutant, but not in individual single mutants. Our studies highlight the role of mmpL genes in mycolic acid metabolism and identify potential new targets for anti-TB drug development.

Involvement of two latex-clearing proteins during rubber degradation and insights into the subsequent degradation pathway revealed by the genome sequence of Gordonia polyisoprenivorans strain VH2

The increasing production of synthetic and natural poly(cis-1,4-isoprene) rubber leads to huge challenges in waste management. Only a few bacteria are known to degrade rubber, and little is known about the mechanism of microbial rubber degradation. The genome of Gordonia polyisoprenivorans strain VH2, which is one of the most effective rubber-degrading bacteria, was sequenced and annotated to elucidate the degradation pathway and other features of this actinomycete. The genome consists of a circular chromosome of 5,669,805 bp and a circular plasmid of 174,494 bp with average GC contents of 67.0% and 65.7%, respectively. It contains 5,110 putative protein-coding sequences, including many candidate genes responsible for rubber degradation and other biotechnically relevant pathways. Furthermore, we detected two homologues of a latex-clearing protein, which is supposed to be a key enzyme in rubber degradation. The deletion of these two genes for the first time revealed clear evidence that latex-clearing protein is essential for the microbial utilization of rubber. Based on the genome sequence, we predict a pathway for the microbial degradation of rubber which is supported by previous and current data on transposon mutagenesis, deletion mutants, applied comparative genomics, and literature search.

The Mycobacterium tuberculosis FAS-II dehydratases and methyltransferases define the specificity of the mycolic acid elongation complexes

BACKGROUND

The human pathogen Mycobacterium tuberculosis (Mtb) has the originality of possessing a multifunctional mega-enzyme FAS-I (Fatty Acid Synthase-I), together with a multi-protein FAS-II system, to carry out the biosynthesis of common and of specific long chain fatty acids: the mycolic acids (MA). MA are the main constituents of the external mycomembrane that represents a tight permeability barrier involved in the pathogenicity of Mtb. The MA biosynthesis pathway is essential and contains targets for efficient antibiotics. We have demonstrated previously that proteins of FAS-II interact specifically to form specialized and interconnected complexes. This finding suggested that the organization of FAS-II resemble to the architecture of multifunctional mega-enzyme like the mammalian mFAS-I, which is devoted to the fatty acid biosynthesis.

PRINCIPAL FINDINGS

Based on conventional and reliable studies using yeast-two hybrid, yeast-three-hybrid and in vitro Co-immunoprecipitation, we completed here the analysis of the composition and architecture of the interactome between the known components of the Mtb FAS-II complexes. We showed that the recently identified dehydratases HadAB and HadBC are part of the FAS-II elongation complexes and may represent a specific link between the core of FAS-II and the condensing enzymes of the system. By testing four additional methyltransferases involved in the biosynthesis of mycolic acids, we demonstrated that they display specific interactions with each type of complexes suggesting their coordinated action during MA elongation.

SIGNIFICANCE

These results provide a global update of the architecture and organization of a FAS-II system. The FAS-II system of Mtb is organized in specialized interconnected complexes and the specificity of each elongation complex is given by preferential interactions between condensing enzymes and dehydratase heterodimers. This study will probably allow defining essential and specific interactions that correspond to promising targets for Mtb FAS-II inhibitors.

AccD6, a key carboxyltransferase essential for mycolic acid synthesis in Mycobacterium tuberculosis, is dispensable in a nonpathogenic strain

Acetyl coenzyme A carboxylase (ACC) is a key enzyme providing a substrate for mycolic acid biosynthesis. Although in vitro studies have demonstrated that the protein encoded by accD6 (Rv2247) may be a functional carboxyltransferase subunit of ACC in Mycobacterium tuberculosis, the in vivo function and regulation of accD6 in slow- and fast-growing mycobacteria remain elusive. Here, directed mutagenesis demonstrated that although accD6 is essential for M. tuberculosis, it can be deleted in Mycobacterium smegmatis without affecting its cell envelope integrity. Moreover, we showed that although it is part of the type II fatty acid synthase operon, the accD6 gene of M. tuberculosis, but not that of M. smegmatis, possesses its own additional promoter (P(acc)). The expression level of accD6(Mtb) placed only under the control of P(acc) is 10-fold lower than that in wild-type M. tuberculosis but is sufficient to sustain cell viability. Importantly, this limited expression level affects growth, mycolic acid content, and cell morphology. These results provide the first in vivo evidence for AccD6 as a key player in the mycolate biosynthesis of M. tuberculosis, implicating AccD6 as the essential ACC subunit in pathogenic mycobacteria and an excellent target for new antitubercular compounds. Our findings also highlight important differences in the mechanism of acetyl carboxylation between pathogenic and nonpathogenic mycobacterial species.

