Mycobacterium tuberculosis antigen 85A and 85C structures confirm binding orientation and conserved substrate specificity

Unlocking Opportunities for Mycobacterium leprae and Mycobacterium ulcerans

In the recent decade, scientific communities have toiled to tackle the emerging burden of drug-resistant tuberculosis (DR-TB) and rapidly growing opportunistic nontuberculous mycobacteria (NTM). Among these, two neglected mycobacteria species of the Acinetobacter family, Mycobacterium leprae and Mycobacterium ulcerans, are the etiological agents of leprosy and Buruli ulcer infections, respectively, and fall under the broad umbrella of neglected tropical diseases (NTDs). Unfortunately, lackluster drug discovery efforts have been made against these pathogenic bacteria in the recent decade, resulting in the discovery of only a few countable hits and majorly repurposing anti-TB drug candidates such as telacebec (Q203), P218, and TB47 for current therapeutic interventions. Major ignorance in drug candidate identification might aggravate the dramatic consequences of rapidly spreading mycobacterial NTDs in the coming days. Therefore, this Review focuses on an up-to-date account of drug discovery efforts targeting selected druggable targets from both bacilli, including the accompanying challenges that have been identified and are responsible for the slow drug discovery. Furthermore, a succinct discussion of the all-new possibilities that could be alternative solutions to mitigate the neglected mycobacterial NTD burden and subsequently accelerate the drug discovery effort is also included. We anticipate that the state-of-the-art strategies discussed here may attract major attention from the scientific community to navigate and expand the roadmap for the discovery of next-generation therapeutics against these NTDs.

Engineering a dual vaccine against COVID-19 and tuberculosis

The COVID-19 pandemic, caused by SARS-CoV-2 virus, has been one of the top public health threats across the world over the past three years. Mycobacterium bovis BCG is currently the only licensed vaccine for tuberculosis, one of the deadliest infectious diseases in the world, that is caused by Mycobacterium tuberculosis. In the past decades, recombinant M.bovis BCG has been studied as a novel vaccine vector for other infectious diseases in humans besides tuberculosis, such as viral infections. In the current study, we generated a recombinant M. bovis BCG strain AspikeRBD that expresses a fusion protein consisting of M. tb Ag85A protein and the receptor-binding domain (RBD) of the SARS-CoV-2 spike protein using synthetic biology technique. Our results show that the recombinant M. bovis BCG strain successfully expressed this fusion protein. Interestingly, the recombinant M. bovis BCG strain AspikeRBD significantly induced SARS-CoV-2 spike-specific T cell activation and IgG production in mice when compared to the parental M.bovis BCG strain, and was more potent than the recombinant M.bovis BCG strain expressing SARS-CoV-2 spike RBD alone. As expected, the recombinant M. bovis BCG strain AspikeRBD activated an increased number of M. tb Ag85A-specific IFNγ-releasing T cells and enhanced IgG production in mice when compared to the parental M.bovis BCG strain or the BCG strain expressing SARS-CoV-2 spike RBD alone. Taken together, our results indicate a potential application of the recombinant M. bovis BCG strain AspikeRBD as a novel dual vaccine against SARS-CoV-2 and M. tb in humans.

Improving the biotransformation efficiency of soybean phytosterols in Mycolicibacterium neoaurum by the combined deletion of fbpC3 and embC in cell envelope synthesis

Biotransformation of soybean phytosterols into 9α-hydroxy-4-androstene-3,17-dione (9-OHAD) by mycobacteria is the core step in the synthesis of adrenocortical hormone. However, the low permeability of the dense cell envelope largely inhibits the overall conversion efficiency of phytosterols. The antigen 85 (Ag85) complex encoded by fbpA, fbpB, and fbpC was proposed as the key factor in the combined catalysis of mycoloyl for producing mycolyl-arabinogalactan (m-AG) and trehalose dimycolate (TDM) in mycobacterial cell envelope. Herein, we confirmed that fbpC3 was essential for the biotransformation of trehalose monomycolate (TMM) to TDM in Mycolicibacterium neoaurum. The deficiency of this gene raised the cell permeability, thereby enhancing the steroid uptake and utilization. The 9-OHAD yield in the fbpC3-deficient 9-OHAD-producing strain was increased by 21.3%. Moreover, the combined deletion of fbpC3 and embC further increased the 9-OHAD yield compared to the single deletion of fbpC3. Finally, after 96 h of bioconversion in industrial resting cells, the 9-OHAD yield of 11.2 g/L was achieved from 20 g/L phytosterols and the productivity reached 0.116 g/L/h. In summary, this study suggested the critical role of the fbpC3 gene in the synthesis of TDM in M. neoaurum and verified the feasibility of improving the bioconversion efficiency of phytosterols through the cell envelope engineering strategy.

Total Synthesis of Tetrahydrolipstatin, Its Derivatives, and Evaluation of Their Ability to Potentiate Multiple Antibiotic Classes against Mycobacterium Species

Tetrahydrolipstatin (THL, 1a) has been shown to inhibit both mammalian and bacterial α/β hydrolases. In the case of bacterial systems, THL is a known inhibitor of several Mycobacterium tuberculosis hydrolases involved in mycomembrane biosynthesis. Herein we report a highly efficient eight-step asymmetric synthesis of THL using a route that allows modification of the THL α-chain substituent to afford compounds 1a through 1e. The key transformation in the synthesis was use of a (TPP)CrCl/Co2(CO)8-catalyzed regioselective and stereospecific carbonylation on an advanced epoxide intermediate to yield a trans-β-lactone. These compounds are modest inhibitors of Ag85A and Ag85C, two α/β hydrolases of M. tuberculosis involved in the biosynthesis of the mycomembrane. Among these compounds, 10d showed the highest inhibitory effect on Ag85A (34 ± 22 μM) and Ag85C (66 ± 8 μM), and its X-ray structure was solved in complex with Ag85C to 2.5 Å resolution. In contrast, compound 1e exhibited the best-in-class MICs of 50 μM (25 μg/mL) and 16 μM (8.4 μg/mL) against M. smegmatis and M. tuberculosis H37Ra, respectively, using a microtiter assay plate. Combination of 1e with 13 well-established antibiotics synergistically enhanced the potency of few of these antibiotics in M. smegmatis and M. tuberculosis H37Ra. Compound 1e applied at concentrations 4-fold lower than its MIC enhanced the MIC of the synergistic antibiotic by 2-256-fold. In addition to observing synergy with first-line drugs, rifamycin and isoniazid, the MIC of vancomycin against M. tuberculosis H37Ra was 65 μg/mL; however, the MIC was lowered to 0.25 μg/mL in the presence of 2.1 μg/mL 1e demonstrating the potential of targeting mycobacterial hydrolases involved in mycomembrane and peptidoglycan biosynthesis.

