The Methylerythritol Phosphate Pathway and its Significance as a Novel Drug Target

Ensiled Mixed Vegetables Enriched Carbohydrate Metabolism in Heterofermentative Lactic Acid Bacteria

This study evaluated the fermentation quality, nutritive profile, in vitro fermentation, and microbial communities colonising sorghum ensiled with an unsalable vegetable mixture (chopped beans, carrot, and onion (1:1:1) ) including: (1)−100% sorghum; (2)−80% sorghum + 20% vegetable mix or (3)−60% sorghum + 40% vegetable mix, on a dry matter (DM) basis, with or without a probiotic inoculant. Samples were obtained across 0, 1, 3, 5,7, and 101 days ensiling and after 14 d aerobic exposure. The V4 region of the 16S rRNA gene and the ITS1 region were sequenced to profile bacterial, archaeal, and fungal communities. Compared to the 0% DM, ethanol increased (p < 0.01) from 8.42 to 20.4 ± 1.32 mM with 40% DM vegetable mix inclusion, while lactate decreased from 5.93 to 2.24 ± 0.26 mM. Linear discriminant analysis revealed that relative abundances of 12 bacterial taxa were influenced by silage treatments (log LDA score ≥ 4.02; p ≤ 0.03), while predicted functional pathways of alternative carbohydrate metabolism (hexitol, sulfoquinovose and glycerol degradation; N-acetyl glucosamine biosynthesis; log LDA score ≥ 2.04; p ≤ 0.02) were similarly enriched. This study indicated that carbohydrate metabolism by heterofermentative lactic acid bacteria can increase the feed value of sorghum when ensiled with an unsalable vegetable mixture at 40%DM, without requiring a high quantity of lactate.

Comparative in-silico proteomic analysis discerns potential granuloma proteins of Yersinia pseudotuberculosis

Yersinia pseudotuberculosis is one of the three pathogenic species of the genus Yersinia. Most studies regarding pathogenesis of Y. pseudotuberculosis are based on the proteins related to Type III secretion system, which is a well-known primary virulence factor in pathogenic Gram-negative bacteria, including Y. pseudotuberculosis. Information related to the factors involved in Y. pseudotuberculosis granuloma formation is scarce. In the present study we have used a computational approach to identify proteins that might be potentially involved in formation of Y. pseudotuberculosis granuloma. A comparative proteome analysis and conserved orthologous protein identification was performed between two different genera of bacteria - Mycobacterium and Yersinia, their only common pathogenic trait being ability to form necrotizing granuloma. Comprehensive analysis of orthologous proteins was performed in proteomes of seven bacterial species. This included M. tuberculosis, M. bovis and M. avium paratuberculosis - the known granuloma forming Mycobacterium species, Y. pestis and Y. frederiksenii - the non-granuloma forming Yersinia species and, Y. enterocolitica - that forms micro-granuloma and, Y. pseudotuberculosis - a prominent granuloma forming Yersinia species. In silico proteome analysis indicated that seven proteins (UniProt id A0A0U1QT64, A0A0U1QTE0, A0A0U1QWK3, A0A0U1R1R0, A0A0U1R1Z2, A0A0U1R2S7, A7FMD4) might play some role in Y. pseudotuberculosis granuloma. Validation of the probable involvement of the seven proposed Y. pseudotuberculosis granuloma proteins was done using transcriptome data analysis and, by mapping on a composite protein-protein interaction map of experimentally proved M. tuberculosis granuloma proteins (RD1 locus proteins, ESAT-6 secretion system proteins and intra-macrophage secreted proteins). Though, additional experiments involving knocking out of each of these seven proteins are required to confirm their role in Y. pseudotuberculosis granuloma our study can serve as a basis for further studies on Y. pseudotuberculosis granuloma.

