Asha Chaubey

Engineering synthetic recursive pathways to generate non-natural small molecules.

Recursive pathways are broadly defined as those that catalyze a series of reactions such that the key, bond-forming functional group of the substrate is always regenerated in each cycle, allowing for a new cycle of reactions to begin. Recursive carbon-chain elongation pathways in nature produce fatty acids, polyketides, isoprenoids and α-keto acids (αKAs), which all use modular or iterative approaches for chain elongation. Recently, an artificial pathway for αKA elongation has been built that uses an engineered isopropylmalate synthase to recursively condense acetyl-CoA with αKAs. This synthetic approach expands the possibilities for recursive pathways beyond the modular or iterative synthesis of natural products and serves as a case study for understanding the challenges of building recursive pathways from nonrecursive enzymes. There exists the potential to design synthetic recursive pathways far beyond what nature has evolved.

Secondary Metabolites from Endophytic Fungus Penicillium pinophilum Induce ROS-Mediated Apoptosis through Mitochondrial Pathway in Pancreatic Cancer Cells

The endophytic fungus strain MRCJ-326, isolated from Allium schoenoprasum, which is also known as Snow Mountain Garlic or Kashmiri garlic, was identified as Penicillium pinophilum on the basis of morphological characteristics and internal transcribed spacer region nucleotide sequence analysis. The endophytic fungus extract was subjected to 2D-SEPBOX bioactivity-guided fractionation and purification. The anthraquinone class of the bioactive secondary metabolites were isolated and characterized as oxyskyrin (1), skyrin (2), dicatenarin (3), and 1,6,8-trihydroxy-3-hydroxy methylanthraquinone (4) by spectral analysis. Dicatenarin and skyrin showed marked growth inhibition against the NCI60/ATCC panel of human cancer cell lines with least IC50 values of 12 µg/mL and 27 µg/mL, respectively, against the human pancreatic cancer (MIA PaCa-2) cell line. The phenolic hydroxyl group in anthraquinones plays a crucial role in the oxidative process and bioactivity. Mechanistically, these compounds, i.e., dicatenarin and skyrin, significantly induce apoptosis and transmit the apoptotic signal via intracellular reactive oxygen species generation, thereby inducing a change in the mitochondrial transmembrane potential and induction of the mitochondrial-mediated apoptotic pathway. Our data indicated that dicatenarin and skyrin induce reactive oxygen species-mediated mitochondrial permeability transition and resulted in an increased induction of caspase-3 apoptotic proteins in human pancreatic cancer (MIA PaCa-2) cells. Dicatenarin showed a more pronounced cytotoxic/proapopotic effect than skyrin due to the presence of an additional phenolic hydroxyl group at C-4, which increases oxidative reactive oxygen species generation. This is the first report from P. pinophilum secreating these cytotoxic/proapoptotic secondary metabolites.

Arthrobacter sp. lipase immobilization for preparation of enantiopure masked β-amino alcohols

Recent reports on immobilization of lipase from Arthrobacter sp. (ABL, MTCC 5125; IIIM isolate) on insoluble polymers have shown altered properties including stability and enantioselectivity. Present work demonstrates a facile method for the preparation of enantiopure beta-amino alcohols by modulation of ABL enzyme properties via immobilization on insoluble as well as soluble supports using entrapment/covalent binding techniques. Efficacies of immobilized ABL on insoluble supports prepared from tetraethylorthosilicate/aminopropyltriethoxy silane and soluble supports derived from copolymerization of N-vinyl pyrrolidone-allylglycidyl ether (ANP type)/N-vinyl pyrrolidone-glycidyl methacrylate (GNP type) for kinetic resolution of masked beta-amino alcohols have been studied vis-à-vis free ABL enzyme/wet cell biomass. The immobilized lipase on different insoluble/soluble supports has shown 21-110 mg/g protein binding and 30-700 U/g activity for hydrolyzing tributyrin substrate. The findings have shown a significant enhancement in enantioselectivity (ee 99%) vis-à-vis wet cell biomass providing ee 70-90% for resolution of beta-amino alcohols.