Chemical Tools for Illumination of Tuberculosis Biology, Virulence Mechanisms, and Diagnosis

Tuberculosis (TB) remains one of the deadliest infectious diseases and begs the scientific community to up the ante for research and exploration of completely novel therapeutic avenues. Chemical biology-inspired design of tunable chemical tools has aided in clinical diagnosis, facilitated discovery of therapeutics, and begun to enable investigation of virulence mechanisms at the host-pathogen interface of Mycobacterium tuberculosis. This Perspective highlights chemical tools specific to mycobacterial proteins and the cell lipid envelope that have furnished rapid and selective diagnostic strategies and provided unprecedented insights into the function of the mycobacterial proteome and lipidome. We discuss chemical tools that have enabled elucidating otherwise intractable biological processes by leveraging the unique lipid and metabolite repertoire of mycobacterial species. Some of these probes represent exciting starting points with the potential to illuminate poorly understood aspects of mycobacterial pathogenesis, particularly the host membrane-pathogen interactions.

Weak spots inhibition in the Mycobacterium tuberculosis antigen 85C target for antitubercular drug design through selective irreversible covalent inhibitor-SER124

Mycobacterium tuberculosis (Mtb) encoded secreted antigen 85 enzymes (Ag85A/Ag85B/Ag85C) play that critical roles in the virulence, survival and drug-resistant TB of the pathogen. Ag85 proteins are potential antitubercular drug targets because they are essential in the catalytic synthesis of trehalose moieties and mycolic acid attachment to the Mtb cell wall. Recently, experimental protocols led to the discovery of a selective covalent Ag85 inhibitor, β-isomer monocyclic enolphosphorus Cycliphostin (CyC_8β) compound, which targets the Ag85 serine 124 to exhibit a promising therapeutic activity. For the first time, our study unravelled the structural features among Mtb Ag85C homologs and motions and dynamics of Ag85C when the CyC_8β bound covalently and in open model conformations to the protein using bioinformatics tools and integrated Molecular dynamics simulations. Comparative Ag85C sequence analysis revealed conserved regions; 70% active site, 90% Adeniyi loop L1 and 50% loop L2, which acts as a switch between open and closed conformations. The average C-α atoms RMSD (2.05 Å) and RMSF (0.9 Å) revealed instability and high induced flexibility in the CyC_8β covalent-bound compared to the apo and open model systems, which displayed more stability and lower fluctuations. DSSP showed structural transitions of α-helices to bend and loops to 3₁₀-helices in the bound systems. SASA of CyC_8β covalent bound showed active site hydrophobic residues exposure to huge solvent. Therefore, these findings present the potential opportunity hotspots in Ag85C protein that would aid the structure-based design of novel chemical entities capable of resulting in potent antitubercular drugs.Communicated by Ramaswamy H. Sarma.

An evaluation of different classification algorithms for protein sequence-based reverse vaccinology prediction

Previous work has shown that proteins that have the potential to be vaccine candidates can be predicted from features derived from their amino acid sequences. In this work, we make an empirical comparison across various machine learning classifiers on this sequence-based inference problem. Using systematic cross validation on a dataset of 200 known vaccine candidates and 200 negative examples, with a set of 525 features derived from the AA sequences and feature selection applied through a greedy backward elimination approach, we show that simple classification algorithms often perform as well as more complex support vector kernel machines. The work also includes a novel cross validation applied across bacterial species, i.e. the validation proteins all come from a specific species of bacterium not represented in the training set. We termed this type of validation Leave One Bacteria Out Validation (LOBOV).

First access to a mycolic acid-based bioorthogonal reporter for the study of the mycomembrane and mycoloyltransferases in Corynebacteria

In this study we describe the first synthesis of an alkyne-based trehalose monomycolate probe closely mimicking the complex pattern of mycolic acids and its utility for the study of mycomembrane and mycoloyltransferases in Corynebacteria.

Imaging mycobacterial growth and division with a fluorogenic probe

Control and manipulation of bacterial populations requires an understanding of the factors that govern growth, division, and antibiotic action. Fluorescent and chemically reactive small molecule probes of cell envelope components can visualize these processes and advance our knowledge of cell envelope biosynthesis (e.g., peptidoglycan production). Still, fundamental gaps remain in our understanding of the spatial and temporal dynamics of cell envelope assembly. Previously described reporters require steps that limit their use to static imaging. Probes that can be used for real-time imaging would advance our understanding of cell envelope construction. To this end, we synthesized a fluorogenic probe that enables continuous live cell imaging in mycobacteria and related genera. This probe reports on the mycolyltransferases that assemble the mycolic acid membrane. This peptidoglycan-anchored bilayer-like assembly functions to protect these cells from antibiotics and host defenses. Our probe, quencher-trehalose-fluorophore (QTF), is an analog of the natural mycolyltransferase substrate. Mycolyltransferases process QTF by diverting their normal transesterification activity to hydrolysis, a process that unleashes fluorescence. QTF enables high contrast continuous imaging and the visualization of mycolyltransferase activity in cells. QTF revealed that mycolyltransferase activity is augmented before cell division and localized to the septa and cell poles, especially at the old pole. This observed localization suggests that mycolyltransferases are components of extracellular cell envelope assemblies, in analogy to the intracellular divisomes and polar elongation complexes. We anticipate QTF can be exploited to detect and monitor mycobacteria in physiologically relevant environments.

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.

Mycolyltransferase from Mycobacterium tuberculosis in covalent complex with tetrahydrolipstatin provides insights into antigen 85 catalysis

Mycobacterium tuberculosis antigen 85 (Ag85) enzymes catalyze the transfer of mycolic acid (MA) from trehalose monomycolate to produce the mycolyl arabinogalactan (mAG) or trehalose dimycolate (TDM). These lipids define the protective mycomembrane of mycobacteria. The current model of substrate binding within the active sites of Ag85s for the production of TDM is not sterically and geometrically feasible; additionally, this model does not account for the production of mAG. Furthermore, this model does not address how Ag85s limit the hydrolysis of the acyl-enzyme intermediate while catalyzing acyl transfer. To inform an updated model, we obtained an Ag85 acyl-enzyme intermediate structure that resembles the mycolated form. Here, we present a 1.45-Å X-ray crystal structure of M. tuberculosis Ag85C covalently modified by tetrahydrolipstatin (THL), an esterase inhibitor that suppresses M. tuberculosis growth and mimics structural attributes of MAs. The mode of covalent inhibition differs from that observed in the reversible inhibition of the human fatty-acid synthase by THL. Similarities between the Ag85-THL structure and previously determined Ag85C structures suggest that the enzyme undergoes structural changes upon acylation, and positioning of the peptidyl arm of THL limits hydrolysis of the acyl-enzyme adduct. Molecular dynamics simulations of the modeled mycolated-enzyme form corroborate the structural analysis. From these findings, we propose an alternative arrangement of substrates that rectifies issues with the previous model and suggest a direct role for the β-hydroxy of MA in the second half-reaction of Ag85 catalysis. This information affords the visualization of a complete mycolyltransferase catalytic cycle.