Negative regulation by Ser/Thr phosphorylation of HadAB and HadBC dehydratases from Mycobacterium tuberculosis type II fatty acid synthase system

The type II fatty acid synthase system of mycobacteria is involved in the biosynthesis of major and essential lipids, mycolic acids, key-factors of Mycobacterium tuberculosis pathogenicity. One reason of the remarkable survival ability of M. tuberculosis in infected hosts is partly related to the presence of cell wall-associated mycolic acids. Despite their importance, the mechanisms that modulate synthesis of these lipids in response to environmental changes are unknown. We demonstrate here that HadAB and HadBC dehydratases of this system are phosphorylated by Ser/Thr protein kinases, which negatively affects their enzymatic activity. The phosphorylation of HadAB/BC is growth phase-dependent, suggesting that it represents a mechanism by which mycobacteria might tightly control mycolic acid biosynthesis under non-replicating condition.

Phosphorylation of the Mycobacterium tuberculosis beta-ketoacyl-acyl carrier protein reductase MabA regulates mycolic acid biosynthesis

Mycolic acids are key cell wall components for the survival, pathogenicity, and antibiotic resistance of the human tubercle bacillus. Although it was thought that Mycobacterium tuberculosis tightly regulates their production to adapt to prevailing environmental conditions, the molecular mechanisms governing mycolic acid biosynthesis remained extremely obscure. Meromycolic acids, the direct precursors of mycolic acids, are synthesized by a type II fatty acid synthase from acyl carrier protein-bound substrates that are extended iteratively, with a reductive cycle in each round of extension, the second step of which is catalyzed by the essential beta-ketoacyl-acyl carrier protein reductase, MabA. In this study, we investigated whether post-translational modifications of MabA might represent a strategy employed by M. tuberculosis to regulate mycolic acid biosynthesis. Indeed, we show here that MabA was efficiently phosphorylated in vitro by several M. tuberculosis Ser/Thr protein kinases, including PknB, as well as in vivo in mycobacteria. Mass spectrometric analyses using LC-ESI/MS/MS and site-directed mutagenesis identified three phosphothreonines, with Thr(191) being the primary phosphor-acceptor. A MabA_T191D mutant, designed to mimic constitutive phosphorylation, exhibited markedly decreased ketoacyl reductase activity compared with the wild-type protein, as well as impaired binding of the NADPH cofactor, as demonstrated by fluorescence spectroscopy. The hypothesis that phosphorylation of Thr(191) alters the enzymatic activity of MabA, and subsequently mycolic acid biosynthesis, was further supported by the fact that constitutive overexpression of the mabA_T191D allele in Mycobacterium bovis BCG strongly impaired mycobacterial growth. Importantly, conditional expression of the phosphomimetic MabA_T191D led to a significant inhibition of de novo biosynthesis of mycolic acids. This study provides the first information on the molecular mechanism(s) involved in mycolic acid regulation through Ser/Thr protein kinase-dependent phosphorylation of a type II fatty acid synthase enzyme.

Triclosan inhibition of mycobacterial InhA in Saccharomyces cerevisiae: yeast mitochondria as a novel platform for in vivo antimycolate assays

AIMS

To demonstrate the suitability of yeast to act as a novel biotechnological platform for conducting in vivo inhibition assays using drugs with low efficacies towards their mycobacterial targets, such as occurs in the situation with triclosan and InhA.

METHODS AND RESULTS

A surrogate yeast host represented by Saccharomyces cerevisiae etr1Delta cells lacking Etr1p, the 2-trans-enoyl-thioester reductase of mitochondrial type 2 fatty acid synthase (FASII), was designed to rely on the Mycobacterium tuberculosis FASII enzyme InhA. Although InhA is 10,000 times less sensitive to the antimicrobial drug triclosan than is bacterial FabI, the respiratory growth of yeast cells depending on InhA was severely affected on glycerol medium containing triclosan.

CONCLUSIONS

The yeast system could detect enzyme inhibition despite the use of a drug with only low efficacy.