Phialomustin A–D, new antimicrobial and cytotoxic metabolites from an endophytic fungus, Phialophora mustea

Phialomustin A–D (1–4), four new bioactive metabolites, with an unprecedented azaphilone derived skeleton, were isolated and characterized from an endophytic fungus isolated from Crocus sativus. The ITS-5.8S-ITS2 ribosomal gene sequence of the endophyte displayed a sequence similarity of more than 99% with Phialophora mustea. The structural determinations of compounds (1–4) were authenticated by spectroscopic and chemical analysis. The absolute configuration of the stereogenic centers of 1, 3 and 4 were determined by electronic circular dichroism spectroscopy. Compounds 3 and 4 showed promising antifungal activities against Candida albicans, with IC50 values of 14.3 and 73.6 μM, whereas compound 2 exhibited remarkable cytotoxic activity against the human breast cancer cell line, T47D, with an IC50 of 1 μM.

An Insight into the Secondary Metabolism of Muscodor yucatanensis: Small-Molecule Epigenetic Modifiers Induce Expression of Secondary Metabolism-Related Genes and Production of New Metabolites in the Endophyte

Muscodor spp. are proficient producers of bioactive volatile organic compounds (VOCs) with many potential applications. However, all members of this genus produce varying amounts and types of VOCs which suggests the involvement of epigenetics as a possible explanation. The members of this genus are poorly explored for the production of soluble compounds (extrolites). In this study, the polyketide synthase (PKS) and non-ribosomal peptide synthetase (NRPS) genes from an endophyte, Muscodor yucatanensis Ni30, were cloned and sequenced. The PKS genes belonged to reduced, partially reduced, non-reduced, and highly reduced subtypes. Strains over-expressing PKS genes were developed through the use of small-molecule epigenetic modifiers (suberoylanilide hydroxamic acid (SAHA) and 5-azacytidine). The putative epigenetic variants of this organism differed considerably from the wild type in morphological features and cultural characteristics as well as metabolites that were produced. Each variant produced a different set of VOCs distinct from the wild type, and several VOCs including methyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)hexane-2,4-diol and 2-carboxymethyl-3-n-hexylmaleic appeared in the variant strains, the production of which could be attributed to the activity of otherwise silent PKS genes. The bioactive extrolite brefeldin A was isolated and characterized from the wild type. However, this metabolite was not detected in EV-1, but instead, two other products were isolated and characterized as ergosterol and xylaguaianol C. Hence, M. yucatanensis has the genetic potential to produce several previously undetectable VOCs and organic solvent soluble products. It is also the case that small-molecule epigenetic modifiers can be used to produce stable variant strains of fungi with the potential to produce new molecules. Finally, this work hints to the prospect that the epigenetics of an endophytic microorganism can be influenced by any number of environmental and chemical factors associated with its host plant which may help to explain the enormous chemical diversity of secondary metabolic products found in Muscodor spp.

Escherichia coli N-Acetylglucosamine-1-Phosphate-Uridyltransferase/Glucosamine-1-Phosphate-Acetyltransferase (GlmU) Inhibitory Activity of Terreic Acid Isolated from Aspergillus terreus

Secondary metabolite of Aspergillus terreus, terreic acid, is a reported potent antibacterial that was identified more than 60 years ago, but its cellular target(s) are still unknown. Here we screen its activity against the acetyltransferase domain of a bifunctional enzyme, Escherichia coli N-acetylglucosamine-1-phosphate-uridyltransferase/glucosamine-1-phosphate-acetyltransferase (GlmU). An absorbance-based assay was used to screen terreic acid against the acetyltransferase activity of E. coli GlmU. Terreic acid was found to inhibit the acetyltransferase domain of E. coli GlmU with an IC50 of 44.24 ± 1.85 µM. Mode of inhibition studies revealed that terreic acid was competitive with AcCoA and uncompetitive with GlcN-1-P. It also exhibited concentration-dependent killing of E. coli ATCC 25922 up to 4× minimum inhibitory concentration and inhibited the growth of biofilms generated by E. coli. Characterization of resistant mutants established mutation in the acetyltransferase domain of GlmU. Terreic acid was also found to be metabolically stable in the in vitro incubations with rat liver microsome in the presence of a NADPH regenerating system. The studies reported here suggest that terreic acid is a potent antimicrobial agent and support that E. coli GlmU acetyltransferase is a molecular target of terreic acid, resulting in its antibacterial activity.

Epigenetic modifier induced enhancement of fumiquinazoline C production in Aspergillus fumigatus (GA-L7): an endophytic fungus from Grewia asiatica L.