Effect of Mycobacterium tuberculosis Rv3717 on cell division and cell adhesion

Mycobacterium tuberculosis Rv3717 has been identified as a zinc-dependent amidase which can hydrolyze peptidoglycan (PG). To demonstrate the relationship of Rv3717 and cell division, in this study, Rv3717 gene was first amplified and expressed and the resulting protein was purified by using a His-tagged approach. M. smegmatis mc²155, a fast-growing and nonpathogenic mycobacterium was used to evaluate the effect of Rv3717 on cell division. Scan electron microscope (SEM) results indicated that M. smegmatis with division site was more exhibited and some of the cells turned larger in size after Rv3717 treatment. Transmission electron microscope (TEM) results revealed that MSMEG_6281 gene knockout strain named M sm-ΔM_6281 (MSMEG_6281 in M. smegmatis mc²155 is the homologous gene of Rv3717) tended to have a division defect with a severely abnormal morphology, and division septa were distorted. Gene expression analysis indicated also that the gene involved in cell division such as M. smegmatis ftsZ was significantly up-regulated with treatment time. The findings demonstrated that physiological role of Rv3717 was related to cell division and regulated possibly division septum formation. Further, fibronectin (Fn) binding ability of Rv3717 was evaluated by protein binding experiment, and the results confirmed the interaction of Rv3717 with Fn in a dose dependent manner. We found also that the invasion rate of M. sm-ΔM_6281 to A549 cells was reduced by 59% compared to the control strain, and the invasion defect could be rescued by Rv3717 addition. RT-PCR results showed that M. smegmatis fbpC were up-regulated after Rv3717 addition. These clues may be significant to explore roles of Rv3717 in growth and colonization of mycobacteria.

Cyclipostins and cyclophostin analogs inhibit the antigen 85C from Mycobacterium tuberculosis both in vitro and in vivo

An increasing prevalence of cases of drug-resistant tuberculosis requires the development of more efficacious chemotherapies. We previously reported the discovery of a new class of cyclipostins and cyclophostin (CyC) analogs exhibiting potent activity against Mycobacterium tuberculosis both in vitro and in infected macrophages. Competitive labeling/enrichment assays combined with MS have identified several serine or cysteine enzymes in lipid and cell wall metabolism as putative targets of these CyC compounds. These targets included members of the antigen 85 (Ag85) complex (i.e. Ag85A, Ag85B, and Ag85C), responsible for biosynthesis of trehalose dimycolate and mycolylation of arabinogalactan. Herein, we used biochemical and structural approaches to validate the Ag85 complex as a pharmacological target of the CyC analogs. We found that CyC_7β, CyC_8β, and CyC₁₇ bind covalently to the catalytic Ser¹²⁴ residue in Ag85C; inhibit mycolyltransferase activity (i.e. the transfer of a fatty acid molecule onto trehalose); and reduce triacylglycerol synthase activity, a property previously attributed to Ag85A. Supporting these results, an X-ray structure of Ag85C in complex with CyC_8β disclosed that this inhibitor occupies Ag85C's substrate-binding pocket. Importantly, metabolic labeling of M. tuberculosis cultures revealed that the CyC compounds impair both trehalose dimycolate synthesis and mycolylation of arabinogalactan. Overall, our study provides compelling evidence that CyC analogs can inhibit the activity of the Ag85 complex in vitro and in mycobacteria, opening the door to a new strategy for inhibiting Ag85. The high-resolution crystal structure obtained will further guide the rational optimization of new CyC scaffolds with greater specificity and potency against M. tuberculosis.

A structural overview of mycobacterial adhesins: Key biomarkers for diagnostics and therapeutics

Adherence, colonization, and survival of mycobacteria in host cells require surface adhesins, which are attractive pharmacotherapeutic targets. A large arsenal of pilus and non-pilus adhesins have been identified in mycobacteria. These adhesins are capable of interacting with host cells, including macrophages and epithelial cells and are essential to microbial pathogenesis. In the last decade, several structures of mycobacterial adhesins responsible for adhesion to either macrophages or extra cellular matrix proteins have been elucidated. In addition, key structural and functional information have emerged for the process of mycobacterial adhesion to epithelial cells, mediated by the Heparin-binding hemagglutinin (HBHA). In this review, we provide an overview of the structural and functional features of mycobacterial adhesins and discuss their role as important biomarkers for diagnostics and therapeutics. Based on the reported data, it appears clear that adhesins are endowed with a variety of different structures and functions. Most adhesins play important roles in the cell life of mycobacteria and are key virulence factors. However, they have adapted to an extracellular life to exert a role in host-pathogen interaction. The type of interactions they form with the host and the adhesin regions involved in binding is partly known and is described in this review.

An Antibacterial β-Lactone Kills Mycobacterium tuberculosis by Disrupting Mycolic Acid Biosynthesis

The spread of antibiotic resistance is a major challenge for the treatment of Mycobacterium tuberculosis infections. In addition, the efficacy of drugs is often limited by the restricted permeability of the mycomembrane. Frontline antibiotics inhibit mycomembrane biosynthesis, leading to rapid cell death. Inspired by this mechanism, we exploited β-lactones as putative mycolic acid mimics to block serine hydrolases involved in their biosynthesis. Among a collection of β-lactones, we found one hit with potent anti-mycobacterial and bactericidal activity. Chemical proteomics using an alkynylated probe identified Pks13 and Ag85 serine hydrolases as major targets. Validation through enzyme assays and customized ¹³ C metabolite profiling showed that both targets are functionally impaired by the β-lactone. Co-administration with front-line antibiotics enhanced the potency against M. tuberculosis by more than 100-fold, thus demonstrating the therapeutic potential of targeting mycomembrane biosynthesis serine hydrolases.