SIGNIFICANCE AND IMPACT OF THE STUDY

Tuberculosis affects a third of the human population, and InhA is a major drug target for combating this disease. InhA is inhibited by isoniazid, but triclosan-derived compounds are presently being developed as antimycolates. A demonstration of triclosan inhibition of InhA in yeast represents a meaningful variation in studying this effect in mycobacteria, because it occurred without the potentially confusing aspects of perturbing protein-protein interactions which are presumed vital to mycobacterial FASII, inactivating other important enzymes or eliciting a dedicated transcriptional response in Myco. tuberculosis.

Platensimycin activity against mycobacterial beta-ketoacyl-ACP synthases

Background

There is an urgent need for the discovery and development of new drugs against Mycobacterium tuberculosis, the causative agent of tuberculosis, especially due to the recent emergence of multi-drug and extensively-drug resistant strains. Herein, we have examined the susceptibility of mycobacteria to the natural product platensimycin.

Methods and Findings

We have demonstrated that platensimycin has bacteriostatic activity against the fast growing Mycobacterium smegmatis (MIC = 14 µg/ml) and against Mycobacterium tuberculosis (MIC = 12 µg/ml). Growth in the presence of paltensimycin specifically inhibited the biosynthesis of mycolic acids suggesting that the antibiotic targeted the components of the mycolate biosynthesis complex. Given the inhibitory activity of platensimycin against β-ketoacyl-ACP synthases from Staphylococcus aureus, M. tuberculosis KasA, KasB or FabH were overexpressed in M. smegmatis to establish whether these mycobacterial KAS enzymes were targets of platensimycin. In M. smegmatis overexpression of kasA or kasB increased the MIC of the strains from 14 µg/ml, to 30 and 124 µg/ml respectively. However, overexpression of fabH on did not affect the MIC. Additionally, consistent with the overexpression data, in vitro assays using purified proteins demonstrated that platensimycin inhibited Mt-KasA and Mt-KasB, but not Mt-FabH.

Significance

Our results have shown that platensimycin is active against mycobacterial KasA and KasB and is thus an exciting lead compound against M. tuberculosis and the development of new synthetic analogues.

Unique roles of DosT and DosS in DosR regulon induction and Mycobacterium tuberculosis dormancy

In Mycobacterium tuberculosis, the sensor kinases DosT and DosS activate the transcriptional regulator DosR, resulting in the induction of the DosR regulon, which is important for anaerobic survival and perhaps latent infection. The individual and collective roles of these sensors have been postulated biochemically, but their roles in vivo have remained unclear. This work demonstrates distinct and additive roles for each sensor during anaerobic dormancy. Both sensors are necessary for wild-type levels of DosR regulon induction, and concomitantly, full induction of the regulon is required for wild-type anaerobic survival. In the anaerobic model, DosT plays an early role, responding to hypoxia. DosT then induces the regulon and with it DosS, which sustains and further induces the regulon. DosT then loses its functionality as oxygen becomes limited, and DosS alone maintains induction of the genes from that point forward. Thus, M. tuberculosis has evolved a system whereby it responds to hypoxic conditions in a stepwise fashion as it enters an anaerobic state.

Heterologous expression of mycobacterial proteins in Saccharomyces cerevisiae reveals two physiologically functional 3-hydroxyacyl-thioester dehydratases, HtdX and HtdY, in addition to HadABC and HtdZ

We report on Mycobacterium tuberculosis Rv0241c and Rv3389c, representing two physiologically functional 3-hydroxyacyl-thioester dehydratases (Htd). These enzymes are potentially entrained in type 2 fatty acid synthase (FASII). Mycobacterial FASII is involved in the synthesis of mycolic acids, which are the major constituents of the protective layer around the pathogen, shielding it from noxious chemicals and the host's immune system. Mycolic acids are additionally associated with the virulence and resilience of M. tuberculosis. Here, Rv0241c and Rv3389c, which are distinct from the previously identified heterodimers Rv0635-Rv0636 (HadAB) and Rv0636-Rv0637 (HadBC) but also the homodimer Rv0130 (HtdZ), were identified by expressing the corresponding candidate open reading frames in Saccharomyces cerevisiae htd2Delta cells lacking mitochondrial 3-hydroxyacyl-acyl carrier protein dehydratase activity, followed by scoring for phenotype rescue. The htd2Delta mutant fails to produce sufficient levels of lipoic acid and does not respire or grow on nonfermentable carbon sources. Soluble protein extracts made from mutant htd2Delta cells expressing mitochondrially targeted Rv0241c or Rv3389c contained 3-hydroxyacyl-thioester hydratase activity. Moreover, mutant yeast cells expressing Rv0241c or Rv3389c were able to recover their respiratory growth on glycerol medium and efficiently reduce 2,3,5-triphenyltetrazolium chloride. Additionally, expression of mitochondrial Rv0241c or Rv3389c in htd2Delta cells also restored de novo lipoic acid synthesis to 92 and 40% of the level in the wild-type strain, respectively. We propose naming Rv0241c and Rv3389c as HtdX and HtdY, respectively, and discuss the implications of our finding with reference to Rv0098, a candidate mycobacterial FabZ homologue with intrinsic thioesterase and hydratase activities that lacks the eukaryotic-like hydratase-2 motif.