Present study relates to the effect of valproic acid, an epigenetic modifier on the metabolic profile of Aspergillus fumigatus (GA-L7), an endophytic fungus isolated from Grewia asiatica L. Seven secondary metabolites were isolated from A. fumigatus (GA-L7) which were identified as: pseurotin A, pseurotin D, pseurotin F2, fumagillin, tryprostatin C, gliotoxin and bis(methylthio)gliotoxin. Addition of valproic acid in the growth medium resulted in the alteration of secondary metabolic profile with an enhanced production of a metabolite, fumiquinazoline C by tenfolds. In order to assess the effect of valproic acid on the biosynthetic pathway of fumiquinazoline C, we studied the expression of the genes involved in its biosynthesis, both in the valproic acid treated and untreated control culture. The results revealed that all the genes i.e. Afua_6g 12040, Afua_6g 12050, Afua_6g 12060, Afua_6g 12070 and Afua_6g 12080, involved in the biosynthesis of fumiquinazoline C were overexpressed significantly by 7.5, 8.8, 3.4, 5.6 and 2.1 folds respectively, resulting in overall enhancement of fumiquinazoline C production by about tenfolds.

Revelation and cloning of valinomycin synthetase genes in Streptomyces lavendulae ACR-DA1 and their expression analysis under different fermentation and elicitation conditions

Streptomyces species are amongst the most exploited microorganisms due to their ability to produce a plethora of secondary metabolites with bioactive potential, including several well known drugs. They are endowed with immense unexplored potential and substantial efforts are required for their isolation as well as characterization for their bioactive potential. Unexplored niches and extreme environments are host to diverse microbial species. In this study, we report Streptomyces lavendulae ACR-DA1, isolated from extreme cold deserts of the North Western Himalayas, which produces a macrolactone antibiotic, valinomycin. Valinomycin is a K+ ionophoric non-ribosomal cyclodepsipeptide with a broad range of bioactivities including antibacterial, antifungal, antiviral and cytotoxic/anticancer activities. Production of valinomycin by the strain S. lavendulae ACR-DA1 was studied under different fermentation conditions like fermentation medium, temperature and addition of biosynthetic precursors. Synthetic medium at 10°C in the presence of precursors i.e. valine and pyruvate showed enhanced valinomycin production. In order to assess the impact of various elicitors, expression of the two genes viz. vlm1 and vlm2 that encode components of heterodimeric valinomycin synthetase, was analyzed using RT-PCR and correlated with quantity of valinomycin using LC-MS/MS. Annelid, bacterial and yeast elicitors increased valinomycin production whereas addition of fungal and plant elicitors down regulated the biosynthetic genes and reduced valinomycin production. This study is also the first report of valinomycin biosynthesis by Streptomyces lavendulae.

Lipovelutibols A–D: Cytotoxic Lipopeptaibols from the Himalayan Cold Habitat Fungus Trichoderma velutinum

Four novel lipovelutibols A (1), B (2), C (3), and D (4) containing six amino acid residues with leucinol at the C-terminus and a fatty acyl moiety (n-octanoyl) at its N-terminus were isolated from the psychrotrophic fungus Trichoderma velutinum collected from the Himalayan cold habitat. The structures (1-4) were determined by NMR and MS/MS, and the stereochemistry of amino acids by Marfeys method. Lipopeptaibols 2 and 4 were found to contain d-isovaline, a nonproteinogenic amino acid, but lacked α-aminoisobutyric acid, characteristic of peptaibols. Cytotoxic activity of 2 and 4 was observed against HL-60, LS180, MDA-MB-231, and A549 cancer cell lines.

Immobilization of enantioselective lipase on soluble supports for kinetic resolution of drug intermediates

The microbial lipase, Arthrobacter sp. lipase (MTCC 5125), from the Indian Institute of Integrative Medicine repository, is known as an effective catalyst for high enantioselective kinetic resolution of drug intermediates. The ABL was immobilized on water-soluble linear supports by covalently binding it to the epoxy groups on the N-vinyl pyrrolidone/allyl glycidyl ether and N-vinyl pyrrolidone/glycidyl methacrylate copolymers. The immobilized lipase, on different soluble supports, had 90–110 mg/g protein binding and 500–700 U/g hydrolysis activities for tributyrin substrate. These copolymers had soluble/insoluble characteristics in different pH ranges, which is an advantage over insoluble copolymers. A soluble polymer at neutral pH provided better accessibility to the immobilized enzyme, which was recovered by precipitation at pH 2–3 for reuse. Kinetic resolution of racemic acyl derivatives of chiral auxiliaries and drug intermediates, namely, phenyl ethanol, aminoalcohol, and fluoxetine intermediate resulted in a significant enhancement in enantioselectivity (99%).