Imaging Mycobacterial Trehalose Glycolipids

Cell surface trehalose mycolates are important modulators of mycobacterial pathogenesis and host immune response. We discuss the use of fluorescent and fluorogenic trehalose probes for the detection of the mycobacterial trehalose glycolipids. These probes enable real-time imaging of trehalose mycolate biosynthesis and mycomembrane dynamics in the laboratory as well as in clinical settings for the detection of mycobacteria in patient samples.

The Rv3799-Rv3807 Gene Cluster in Mycobacterium Tuberculosis Genome Corresponds to the ‘Ancient Conserved Region’ in CMN Mycolyltransferases

We have identified based on gene cluster analysis that the genes between Rv3799–Rv3807 in M. tuberculosis have orthologs in Corynebacteria, Mycobacteria and Nocardia (CMN) genomes. Therefore, this gene cluster possibly corresponds to the ‘Ancient Conserved Region’ of CMN mycolyltransferases. The evolutionary trace analysis suggests that twelve amino acid residues; Leu39, Trp51, Pro71, Trp82, Trp97, Phe100, Gly124, Ser126, Asp192, Glu230, Gly260 and Trp264 are ‘absolutely conserved’. These amino acid residues constitute the active site and conserved hydrophobic tunnel in CMN mycolyltransferases.

Immuno-PCR, a new technique for the serodiagnosis of tuberculosis

Rapid and accurate diagnosis of tuberculosis (TB) is essential to control the disease. The conventional microbiological tests have limitations and there is an urgent need to devise a simple, rapid and reliable point-of-care (POC) test. The failure of TB diagnostic tests based on antibody detection due to inconsistent and imprecise results has stimulated renewed interest in the development of rapid antigen detection methods. However, the World Health Organization (WHO) has emphasized to continue research for designing new antibody-based detection tests with improved accuracy. Immuno-polymerase chain reaction (I-PCR) combines the simplicity and versatility of enzyme-linked immunosorbent assay (ELISA) with the exponential amplification capacity and sensitivity of PCR thus leading to several-fold increase in sensitivity in comparison to analogous ELISA. In this review, we have described the serodiagnostic potential of I-PCR assays for an early diagnosis of TB based on the detection of potential mycobacterial antigens and circulating antibodies in body fluids of TB patients.

Exploring Covalent Allosteric Inhibition of Antigen 85C from Mycobacterium tuberculosis by Ebselen Derivatives

Previous studies identified ebselen as a potent in vitro and in vivo inhibitor of the Mycobacterium tuberculosis (Mtb) antigen 85 (Ag85) complex, comprising three homologous enzymes required for the biosynthesis of the mycobacterial cell wall. In this study, the Mtb Ag85C enzyme was cocrystallized with azido and adamantyl ebselen derivatives, resulting in two crystallographic structures of 2.01 and 1.30 Å resolution, respectively. Both structures displayed the anticipated covalent modification of the solvent accessible, noncatalytic Cys209 residue forming a selenenylsulfide bond. Continuous difference density for both thiol modifiers allowed for the assessment of interactions that influence ebselen binding and inhibitor orientation that were unobserved in previous Ag85C ebselen structures. The k_inact/K_I values for ebselen, adamantyl ebselen, and azido ebselen support the importance of observed constructive chemical interactions with Arg239 for increased in vitro efficacy toward Ag85C. To better understand the in vitro kinetic properties of these ebselen derivatives, the energetics of specific protein-inhibitor interactions and relative reaction free energies were calculated for ebselen and both derivatives using density functional theory. These studies further support the different in vitro properties of ebselen and two select ebselen derivatives from our previously published ebselen library with respect to kinetics and protein-inhibitor interactions. In both structures, the α9 helix was displaced farther from the enzyme active site than the previous Ag85C ebselen structure, resulting in the restructuring of a connecting loop and imparting a conformational change to residues believed to play a role in substrate binding specific to Ag85C. These notable structural changes directly affect protein stability, reducing the overall melting temperature by up to 14.5 °C, resulting in the unfolding of protein at physiological temperatures. Additionally, this structural rearrangement due to covalent allosteric modification creates a sizable solvent network that encompasses the active site and extends to the modified Cys209 residue. In all, this study outlines factors that influence enzyme inhibition by ebselen and its derivatives while further highlighting the effects of the covalent modification of Cys209 by said inhibitors on the structure and stability of Ag85C. Furthermore, the results suggest a strategy for developing new classes of Ag85 inhibitors with increased specificity and potency.

Mycoloyltransferases: A large and major family of enzymes shaping the cell envelope of Corynebacteriales

Mycobacterium and Corynebacterium are important genera of the Corynebacteriales order, the members of which are characterized by an atypical diderm cell envelope. Indeed the cytoplasmic membrane of these bacteria is surrounded by a thick mycolic acid-arabinogalactan-peptidoglycan (mAGP) covalent polymer. The mycolic acid-containing part of this complex associates with other lipids (mainly trehalose monomycolate (TMM) and trehalose dimycolate (TDM)) to form an outer membrane. The metabolism of mycolates in the cell envelope is governed by esterases called mycoloyltransferases that catalyze the transfer of mycoloyl chains from TMM to another TMM molecule or to other acceptors such as the terminal arabinoses of arabinogalactan or specific polypeptides. In this review we present an overview of this family of Corynebacteriales enzymes, starting with their expression, localization, structure and activity to finally discuss their putative functions in the cell. In addition, we show that Corynebacteriales possess multiple mycoloyltransferases encoding genes in their genome. The reason for this multiplicity is not known, as their function in mycolates biogenesis appear to be only partially redundant. It is thus possible that, in some species living in specific environments, some mycoloyltransferases have evolved to gain some new functions. In any case, the few characterized mycoloyltransferases are very important for the bacterial physiology and are also involved in adaptation in the host where they constitute major secreted antigens. Although not discussed in this review, all these functions make them interesting targets for the discovery of new antibiotics and promising vaccines candidates. This article is part of a Special Issue entitled "Science for Life" Guest Editor: Dr. Austen Angell, Dr. Salvatore Magazù and Dr. Federica Migliardo.

Targeting the trehalose utilization pathways of Mycobacterium tuberculosis

Tuberculosis (TB) is an epidemic disease and the growing burden of multidrug-resistant (MDR) TB world wide underlines the need to discover new drugs to treat the disease. Mycobacterium tuberculosis (Mtb) is the etiological agent of most cases of TB. Mtb is difficult to treat, in part, due to the presence of a sturdy hydrophobic barrier that prevents penetration of drugs through the cell wall. Mtb can also survive in a non-replicative state for long periods of time avoiding the action of common antibiotics. Trehalose is an essential metabolite in mycobacteria since it plays key roles in cell wall synthesis, transport of cell wall glycolipids, and energy storage. It is also known for its stress protective roles such as: protection from desiccation, freezing, starvation and osmotic stress in bacteria. In this review we discuss the drug discovery efforts against enzymes involved in the trehalose utilization pathways (TUPs) and their likelihood of becoming drug targets.