The Mycobacterium tuberculosis beta-ketoacyl-acyl carrier protein synthase III activity is inhibited by phosphorylation on a single threonine residue

Mycolic acids are hallmark features of the Mycobacterium tuberculosis cell wall. They are synthesized by the condensation of two fatty acids, a C56-64-meromycolyl chain and a C24-26-fatty acyl chain. Meromycolates are produced via the combination of type I and type II fatty acid synthases (FAS-I and FAS-II). The beta-ketoacyl-acyl carrier protein (ACP) synthase III (mtFabH) links FAS-I and FAS-II, catalyzing the condensation of FAS-I-derived acyl-CoAs with malonyl-ACP. Because mtFabH represents a potential regulatory key point of the mycolic acid pathway, we investigated the hypothesis that phosphorylation of mtFabH controls its activity. Phosphorylation of proteins by Ser/Thr protein kinases (STPKs) has recently emerged as a major physiological mechanism of regulation in prokaryotes. We demonstrate here that mtFabH was efficiently phosphorylated in vitro by several mycobacterial STPKs, particularly by PknF and PknA, as well as in vivo in mycobacteria. Analysis of the phosphoamino acid content indicated that mtFabH was phosphorylated exclusively on threonine residues. Mass spectrometry analyses using liquid chromatography-electrospray ionization/tandem mass spectrometry identified Thr45 as the unique phosphoacceptor. This was further supported by complete loss of PknF- or PknA-dependent phosphorylation of a mtFabH mutant. Mapping Thr45 on the crystal structure of mtFabH illustrates that this residue is located at the entrance of the substrate channel, suggesting that the phosphate group may alter accessibility of the substrate and thus affect mtFabH enzymatic activity. A T45D mutant of mtFabH, designed to mimic constitutive phosphorylation, exhibited markedly decreased transacylation, malonyl-AcpM decarboxylation, and condensing activities compared with the wild-type protein or the T45A mutant. Together, these findings not only represent the first demonstration of phosphorylation of a beta-ketoacyl-ACP synthase III enzyme but also indicate that phosphorylation of mtFabH inhibits its enzymatic activity, which may have important consequences in regulating mycolic acid biosynthesis.

Loss of a mycobacterial gene encoding a reductase leads to an altered cell wall containing beta-oxo-mycolic acid analogs and accumulation of ketones

Mycolic acids are essential components of the mycobacterial cell wall. In this study, we show that a gene encoding a reductase involved in the final step of mycolic acid biosynthesis can be deleted in Mycobacterium smegmatis without affecting cell viability. Deletion of MSMEG4722 (ortholog of Mycobacterium tuberculosis Rv2509) altered culture characteristics and antibiotic sensitivity. The DeltaMSMEG4722 strain synthesized alpha-alkyl, beta-oxo intermediates of mycolic acids, which were found esterified to cell wall arabinogalactan. While the precursors could not be isolated directly due to their inherent instability during base treatment, their presence was established by prior reduction of the beta-oxo group by sodium borohydride. Interestingly, the mutant also accumulated unsaturated ketones, similar to tuberculenone from M. tuberculosis, which were shunt products derived from spontaneous decarboxylation of alpha-alkyl, beta-oxo fatty acid precursors of mycolic acids.

