Design and synthesis of substrate-mimic inhibitors of mycothiol-S-conjugate amidase from Mycobacterium tuberculosis

A novel scaffold to fight Pseudomonas aeruginosa pyocyanin production: early steps to novel antivirulence drugs

Aim: Although bacterial resistance is a growing concern worldwide, the development of antibacterial drugs has been steadily decreasing. One alternative to fight this issue relies on reducing the bacteria virulence without killing it. PhzS plays a pivotal role in pyocyanin production in Pseudomonas aeruginosa. Results: A total of 31 thiazolidinedione derivatives were evaluated as putative PhzS inhibitors, using thermo shift assays. Compounds that significantly shifted PhzS's Tm had their mode of inhibition (cofactor competitor) and affinity calculated by thermo shift assays as well. The most promising compound (E)-5-(4-((4-oxo-3-phenyl-3,4-dihydroquinazolin-2-yl)methoxy)benzylidene)thiazolidine-2,4-dione had their affinity confirmed by microscale thermophoresis (K_d = 18 μM). Cellular assays suggest this compound reduces pyocyanin production in vitro, but does not affect P. aeruginosa viability. Conclusion: The first inhibitor of PhzS is described.

Homology modeling, structural insights and in-silico screening for selective inhibitors of mycobacterial CysE

Tuberculosis posses a major threat for health practitioners due to lengthy treatment regimen, increase in the drug-resistant strains of Mycobacterium tuberculosis (M. tb) and unavailability of drugs for its persistent form. Therefore, there is an urgent need for discovery of new and improved anti-tubercular drugs. In M. tb, the two step de novo biosynthesis of L-cysteine, an essential metabolic pathway is reported to be up-regulated in the persistent phase of the organism, involves two enzymes CysE and CysK. Although, structural insights for rational drug discovery are available for the later, not much information is known for the former. This study proposes a 3-dimensional model of M. tb CysE followed by in-silico screening of 67,030 anti-tuberculosis bioactive compounds. Subsequently, post-processing of 1000 best hits was carried out and top 200 compounds thus obtained were docked into the active site cleft of E. coli homologue as a control, but revealed unexpected results. Differences in the active site architectures and comparative analysis of molecular electrostatic potentials between the two CysEs provide molecular basis for the compounds C1, C3, C4 and C7 exhibiting preferential binding for M. tb CysE. In addition, shorter N-terminus along with positive and irregular trimeric base of M. tb CysE indicates its biological assembly as trimer. Based on mapping of residues involved in cysteine sensitivity on to the model structure of M. tb CysE, it is hypothesized that feedback inhibition of this homologue by cysteine may be affected.Communicated by Ramaswamy H. Sarma.

Nonclassical Routes for Amide Bond Formation

The present review offers an overview of nonclassical (e.g., with no pre- or in situ activation of a carboxylic acid partner) approaches for the construction of amide bonds. The review aims to comprehensively discuss relevant work, which was mainly done in the field in the last 20 years. Organization of the data follows a subdivision according to substrate classes: catalytic direct formation of amides from carboxylic and amines ( section 2 ); the use of carboxylic acid surrogates ( section 3 ); and the use of amine surrogates ( section 4 ). The ligation strategies (NCL, Staudinger, KAHA, KATs, etc.) that could involve both carboxylic acid and amine surrogates are treated separately in section 5 .

The Physiology and Genetics of Oxidative Stress in Mycobacteria

During infection, Mycobacterium tuberculosis is exposed to a diverse array of microenvironments in the human host, each with its own unique set of redox conditions. Imbalances in the redox environment of the bacillus or the host environment serve as stimuli, which could regulate virulence. The ability of M. tuberculosis to evade the host immune response and cause disease is largely owing to the capacity of the mycobacterium to sense changes in its environment, such as host-generated gases, carbon sources, and pathological conditions, and alter its metabolism and redox balance accordingly for survival. In this article we discuss the redox sensors that are, to date, known to be present in M. tuberculosis , such as the Dos dormancy regulon, WhiB family, anti-σ factors, and MosR, in addition to the strategies present in the bacillus to neutralize free radicals, such as superoxide dismutases, catalase-peroxidase, thioredoxins, and methionine sulfoxide reductases, among others. M. tuberculosis is peculiar in that it appears to have a hierarchy of redox buffers, namely, mycothiol and ergothioneine. We discuss the current knowledge of their biosynthesis, function, and regulation. Ergothioneine is still an enigma, although it appears to have distinct and overlapping functions with mycothiol, which enable it to protect against a wide range of toxic metabolites and free radicals generated by the host. Developing approaches to quantify the intracellular redox status of the mycobacterium will enable us to determine how the redox balance is altered in response to signals and environments that mimic those encountered in the host.

Mycothiol: a promising antitubercular target

Tuberculosis (TB) is the world's second commonest cause of death next to HIV/AIDS. The increasing emergence of multi drug resistance and the recalcitrant nature of persistent infections pose an additional challenge for the treatment of TB. Due to the development of resistance to conventional antibiotics there is a need for new therapeutic strategies to combat M. tuberculosis. One such target is Mycothiol (MSH), a major low molecular-mass thiol in mycobacteria, an important cellular anti-oxidant. MSH is present only in actinomycetes and hence is a good target. This review explores mycothiol as a potential target against tuberculosis and various research ongoing worldwide.