Novel human recombinant antibodies against Mycobacterium tuberculosis antigen 85B

Background

Tuberculosis is the leading cause of death due to bacterial infections worldwide, mainly caused by Mycobacterium tuberculosis. The antigen 85 complex comprises a set of major secreted proteins of M. tuberculosis, which are potential biomarkers for diagnostic.

Results

In this work, the first human single chain fragment variable (scFv) antibodies specific for the tuberculosis biomarker 85 B were selected by phage display from naïve antibody gene libraries (HAL7/8). Produced as scFv-Fc in mammalian cells, these antibodies were further characterized and analysed for specificity and applicability in different tuberculosis antigen detection assays. Sandwich detection of recombinant 85 B was successful in enzyme linked immunosorbent assay (ELISA), lateral flow immunoassay and immunoblot. Whereas detection of M. tuberculosis cell extracts and culture filtrates was only possible in direct ELISA and immunoblot assays. It was found that the conformation of 85 B, depending on sample treatment, influenced antigen detection.

Conclusions

Recombinant antibodies, selected by phage display, may be applicable for 85 B detection in various assays. These antibodies are candidates for the development of future point of care tuberculosis diagnostic kits. Using 85 B as a biomarker, the antigen conformation influenced by sample treatment is important.

Inactivation of the Mycobacterium tuberculosis antigen 85 complex by covalent, allosteric inhibitors

The rise of multidrug-resistant and totally drug-resistant tuberculosis and the association with an increasing number of HIV-positive patients developing tuberculosis emphasize the necessity to find new antitubercular targets and drugs. The antigen 85 (Ag85) complex from Mycobacterium tuberculosis plays important roles in the biosynthesis of major components of the mycobacterial cell envelope. For this reason, Ag85 has emerged as an attractive drug target. Recently, ebselen was identified as an effective inhibitor of the Ag85 complex through covalent modification of a cysteine residue proximal to the Ag85 active site and is therefore a covalent, allosteric inhibitor. To expand the understanding of this process, we have solved the x-ray crystal structures of Ag85C covalently modified with ebselen and other thiol-reactive compounds, p-chloromercuribenzoic acid and iodoacetamide, as well as the structure of a cysteine to glycine mutant. All four structures confirm that chemical modification or mutation at this particular cysteine residue leads to the disruption of the active site hydrogen-bonded network essential for Ag85 catalysis. We also describe x-ray crystal structures of Ag85C single mutants within the catalytic triad and show that a mutation of any one of these three residues promotes the same conformational change observed in the cysteine-modified forms. These results provide evidence for active site dynamics that may afford new strategies for the development of selective and potent Ag85 inhibitors.

The three Mycobacterium tuberculosis antigen 85 isoforms have unique substrates and activities determined by non-active site regions

The three isoforms of antigen 85 (A, B, and C) are the most abundant secreted mycobacterial proteins and catalyze transesterification reactions that synthesize mycolated arabinogalactan, trehalose monomycolate (TMM), and trehalose dimycolate (TDM), important constituents of the outermost layer of the cellular envelope of Mycobacterium tuberculosis. These three enzymes are nearly identical at the active site and have therefore been postulated to exist to evade host immunity. Distal to the active site is a second putative carbohydrate-binding site of lower homology. Mutagenesis of the three isoforms at this second site affected both substrate selectivity and overall catalytic activity in vitro. Using synthetic and natural substrates, we show that these three enzymes exhibit unique selectivity; antigen 85A more efficiently mycolates TMM to form TDM, whereas C (and to a lesser extent B) has a higher rate of activity using free trehalose to form TMM. This difference in substrate selectivity extends to the hexasaccharide fragment of cell wall arabinan. Mutation of secondary site residues from the most active isoform (C) into those present in A or B partially interconverts this substrate selectivity. These experiments in combination with molecular dynamics simulations reveal that differences in the N-terminal helix α9, the adjacent Pro(216)-Phe(228) loop, and helix α5 are the likely cause of changes in activity and substrate selectivity. These differences explain the existence of three isoforms and will allow for future work in developing inhibitors.

The molecular biology of mycobacterial trehalose in the quest for advanced tuberculosis therapies

Trehalose is a natural glucose disaccharide identified in the 19th century in fungi and insect cocoons, and later across the three domains of life. In members of the genus Mycobacterium, which includes the tuberculosis (TB) pathogen and over 160 species of nontuberculous mycobacteria (NTM), many of which are opportunistic pathogens, trehalose has been an important focus of research over the last 60 years. It is a crucial player in the assembly and architecture of the remarkable mycobacterial cell envelope as an element of unique highly antigenic glycolipids, namely trehalose dimycolate ('cord factor'). Free trehalose has been detected in the mycobacterial cytoplasm and occasionally in oligosaccharides with unknown function. TB and NTM infection statistics and death toll, the decline in immune responses in the aging population, human immunodeficiency virus/AIDS or other debilitating conditions, and the proliferation of strains with different levels of resistance to the dated drugs in use, all merge into a serious public-health threat urging more effective vaccines, efficient diagnostic tools and new drugs. This review deals with the latest findings on mycobacterial trehalose biosynthesis, catabolism, processing and recycling, as well with the ongoing quest for novel trehalose-related mechanisms to be targeted by novel TB therapeutics. In this context, the drug-discovery pipeline has recently included new lead compounds directed toward trehalose-related targets highlighting the potential of these pathways to stem the tide of rising drug resistance.

Antigen 85 variation across lineages of Mycobacterium tuberculosis-implications for vaccine and biomarker success

Mycobacterium tuberculosis secretes several hundred proteins; many of which elicit immune responses. As a result, many of these proteins have been explored for their potential as diagnostic and vaccine candidates. Of these, the Antigen 85 complex proteins, represented by Antigen85 A, B, and C, are the most studied from the mycobacterial secretome. However, vaccine constructs exploiting Antigen 85 as the sole antigen repertoire have not experienced the pre-clinical and clinical trials success originally anticipated. Anecdotal and biochemical evidence suggests that differences in protein abundance may explain this phenomenon. Here, biochemical, molecular, and mass spectrometry approaches were used to quantify Antigen 85 among six M. tuberculosis strains from four phylogenetically distinct clades. Our data demonstrates that the greatest variation in Antigen 85 is ascribed to protein quantities, whereas few transcriptional differences were found. In addition, the ratio of Antigen 85 A, to B, to C is conserved within clades and phylogenetic neighbors. In contrast, no such relationship between individual protein quantities was observed, and in the case of Antigen85 B, this variation even extends within biological replicates of individual isolates. The relevance of Antigen 85 protein quantities and vaccine efficacy remains to be defined.