Synthesis and biological evaluation of NAS-21 and NAS-91 analogues as potential inhibitors of the mycobacterial FAS-II dehydratase enzyme Rv0636

The identification of potential new anti-tubercular chemotherapeutics is paramount due to the recent emergence of extensively drug-resistant strains of Mycobacterium tuberculosis (XDR-TB). Libraries of NAS-21 and NAS-91 analogues were synthesized and evaluated for their whole-cell activity against Mycobacterium bovis BCG. NAS-21 analogues 1 and 2 demonstrated enhanced whole-cell activity in comparison to the parental compound, and an M. bovis BCG strain overexpressing the dehydratase enzyme Rv0636 was resistant to these analogues. NAS-91 analogues with ortho-modifications gave enhanced whole-cell activity. However, extension with biphenyl modifications compromised the whole-cell activities of both NAS-21 and NAS-91 analogues. Interestingly, both libraries demonstrated in vitro activity against fatty acid synthase II (FAS-II) but not FAS-I in cell-free extracts. In in vitro assays of FAS-II inhibition, NAS-21 analogues 4 and 5 had IC₅₀ values of 28 and 19 μg ml⁻¹, respectively, for the control M. bovis strain, and the M. bovis BCG strain overexpressing Rv0636 showed a marked increase in resistance. In contrast, NAS-91 analogues demonstrated moderate in vitro activity, although increased resistance was again observed in FAS-II activity assays with the Rv0636-overexpressing strain. Fatty acid methyl ester (FAME) and mycolic acid methyl ester (MAME) analysis of M. bovis BCG and the Rv0636-overexpressing strain revealed that the effect of the drug was relieved in the overexpressing strain, further implicating and potentially identifying Rv0636 as the target for these known FabZ dehydratase inhibitors. This study has identified candidates for further development as drug therapeutics against the mycobacterial FAS-II dehydratase enzyme.

Tuberculosis: a balanced diet of lipids and carbohydrates

In spite of effective antibiotics to treat TB (tuberculosis) since the early 1960s, we enter the new millennium with TB currently the leading cause of death from a single infectious agent, killing more than 3 million people worldwide each year. Thus an understanding of drug-resistance mechanisms, the immunobiology of cell wall components to elucidate host–pathogen interactions and the discovery of new drug targets are now required for the treatment of TB. Above the plasma membrane is a classical chemotype IV peptidoglycan to which is attached the macromolecular structure, mycolyl-arabinogalactan via a unique diglycosylphosphoryl bridge. The present review discusses the assembly of the mAGP (mycolyl-arabinogalactan–peptidoglycan) complex and the site of action of EMB (ethambutol), bringing forward a new era in TB research and focus for new drugs to combat multidrug-resistant TB.

Separate entrance and exit portals for ligand traffic in Mycobacterium tuberculosis FabH

Mycobacterium tuberculosis FabH initiates type II fatty acid synthase-catalyzed formation of the long chain (C(16)-C(22)) acyl-coenzyme A (CoA) precursors of mycolic acids, which are major constituents of the bacterial cell envelope. Crystal structures of M. tuberculosis FabH (mtFabH) show the substrate binding site to be a buried, extended L-shaped channel with only a single solvent access portal. Entrance of an acyl-CoA substrate through the solvent portal would require energetically unfavorable reptational threading of the substrate to its reactive position. Using a class of FabH inhibitors, we have tested an alternative hypothesis that FabH exists in an "open" form during substrate binding and product release, and a "closed" form in which catalysis and intermediate steps occur. This hypothesis is supported by mass spectrometric analysis of the product profile and crystal structures of complexes of mtFabH with these inhibitors.

Identification of a novel mycobacterial 3-hydroxyacyl-thioester dehydratase, HtdZ (Rv0130), by functional complementation in yeast

We report on the identification of Mycobacterium tuberculosis HtdZ (Rv0130), representing a novel 3-hydroxyacyl-thioester dehydratase. HtdZ was picked up by the functional complementation of Saccharomyces cerevisiae htd2Delta cells lacking the dehydratase of mitochondrial type II fatty acid synthase. Mutant cells expressing HtdZ contained dehydratase activity, recovered their respiratory ability, and partially restored de novo lipoic acid synthesis.

Antimycobacterial Activity and Mechanism of Action of NAS-91

The antimalarial agents NAS-91 and NAS-21 were found to express potent antimycobacterial activity, NAS-91 being more active than NAS-21. They partially inhibited mycolic acid biosynthesis and profoundly altered oleic acid production. The development of a cell-free assay for Delta 9-desaturase activity allowed direct demonstration of the inhibition of oleic acid biosynthesis by these compounds.