Interactions between Microcystis aeruginosa and coexisting bisphenol A at different nitrogen levels

Microcystis aeruginosa is known as a main contributor of cyanobacterial bloom. However, factors that drive its formation and dispersion remain poorly understood. The cellular-level responses to nutrient drivers of eutrophication were investigated. The results showed that growth rate of M. aeruginosa was significantly enhanced with the increasing bisphenol A (BPA) and nitrogen (N) level. Stress of BPA significantly inhibited cellular density, chlorophyll-a content across all the nutrient conditions, while Fv/Fm and rETRmax value were promoted by BPA. Responses of reactive oxygen species (ROS) value, superoxide dismutase (SOD) activity and malodialdehyde (MDA) content indicated that nitrogen deficiency and BPA caused oxidative stress to M. aeruginosa. Besides, nitrogen and BPA regulated the production and release of microcystins (MCs). M. aeruginosa exposed to BPA caused 95 up-regulated proteins, which was primarily associated with photosynthesis, nitrogen metabolism, glycolysis/glyconeogenesis and carbon fixation in photosynthetic organisms. The 91 down-regulated proteins were related to quorum sensing, longevity regulating and cell cycle-caulobacter, confirming that the driving force of regulating the change of cellular density and genes expression weakened. These findings provide important clues to elucidate the combined regulatory mechanisms of cyanobacterial blooms triggered by endocrine-disrupting compounds and environmental factors and help to effectively prevent and reduce cyanobacterial blooms.

Molecular Mechanism of Action of Antimalarial Benzoisothiazolones: Species-Selective Inhibitors of the Plasmodium spp. MEP Pathway enzyme, IspD

The methylerythritol phosphate (MEP) pathway is an essential metabolic pathway found in malaria parasites, but absent in mammals, making it a highly attractive target for the discovery of novel and selective antimalarial therapies. Using high-throughput screening, we have identified 2-phenyl benzo[d]isothiazol-3(2H)-ones as species-selective inhibitors of Plasmodium spp. 2-C-methyl-_D-erythritol-4-phosphate cytidyltransferase (IspD), the third catalytic enzyme of the MEP pathway. 2-Phenyl benzo[d]isothiazol-3(2H)-ones display nanomolar inhibitory activity against P. falciparum and P. vivax IspD and prevent the growth of P. falciparum in culture, with EC₅₀ values below 400 nM. In silico modeling, along with enzymatic, genetic and crystallographic studies, have established a mechanism-of-action involving initial non-covalent recognition of inhibitors at the IspD binding site, followed by disulfide bond formation through attack of an active site cysteine residue on the benzo[d]isothiazol-3(2H)-one core. The species-selective inhibitory activity of these small molecules against Plasmodium spp. IspD and cultured parasites suggests they have potential as lead compounds in the pursuit of novel drugs to treat malaria.

Quantifying the Metabolites of the Methylerythritol 4-Phosphate (MEP) Pathway in Plants and Bacteria by Liquid Chromatography-Triple Quadrupole Mass Spectrometry

The 2-C-methyl-d-erythritol 4-phosphate (MEP) pathway occurs in the plastids of higher plants and in most economically important prokaryotes where it is responsible for the biosynthesis of the isoprenoid building blocks, isopentenyl diphosphate and dimethylallyl diphosphate. These five-carbon compounds are the substrates for the enormous variety of terpenoid products, including many essential metabolites and substances of commercial value. Increased knowledge of the regulation of the MEP pathway is critical to understanding many aspects of plant and microbial metabolism as well as in developing biotechnological platforms for producing these commercially valuable isoprenoids. To achieve this goal, researchers must have the ability to investigate the in vivo kinetics of the pathway by accurately measuring the concentrations of MEP pathway metabolites. However, the low levels of these metabolites complicate their accurate determination without suitable internal standards. This chapter describes a sensitive method to accurately determine the concentrations of MEP pathway metabolites occurring at trace amounts in biological samples using liquid chromatography coupled to triple quadrupole mass spectrometry. In addition, simple protocols are given for producing stable isotope-labeled internal standards for these analyses.

Biosynthesis of Menaquinone (Vitamin K2) and Ubiquinone (Coenzyme Q)