BIOLOGICAL SIGNIFICANCE

Absolute quantitation via multiple reaction monitoring mass spectrometry was used to determine the exact molar concentrations of Antigen 85A, B, and C; three key immunodominant proteins present in M. tuberculosis. Further, the concentration of these three proteins was compared among various clades of M. tuberculosis, and demonstrated differences in abundance of two of the three proteins. These proteins have been identified as key antigens in multiple vaccine and diagnostic platforms, thus the potential relevance of their abundance in various M. tuberculosis clades to the successful outcome of these interventions is discussed. This article is part of a Special Issue entitled: Trends in Microbial Proteomics.

Tuberculosis diagnosis using immunodominant, secreted antigens of Mycobacterium tuberculosis

Tuberculosis (TB) remains a major public health concern in most low-income countries. Hence, rapid and sensitive TB diagnostics play an important role in detecting and preventing the disease. In addition to established diagnostic methods, several new approaches have been reported. Some techniques are simple but time-consuming, while others require complex instrumentation. One prominent and readily available approach is to detect proteins that Mycobacterium tuberculosis secretes, such as Mpt64, the 6-kDa early secreted antigenic target (Esat6), the 10-kDa culture filtrate protein (Cfp10), and the antigen 85 (Ag85) complex. Although their functions are not fully understood, a growing body of molecular evidence implicates them in M. tuberculosis virulence. Currently these biomarkers are either being used or investigated for use in skin patch tests, biosensor analyses, and immunochromatographic, immunohistochemical, polymerase chain reaction-based, and enzyme-linked immunosorbent assays. This review provides a comprehensive discussion of the roles these immunodominant antigens play in M. tuberculosis pathogenesis and compares diagnostic methods based on the detection of these proteins with more established tests for TB.

Identification of distant co-evolving residues in antigen 85C from Mycobacterium tuberculosis using statistical coupling analysis of the esterase family proteins

A fundamental goal in cellular signaling is to understand allosteric communication, the process by which signals originating at one site in a protein propagate reliably to affect distant functional sites. The general principles of protein structure that underlie this process remain unknown. Statistical coupling analysis (SCA) is a statistical technique that uses evolutionary data of a protein family to measure correlation between distant functional sites and suggests allosteric communication. In proteins, very distant and small interactions between collections of amino acids provide the communication which can be important for signaling process. In this paper, we present the SCA of protein alignment of the esterase family (pfam ID: PF00756) containing the sequence of antigen 85C secreted by Mycobacterium tuberculosis to identify a subset of interacting residues. Clustering analysis of the pairwise correlation highlighted seven important residue positions in the esterase family alignments. These residues were then mapped on the crystal structure of antigen 85C (PDB ID: 1DQZ). The mapping revealed correlation between 3 distant residues (Asp38, Leu123 and Met125) and suggests allosteric communication between them. This information can be used for a new drug against this fatal disease.

Differential cellular recognition pattern to M. tuberculosis targets defined by IFN-γ and IL-17 production in blood from TB + patients from Honduras as compared to health care workers: TB and immune responses in patients from Honduras

Background

A better understanding of the quality of cellular immune responses directed against molecularly defined targets will guide the development of TB diagnostics and identification of molecularly defined, clinically relevant M.tb vaccine candidates.

Methods

Recombinant proteins (n = 8) and peptide pools (n = 14) from M. tuberculosis (M.tb) targets were used to compare cellular immune responses defined by IFN-γ and IL-17 production using a Whole Blood Assay (WBA) in a cohort of 148 individuals, i.e. patients with TB + (n = 38), TB- individuals with other pulmonary diseases (n = 81) and individuals exposed to TB without evidence of clinical TB (health care workers, n = 29).

Results

M.tb antigens Rv2958c (glycosyltransferase), Rv2962c (mycolyltransferase), Rv1886c (Ag85B), Rv3804c (Ag85A), and the PPE family member Rv3347c were frequently recognized, defined by IFN-γ production, in blood from healthy individuals exposed to M.tb (health care workers). A different recognition pattern was found for IL-17 production in blood from M.tb exposed individuals responding to TB10.4 (Rv0288), Ag85B (Rv1886c) and the PPE family members Rv0978c and Rv1917c.

Conclusions

The pattern of immune target recognition is different in regard to IFN-γ and IL-17 production to defined molecular M.tb targets in PBMCs from individuals frequently exposed to M.tb. The data represent the first mapping of cellular immune responses against M.tb targets in TB patients from Honduras.

Mechanism of inhibition of Mycobacterium tuberculosis antigen 85 by ebselen

The increasing prevalence of drug-resistant tuberculosis highlights the need for identifying new antitubercular drugs that can treat these infections. The antigen 85 (Ag85) complex has emerged as an intriguing mycobacterial drug target due to its central role in synthesizing major components of the inner and outer leaflets of the mycobacterial outer membrane. Here we identify ebselen (EBS) as a potent inhibitor of the Mycobacterium tuberculosis Ag85 complex. Mass spectrometry data show that EBS binds covalently to a cysteine residue (C209) located near the Ag85C active site. The crystal structure of Ag85C in the presence of EBS shows that C209 modification restructures the active site, thereby disrupting the hydrogen-bonded network within the active site that is essential for enzymatic activity. C209 mutations display marked decreases in enzymatic activity. These data suggest that compounds using this mechanism of action will strongly inhibit the Ag85 complex and minimize the selection of drug resistance.

Elastin, a novel extracellular matrix protein adhering to mycobacterial antigen 85 complex

The antigen 85 complex (Ag85) consists of three predominantly secreted proteins (Ag85A, Ag85B, and Ag85C), which play a key role in the mycobacterial pathogenesis and also possess enzymatic mycolyltransferase activity involved in cell wall synthesis. Ag85 is not only considered to be a virulence factor because its expression is essential for intracellular survival within macrophages, but also because it contributes to adherence, invasion, and dissemination of mycobacteria in host cells. In this study, we report that the extracellular matrix components, elastin and its precursor (tropoelastin) derived from human aorta, lung, and skin, serve as binding partners of Ag85 from Mycobacterium tuberculosis. The binding affinity of M. tuberculosis Ag85 to human tropoelastin was characterized (K(D) = 0.13 ± 0.006 μm), and a novel Ag85-binding motif, AAAKAA(K/Q)(Y/F), on multiple tropoelastin modules was identified. In addition, the negatively charged Glu-258 of Ag85 was demonstrated to participate in an electrostatic interaction with human tropoelastin. Moreover, binding of Ag85 on elastin siRNA-transfected Caco-2 cells was significantly reduced (34.3%), implying that elastin acts as an important ligand contributing to mycobacterial invasion.