Stereoselective α-glycosylation of C(6)-hydroxyl myo-inositols via nickel catalysis-application to the synthesis of GPI anchor pseudo-oligosaccharides

Glycosylphosphatidyl inositol (GPI) anchors play a key role in many eukaryotic biological pathways. Stereoselective synthesis of GPI anchor analogues have proven to be critical for probing the biosynthesis, structure, and biological properties of these compounds. Challenges that have emerged from these efforts include the preparation of the selectively protected myo-inositol building blocks and the stereoselective construction of glucosamine α-linked myo-inositol containing pseudodisaccharide units. Herein, we describe the effectiveness of the cationic nickel(II) catalyst, Ni(4-F-PhCN)4(OTf)2, at promoting selective formation of 1,2-cis-2-amino glycosidic bonds between the C(2)-N-substituted benzylideneamino trihaloacetimidate donors and C(6)-hydroxyl myo-inositol acceptors. This catalytic coupling process allows rapid access to pseudosaccharides of GPI anchors in good yields and with excellent levels of α-selectivity (α:β=10:1-20:1). In stark contrast, activation of trichloroacetimidate donors containing the C(2)-N-substituted benzylidene group with TMSOTf and BF3(.)OEt2 provided the desired pseudodisaccharides as a 1:1 mixture of α- and β-isomers.

New targets and inhibitors of mycobacterial sulfur metabolism

The identification of new antibacterial targets is urgently needed to address multidrug resistant and latent tuberculosis infection. Sulfur metabolic pathways are essential for survival and the expression of virulence in many pathogenic bacteria, including Mycobacterium tuberculosis. In addition, microbial sulfur metabolic pathways are largely absent in humans and therefore, represent unique targets for therapeutic intervention. In this review, we summarize our current understanding of the enzymes associated with the production of sulfated and reduced sulfur-containing metabolites in Mycobacteria. Small molecule inhibitors of these catalysts represent valuable chemical tools that can be used to investigate the role of sulfur metabolism throughout the Mycobacterial lifecycle and may also represent new leads for drug development. In this light, we also summarize recent progress made in the development of inhibitors of sulfur metabolism enzymes.

Nickel-catalyzed α-glycosylation of C(1)-hydroxyl D-myo-inositol: a formal synthesis of mycothiol

Formal synthesis of mycothiol has been developed via nickel-catalyzed α-glycosylation of the C(1)-hydroxyl group of D-myo-inositols with C(2)-N-substituted benzylideneamino N-phenyl trifluoroacetimidate donors. The pseudo-oligosaccharides were obtained in good yield and with excellent α-selectivity. Removal of the C(2)-N-2-trifluoromethylphenyl-benzylidene group under mild conditions provides a pseudo-disaccharide, completing the formal synthesis of mycothiol.

Intramolecular alpha-glucosaminidation: synthesis of mycothiol

A protected cyclitol aglycon was tethered to an (N-arylsulfonyl)glucosamine donor by a methylene linker; the exclusively alpha-selective intramolecular glycosylation reaction was then initiated by electrophilic activation of the thioglycoside donor portion. Further transformations of the glycosylation product to give the M. tuberculosis detoxifier mycothiol and its oxidized congener, the disulfide mycothione, are detailed.

Biosynthesis and functions of mycothiol, the unique protective thiol of Actinobacteria

Mycothiol (MSH; AcCys-GlcN-Ins) is the major thiol found in Actinobacteria and has many of the functions of glutathione, which is the dominant thiol in other bacteria and eukaryotes but is absent in Actinobacteria. MSH functions as a protected reserve of cysteine and in the detoxification of alkylating agents, reactive oxygen and nitrogen species, and antibiotics. MSH also acts as a thiol buffer which is important in maintaining the highly reducing environment within the cell and protecting against disulfide stress. The pathway of MSH biosynthesis involves production of GlcNAc-Ins-P by MSH glycosyltransferase (MshA), dephosphorylation by the MSH phosphatase MshA2 (not yet identified), deacetylation by MshB to produce GlcN-Ins, linkage to Cys by the MSH ligase MshC, and acetylation by MSH synthase (MshD), yielding MSH. Studies of MSH mutants have shown that the MSH glycosyltransferase MshA and the MSH ligase MshC are required for MSH production, whereas mutants in the MSH deacetylase MshB and the acetyltransferase (MSH synthase) MshD produce some MSH and/or a closely related thiol. Current evidence indicates that MSH biosynthesis is controlled by transcriptional regulation mediated by sigma(B) and sigma(R) in Streptomyces coelicolor. Identified enzymes of MSH metabolism include mycothione reductase (disulfide reductase; Mtr), the S-nitrosomycothiol reductase MscR, the MSH S-conjugate amidase Mca, and an MSH-dependent maleylpyruvate isomerase. Mca cleaves MSH S-conjugates to generate mercapturic acids (AcCySR), excreted from the cell, and GlcN-Ins, used for resynthesis of MSH. The phenotypes of MSH-deficient mutants indicate the occurrence of one or more MSH-dependent S-transferases, peroxidases, and mycoredoxins, which are important targets for future studies. Current evidence suggests that several MSH biosynthetic and metabolic enzymes are potential targets for drugs against tuberculosis. The functions of MSH in antibiotic-producing streptomycetes and in bioremediation are areas for future study.

Mycothiol: synthesis, biosynthesis and biological functions of the major low molecular weight thiol in actinomycetes

Actinomycetes produce mycothiol as their major low molecular weight thiol, which parallels the functions of glutathione found in prokaryotes and most Gram-negative bacteria. This review covers progress that has so far been made in terms of its distribution, biosynthesis and metabolic functions, as well as chemical syntheses of mycothiol and alternative substrates and inhibitors of mycothiol biosynthesis and mycothiol-dependent enzymes. 152 references are cited.