Escherichia coli and Salmonella contain the naphthoquinones menaquinone (MK; vitamin K2) and demethylmenaquinone and the benzoquinone ubiquinone (coenzyme Q; Q). Both quinones are derived from the shikimate pathway, which has been called a "metabolic tree with many branches." There are two different pathways for the biosynthesis of the naphthoquinones. The vast majority of prokaryotes, including E. coli and Salmonella, and the plants use the o-succinylbenzoate pathway, while a minority uses the futalosine pathway. The quinone nucleus of Q is derived directly from chorismate, while that of MK is derived from chorismate via isochorismate. The prenyl side chains of both quinones are from isopentenyl diphosphate formed by the 2-C-methyl-D-erythritol 4-phosphate (non-mevalonate) pathway and the methyl groups are from S-adenosylmethionine. In addition, MK biosynthesis requires 2-ketoglutarate and cofactors ATP, coenzyme A, and thiamine pyrophosphate. Despite the fact that both quinones originate from the shikimate pathway, there are important differences in their biosyntheses. The prenyl side chain in MK biosynthesis is introduced at the penultimate step, accompanied by decarboxylation, whereas in Q biosynthesis it is introduced at the second step, with retention of the carboxyl group. In MK biosynthesis, all the reactions of the pathway up to prenylation are carried out by soluble enzymes, whereas all the enzymes involved in Q biosynthesis except the first are membrane bound. In MK biosynthesis, the last step is a C-methylation; in Q biosynthesis, the last step is an O-methylation. In Q biosynthesis a second C-methylation and O-methylation take place in the middle part of the pathway. Despite the fact that Q and MK biosyntheses diverge at chorismate, the C-methylations in both pathways are carried out by the same methyltransferase.

Optimization of the IPP Precursor Supply for the Production of Lycopene, Decaprenoxanthin and Astaxanthin by Corynebacterium glutamicum

The biotechnologically relevant bacterium Corynebacterium glutamicum, currently used for the million ton-scale production of amino acids for the food and feed industries, is pigmented due to synthesis of the rare cyclic C50 carotenoid decaprenoxanthin and its glucosides. The precursors of carotenoid biosynthesis, isopenthenyl pyrophosphate (IPP) and its isomer dimethylallyl pyrophosphate, are synthesized in this organism via the methylerythritol phosphate (MEP) or non-mevalonate pathway. Terminal pathway engineering in recombinant C. glutamicum permitted the production of various non-native C50 and C40 carotenoids. Here, the role of engineering isoprenoid precursor supply for lycopene production by C. glutamicum was characterized. Overexpression of dxs encoding the enzyme that catalyzes the first committed step of the MEP-pathway by chromosomal promoter exchange in a prophage-cured, genome-reduced C. glutamicum strain improved lycopene formation. Similarly, an increased IPP supply was achieved by chromosomal integration of two artificial operons comprising MEP pathway genes under the control of a constitutive promoter. Combined overexpression of dxs and the other six MEP pathways genes in C. glutamicum strain LYC3-MEP was not synergistic with respect to improving lycopene accumulation. Based on C. glutamicum strain LYC3-MEP, astaxanthin could be produced in the milligrams per gram cell dry weight range when the endogenous genes crtE, crtB, and crtI for conversion of geranylgeranyl pyrophosphate to lycopene were coexpressed with the genes for lycopene cyclase and β-carotene hydroxylase from Pantoea ananatis and carotene C(4) oxygenase from Brevundimonas aurantiaca.

Sub-inhibitory fosmidomycin exposures elicits oxidative stress in Salmonella enterica serovar Typhimurium LT2

Fosmidomycin is a time-dependent nanomolar inhibitor of methylerythritol phosphate (MEP) synthase, which is the enzyme that catalyzes the first committed step in the MEP pathway to isoprenoids. Importantly, fosmidomycin is one of only a few MEP pathway-specific inhibitors that exhibits antimicrobial activity. Most inhibitors identified to date only exhibit activity against isolated pathway enzymes. The MEP pathway is the sole route to isoprenoids in many bacteria, yet has no human homologs. The development of inhibitors of this pathway holds promise as novel antimicrobial agents. Similarly, analyses of the bacterial response toward MEP pathway inhibitors provides valuable information toward the understanding of how emergent resistance may ultimately develop to this class of antibiotics. We have examined the transcriptional response of Salmonella enterica serovar typhimurium LT2 to sub-inhibitory concentrations of fosmidomycin via cDNA microarray and RT-PCR. Within the regulated genes identified by microarray were a number of genes encoding enzymes associated with the mediation of reactive oxygen species (ROS). Regulation of a panel of genes implicated in the response of cells to oxidative stress (including genes for catalases, superoxide dismutases, and alkylhydrogen peroxide reductases) was investigated and mild upregulation in some members was observed as a function of fosmidomycin exposure over time. The extent of regulation of these genes was similar to that observed for comparable exposures to kanamycin, but differed significantly from tetracycline. Furthermore, S. typhimurium exposed to sub-inhibitory concentrations of fosmidomycin displayed an increased sensitivity to exogenous H2O2 relative to either untreated controls or kanamycin-treated cells. Our results suggest that endogenous oxidative stress is one consequence of exposures to fosmidomycin, likely through the temporal depletion of intracellular isoprenoids themselves, rather than other mechanisms that have been proposed to facilitate ROS accumulation in bacteria (e.g. cell death processes or the ability of the antibiotic to redox cycle).