Design, synthesis, and X-ray analysis of a glycoconjugate bound to Mycobacterium tuberculosis antigen 85C

Tuberculosis (TB) is a global health threat with nearly 500 000 new cases of multidrug-resistant TB estimated to occur every year, so new drugs are desperately needed. A number of current antimycobacterial drugs work by interfering with the biosynthesis of key components of the mycolylarabinogalactan (mAG). In light of this observation, other enzymes involved in the synthesis of the mAG should also serve as targets for antimycobacterial drug development. One potential target is the Antigen 85 (Ag85) complex, a family of mycolyltransferases that are responsible for the transfer of mycolic acids from trehalose monomycolate (TMM) to the arabinogalactan. Virtual thiophenyl-arabinoside conjugates were docked to antigen Ag85C (PDB code: 1va5 ) using Glide. Compounds with good docking scores were synthesized by a Gewald synthesis followed by linking to 5-thioarabinofuranosides. The resulting thiophenyl-thioarabinofuranosides were assayed for inhibition of mycoyltransferase activity using a 4-methylumbelliferyl butyrate fluorescence assay. The conjugates showed K(i) values ranging from 18.2 to 71.0 μM. The most potent inhibitor was soaked into crystals of Mycobacterium tuberculosis antigen 85C and the structure of the complex determined. The X-ray structure shows the compound bound within the active site of the enzyme with the thiophene moiety positioned in the putative α-chain binding site of TMM and the arabinofuranoside moiety within the known carbohydrate-binding site as exhibited for the Ag85B-trehalose crystal structure. Unexpectedly, no specific hydrogen bonding interactions are being formed between the arabinofuranoside and the carbohydrate-binding site of the active site suggesting that the binding of the arabinoside within this structure is driven by shape complementarily between the arabinosyl moiety and the carbohydrate binding site.

Targeting the mycobacterial envelope for tuberculosis drug development

The bacterium that causes tuberculosis, Mycobacterium tuberculosis, possesses a rather unique outer membrane composed largely of lipids that possess long-chain and branched fatty acids, called mycolic acids. These lipids form a permeability barrier that prevents entry of many environmental solutes, thereby making these bacteria acid-fast and able to survive extremely hostile surroundings. Antitubercular drugs must penetrate this layer to reach their target. This review highlights drug development efforts that have added to the slowly growing tuberculosis drug pipeline, identified new enzyme activities to target with drugs and increased the understanding of important biosynthetic pathways for mycobacterial outer membrane and cell wall core assembly. In addition, a portion of this review looks at discovery efforts aimed at weakening this barrier to decrease mycobacterial virulence, decrease fitness in the host or enhance the efficacy of the current drug repertoire by disrupting the permeability barrier.

Antigen 85C inhibition restricts Mycobacterium tuberculosis growth through disruption of cord factor biosynthesis

The antigen 85 (Ag85) protein family, consisting of Ag85A, -B, and -C, is vital for Mycobacterium tuberculosis due to its role in cell envelope biogenesis. The mycoloyl transferase activity of these proteins generates trehalose dimycolate (TDM), an envelope lipid essential for M. tuberculosis virulence, and cell wall arabinogalactan-linked mycolic acids. Inhibition of these enzymes through substrate analogs hinders growth of mycobacteria, but a link to mycolic acid synthesis has not been established. In this study, we characterized a novel inhibitor of Ag85C, 2-amino-6-propyl-4,5,6,7-tetrahydro-1-benzothiophene-3-carbonitrile (I3-AG85). I3-AG85 was isolated from a panel of four inhibitors that exhibited structure- and dose-dependent inhibition of M. tuberculosis division in broth culture. I3-AG85 also inhibited M. tuberculosis survival in infected primary macrophages. Importantly, it displayed an identical MIC against the drug-susceptible H37Rv reference strain and a panel of extensively drug-resistant/multidrug-resistant M. tuberculosis strains. Nuclear magnetic resonance analysis indicated binding of I3-AG85 to Ag85C, similar to its binding to the artificial substrate octylthioglucoside. Quantification of mycolic acid-linked lipids of the M. tuberculosis envelope showed a specific blockade of TDM synthesis. This was accompanied by accumulation of trehalose monomycolate, while the overall mycolic acid abundance remained unchanged. Inhibition of Ag85C activity also disrupted the integrity of the M. tuberculosis envelope. I3-AG85 inhibited the division of and reduced TDM synthesis in an M. tuberculosis strain deficient in Ag85C. Our results indicate that Ag85 proteins are promising targets for novel antimycobacterial drug design.

Discovery of novel MDR-Mycobacterium tuberculosis inhibitor by new FRIGATE computational screen

With 1.6 million casualties annually and 2 billion people being infected, tuberculosis is still one of the most pressing healthcare challenges. Here we report on the new computational docking algorithm FRIGATE which unites continuous local optimization techniques (conjugate gradient method) with an inherently discrete computational approach in forcefield computation, resulting in equal or better scoring accuracies than several benchmark docking programs. By utilizing FRIGATE for a virtual screen of the ZINC library against the Mycobacterium tuberculosis (Mtb) enzyme antigen 85C, we identified novel small molecule inhibitors of multiple drug-resistant Mtb, which bind in vitro to the catalytic site of antigen 85C.

Novel mycobacteria antigen 85 complex binding motif on fibronectin

The members of the antigen 85 protein family (Ag85), consisting of members Ag85A, Ag85B, and Ag85C, are the predominantly secreted proteins of mycobacteria and possess the ability to specifically interact with fibronectin (Fn). Because Fn-binding proteins are likely to be important virulence factors of Mycobacterium spp., Ag85 may contribute to the adherence, invasion, and dissemination of organisms in host tissue. In this study, we reported the Fn binding affinity of Ag85A, Ag85B, and Ag85C from Mycobacterium avium subsp. paratuberculosis (MAP) (K(D) values were determined from 33.6 to 68.4 nm) and mapped the Ag85-binding motifs of Fn. Fn14, a type III module located on the heparin-binding domain II (Hep-2) of Fn, was discovered to interact with Ag85 from MAP. The peptide inhibition assay subsequently demonstrated that a peptide consisting of residues 17-26 from Fn14 ((17)SLLVSWQPPR(26), termed P17-26) could interfere with Ag85B binding to Fn (73.3% reduction). In addition, single alanine substitutions along the sequence of P17-26 revealed that the key residues involved in Ag85-Fn binding likely contribute through hydrophobic and charge interactions. Moreover, binding of Ag85 on Fn siRNA-transfected Caco2 cells was dramatically reduced (44.6%), implying the physiological significance of the Ag85-Fn interaction between mycobacteria and host cells during infection. Our results indicate that Ag85 binds to Fn at a novel motif and plays a critical role in mycobacteria adherence to host cells by initiating infection. Ag85 might serve as an important colonization factor potentially contributing to mycobacterial virulence.