Synthetic Routes to Methylerythritol Phosphate Pathway Intermediates and Downstream Isoprenoids

Isoprenoids constitute the largest class of natural products with greater than 55,000 identified members. They play essential roles in maintaining proper cellular function leading to maintenance of human health, plant defense mechanisms against predators, and are often exploited for their beneficial properties in the pharmaceutical and nutraceutical industries. Most impressively, all known isoprenoids are derived from one of two C5-precursors, isopentenyl diphosphate (IPP) or dimethylallyl diphosphate (DMAPP). In order to study the enzyme transformations leading to the extensive structural diversity found within this class of compounds there must be access to the substrates. Sometimes, intermediates within a biological pathway can be isolated and used directly to study enzyme/pathway function. However, the primary route to most of the isoprenoid intermediates is through chemical catalysis. As such, this review provides the first exhaustive examination of synthetic routes to isoprenoid and isoprenoid precursors with particular emphasis on the syntheses of intermediates found as part of the 2C-methylerythritol 4-phosphate (MEP) pathway. In addition, representative syntheses are presented for the monoterpenes (C10), sesquiterpenes (C15), diterpenes (C20), triterpenes (C30) and tetraterpenes (C40). Finally, in some instances, the synthetic routes to substrate analogs found both within the MEP pathway and downstream isoprenoids are examined.

Semi-automated curation of metabolic models via flux balance analysis: a case study with Mycoplasma gallisepticum

Primarily used for metabolic engineering and synthetic biology, genome-scale metabolic modeling shows tremendous potential as a tool for fundamental research and curation of metabolism. Through a novel integration of flux balance analysis and genetic algorithms, a strategy to curate metabolic networks and facilitate identification of metabolic pathways that may not be directly inferable solely from genome annotation was developed. Specifically, metabolites involved in unknown reactions can be determined, and potentially erroneous pathways can be identified. The procedure developed allows for new fundamental insight into metabolism, as well as acting as a semi-automated curation methodology for genome-scale metabolic modeling. To validate the methodology, a genome-scale metabolic model for the bacterium Mycoplasma gallisepticum was created. Several reactions not predicted by the genome annotation were postulated and validated via the literature. The model predicted an average growth rate of 0.358±0.12, closely matching the experimentally determined growth rate of M. gallisepticum of 0.244±0.03. This work presents a powerful algorithm for facilitating the identification and curation of previously known and new metabolic pathways, as well as presenting the first genome-scale reconstruction of M. gallisepticum.

Flux balance analysis (FBA) is a powerful approach for genome-scale metabolic modeling. It provides metabolic engineers with a tool for manipulating, predicting, and optimizing metabolism for biotechnological and biomedical purposes. However, we posit that it can also be used as tool for fundamental research in understanding and curating metabolic networks. Specifically, by using a genetic algorithm integrated with FBA, we developed a curation approach to identify missing reactions, incomplete reactions, and erroneous reactions. Additionally, it was possible to take advantage of the ensemble information from the genetic algorithm to identify the most critical reactions for curation. We tested our strategy using Mycoplasma gallisepticum as our model organism. Using the genome annotation as the basis, the preliminary genome-scale metabolic model consisted of 446 metabolites involved in 380 reactions. Carrying out our analysis, we found over 80 incorrect reactions and 16 missing reactions. Based upon the guidance of the algorithm, we were able to curate and resolve all discrepancies. The model predicted an average bacterial growth rate of 0.358±0.12 h⁻¹ compared to the experimentally observed 0.244±0.03 h⁻¹. Thus, our approach facilitated the curation of a genome-scale metabolic network and generated a high quality metabolic model.

DXP synthase-catalyzed C-N bond formation: nitroso substrate specificity studies guide selective inhibitor design

1-Deoxy-D-xylulose 5-phosphate (DXP) synthase catalyzes the first step in the nonmammalian isoprenoid biosynthetic pathway to form DXP from pyruvate and D-glyceraldehyde 3-phosphate (D-GAP) in a thiamin diphosphate-dependent manner. Its unique structure and mechanism distinguish DXP synthase from its homologues and suggest that it should be pursued as an anti-infective drug target. However, few reports describe any development of selective inhibitors of this enzyme. Here, we reveal that DXP synthase catalyzes C-N bond formation and exploit aromatic nitroso substrates as active site probes. Substrate specificity studies reveal a high affinity of DXP synthase for aromatic nitroso substrates compared to the related ThDP-dependent enzyme pyruvate dehydrogenase (PDH). Results from inhibition and mutagenesis studies indicate that nitroso substrates bind to E. coli DXP synthase in a manner distinct from that of D-GAP. Our results suggest that the incorporation of aryl acceptor substrate mimics into unnatural bisubstrate analogues will impart selectivity to DXP synthase inhibitors. As a proof of concept, we show selective inhibition of DXP synthase by benzylacetylphosphonate (BnAP).

Design of Potential Bisubstrate Inhibitors against Mycobacterium tuberculosis (Mtb) 1-Deoxy-D-Xylulose 5-Phosphate Reductoisomerase (Dxr)-Evidence of a Novel Binding Mode

In most bacteria, the nonmevalonate pathway is used to synthesize isoprene units. Dxr, the second step in the pathway, catalyzes the NADPH-dependent reductive isomerization of 1-deoxy-D-xylulose-5-phosphate (DXP) to 2-C-methyl-D-erythritol-4-phosphate (MEP). Dxr is inhibited by natural products fosmidomycin and FR900098, which bind in the DXP binding site. These compounds, while potent inhibitors of Dxr, lack whole cell activity against Mycobacterium tuberculosis (Mtb) due to their polarity. Our goal was to use the Mtb Dxr-fosmidomycin co-crystal structure to design bisubstrate ligands to bind to both the DXP and NADPH sites. Such compounds would be expected to demonstrate improved whole cell activity due to increased lipophilicity. Two series of compounds were designed and synthesized. Compounds from both series inhibited Mtb Dxr. The most potent compound (8) has an IC₅₀ of 17.8 µM. Analysis shows 8 binds to Mtb Dxr via a novel, non-bisubstrate mechanism. Further, the diethyl ester of 8 inhibits Mtb growth making this class of compounds interesting lead molecules in the search for new antitubercular agents.

Viable screening targets related to the bacterial cell wall

The synthesis of the bacterial peptidoglycan has been recognized for over 50 years as fertile ground for antibacterial discovery. Initially, empirical screening of natural products for inhibition of bacterial growth detected many chemical classes of antibiotics whose specific mechanisms of action were eventually dissected and defined. Of the nontoxic antibiotics discovered, most were found to be inhibitors of either protein synthesis or cell wall synthesis, which led to more directed screening for inhibitors of these pathways. Directed screening and design programs for cell wall inhibitors have been undertaken since the 1960s. In that time it has become clear that, while certain steps and intermediates have yielded selective inhibitors and are established targets, other potential targets have not yielded inhibitors whose antibacterial activity is proven to be solely due to that inhibition. Why has this search been so problematic? Are the established targets still worth pursuing? This review will attempt to answer these and other questions and evaluate the viability of targets related to peptidoglycan synthesis.

Influence of oxidative and nitrosative stress on accumulation of diphosphate intermediates of the non-mevalonate pathway of isoprenoid biosynthesis in corynebacteria and mycobacteria