The Mycobacterium tuberculosis Ag85A is a novel diacylglycerol acyltransferase involved in lipid body formation

Mycobacterium tuberculosis accumulates large amounts of triacylglycerol (TAG) which acts as storage compounds for energy and carbon. The mycobacterial triacylglycerols stored in the form of intracellular lipid droplets are essential for long-term survival of M. tuberculosis during a dormant state. We report here that when the M. tuberculosis mycolytransferase Ag85A is overexpressed in Mycobacterium smegmatis mc(2)155, cell morphology was changed and the cells became grossly enlarged. A massive formation of lipid bodies and a change in lipid pattern was observed simultaneously. We suspected a possible role of Ag85A in the acyl lipid metabolism and discovered that the enzyme possesses acyl-CoA:diacylglycerol acyltransferase (DGAT) activity in addition to its well-known function as mycolyltransferase. Ag85A mediates the transesterification of diacylglycerol using long-chain acyl-CoA as acyl donors. The K(m) and K(cat) values for palmitoleoyl-coenzyme A were 390 µM and 55.54 min(-1) respectively. A docking model suggests that palmitoleoyl-coenzyme A and 1,2-dipalmitin occupy the same active site as trehalose 6,6'-dimycolate and trehalose 6'-monomycolate. The site-directed Ser126Ala mutation of the active site proved that this residue is involved in the catalytic activity of this enzyme. Although not proven conclusively for dormant stage of M. tuberculosis, our novel finding about the synthesis of TAGs by Ag85A strongly suggests that Ag85A may play a significant role in the formation of lipid storage bodies and thus also in the establishment and maintenance of a persistent tuberculosis infection.

Uptake of unnatural trehalose analogs as a reporter for Mycobacterium tuberculosis

The detection of tuberculosis currently relies upon insensitive and unspecific techniques; newer diagnostics would ideally co-opt specific bacterial processes to provide real-time readouts. The trehalose mycolyltransesterase enzymes (antigens 85A, 85B and 85C (Ag85A, Ag85B, Ag85C)) serve as essential mediators of cell envelope function and biogenesis in Mycobacterium tuberculosis. Through the construction of a systematically varied sugar library, we show here that Ag85 enzymes have exceptionally broad substrate specificity. This allowed exogenously added synthetic probes to be specifically incorporated into M. tuberculosis growing in vitro and within macrophages. Even bulky substituents, such as a fluorescein-containing trehalose probe (FITC-trehalose), were incorporated by growing bacilli, thereby producing fluorescent bacteria; microscopy revealed selective labeling of poles and membrane. Addition of FITC-trehalose to M. tuberculosis-infected macrophages allowed selective, sensitive detection of M. tuberculosis within infected mammalian macrophages. These studies suggest that analogs of trehalose may prove useful as probes of function and for other imaging modalities.

The mycolyltransferase 85A, a putative drug target of Mycobacterium tuberculosis: development of a novel assay and quantification of glycolipid-status of the mycobacterial cell wall

The enzymes of the antigen 85 complex (Ag85A, B, and C) possess mycolyltransferase activity and catalyze the synthesis of the most abundant glycolipid of the mycobacterial cell wall, the cord factor. The cord factor (trehalose 6,6'-dimycolate, TDM) is essential for the integrity of the mycobacterial cell wall and pathogenesis of the bacillus. Thus, TDM biosynthesis is regarded as a potential drug target for control of Mycobacterium tuberculosis infections. Trehalose 6,6'-dimycolate (TDM) is synthesized from two molecules of trehalose-6'-monomycolate (TMM) by antigen 85A. We report here a novel enzyme assay using the natural substrate TMM. The novel colorimetric assay is based on the quantification of glucose from the degradation of trehalose, which is the product from catalytic activity of antigen 85A. Using the new assay, K(m) and K(cat) were determined with values of 129.6+/-8.1 microM and 65.4+/-4.1 min(-1), respectively. This novel assay is also suitable for robust high-throughput screening (HTS) for compound library screening against mycolyltransferase (antigen 85A). The assay is significantly faster and more convenient to use than all assays currently in use. The assay has a very low coefficient of variance (0.04) in 96-well plates and shows a Z' factor of 0.67-0.73, indicating the robustness of the assay. In addition, this new assay is highly suitable for the quantification of total TMM of the mycobacterial cell envelope.

Design, synthesis and biological evaluation of sugar-derived esters, alpha-ketoesters and alpha-ketoamides as inhibitors for Mycobacterium tuberculosis antigen 85C

Peptide-based 1,2-dicarbonyl compounds have emerged as potent inhibitors for serine proteases. Herein, we have designed and synthesized d-arabinose and d-trehalose-based esters, alpha-ketoesters and alpha-ketoamides, and evaluated their inhibitory activity against Mycobacterium tuberculosis (Mtb) antigen 85C (ag85C), an acyltransferase in the serine hydrolase superfamily. In addition the compounds were evaluated for the ability to inhibit the growth of Mycobacterium smegmatis ATCC 14 468, a non-pathogenic surrogate for Mtb. Among the synthetic analogs evaluated only the methyl ester derived from d-arabinose was found to inhibit the acyltransferase activity of ag85C (IC(50) = 25 mM). Based on this weak inhibitory activity it was not surprising that none of the compounds inhibits the growth of M. smegmatis. In spite of the weak inhibitory activity of , X-ray crystallography on crystals of ag85C soaked with suggested the formation of a covalent ester adduct between and the Ser124 side chain hydroxyl moiety found within the catalytic site of ag85C; however, some of the active site electron density appears to result from bound glycerol. The lack of activity associated with the alpha-ketoester and alpha-ketoamide derivatives of d-trehalose may be the result of intramolecular cyclization of the alpha-keto moiety with the nearby C-4/4' hydroxyls leading to the formation of stable bicyclo-ester and amide derivatives.