Artificial generation of oxygen superoxide radicals in actively growing cultures of Mycobacterium tuberculosis, Myc. smegmatis, and Corynebacterium ammoniagenes is followed by accumulation in the bacterial cells of substantial amounts of 2-C-methyl-D-erythritol-2,4-cyclodiphosphate (MEcDP) - an intermediate of the non-mevalonate pathway of isoprenoid biosynthesis (MEP) - most possibly due to the interaction of the oxygen radicals with the 4Fe-4S group in the active center and inhibition of the enzyme (E)-4-oxy-3-methylbut-2-enyl diphosphate synthase (IspG). Cadmium ions known to inhibit IspG enzyme in chloroplasts (Rivasseau, C., Seemann, M., Boisson, A. M., Streb, P., Gout, E., Douce, R., Rohmer, M., and Bligny, R. (2009) Plant Cell Environ., 32, 82-92), when added to culture of Myc. smegmatis, substantially increase accumulation of MEcDP induced by oxidative stress with no accumulation of other organic phosphate intermediates in the cell. Corynebacterium ammoniagenes'', well-known for its ability to synthesize large amounts of MEcDP, was also shown to accumulate this unique cyclodiphosphate in actively growing culture when NO at low concentration is artificially generated in the medium. A possible role of the MEP-pathway of isoprenoid biosynthesis and a role of its central intermediate MEcDP in bacterial response to nitrosative and oxidative stress is discussed.

A whole-cell phenotypic screening platform for identifying methylerythritol phosphate pathway-selective inhibitors as novel antibacterial agents

Isoprenoid biosynthesis is essential for survival of all living organisms. More than 50,000 unique isoprenoids occur naturally, with each constructed from two simple five-carbon precursors: isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). Two pathways for the biosynthesis of IPP and DMAPP are found in nature. Humans exclusively use the mevalonate (MVA) pathway, while most bacteria, including all Gram-negative and many Gram-positive species, use the unrelated methylerythritol phosphate (MEP) pathway. Here we report the development of a novel, whole-cell phenotypic screening platform to identify compounds that selectively inhibit the MEP pathway. Strains of Salmonella enterica serovar Typhimurium were engineered to have separately inducible MEP (native) and MVA (nonnative) pathways. These strains, RMC26 and CT31-7d, were then used to differentiate MVA pathway- and MEP pathway-specific perturbation. Compounds that inhibit MEP pathway-dependent bacterial growth but leave MVA-dependent growth unaffected represent MEP pathway-selective antibacterials. This screening platform offers three significant results. First, the compound is antibacterial and is therefore cell permeant, enabling access to the intracellular target. Second, the compound inhibits one or more MEP pathway enzymes. Third, the MVA pathway is unaffected, suggesting selectivity for targeting the bacterial versus host pathway. The cell lines also display increased sensitivity to two reported MEP pathway-specific inhibitors, further biasing the platform toward inhibitors selective for the MEP pathway. We demonstrate development of a robust, high-throughput screening platform that combines phenotypic and target-based screening that can identify MEP pathway-selective antibacterials simply by monitoring optical density as the readout for cell growth/inhibition.

Thermodynamic Investigation of Inhibitor Binding to 1-Deoxy-D-Xylulose-5-Phosphate Reductoisomerase

Isothermal titration calorimetry (ITC) was used to investigate the binding of six inhibitors to 1-deoxy-D-xylulose-5-phosphate reductoisomerase (DXR), a target for developing novel anti-infectives. The binding of hydroxamate inhibitors to E. coli DXR is Mg(2+)-dependent, highly endothermic (ΔH: 22.7-24.3 kJ/mol) and entropy-driven, while that of non-hydroxamate compounds is metal ion independent and exothermic (ΔH: -19.4- -13.8 kJ/mol), showing hydration/dehydration of the enzyme metal ion binding pocket account for the drastic ΔH change. However, for DXRs from Plasmodium falciparum and Mycobacterium tuberculosis, the binding of all inhibitors is exothermic (ΔH: -24.9 - -9.2 kJ/mol), suggesting the metal ion binding sites of these two enzymes are considerably less hydrated. The dissociation constants measured by ITC are well correlated with those obtained by enzyme inhibition assays (R(2) = 0.75). Given the rapid rise of antibiotic resistance, this work is of interest since it provides novel structural implications for rational development of potent DXR inhibitors.

A raison d'être for two distinct pathways in the early steps of plant isoprenoid biosynthesis?

When compared to other organisms, plants are atypical with respect to isoprenoid biosynthesis: they utilize two distinct and separately compartmentalized pathways to build up isoprene units. The co-existence of these pathways in the cytosol and in plastids might permit the synthesis of many vital compounds, being essential for a sessile organism. While substrate exchange across membranes has been shown for a variety of plant species, lack of complementation of strong phenotypes, resulting from inactivation of either the cytosolic pathway (growth and development defects) or the plastidial pathway (pigment bleaching), seems to be surprising at first sight. Hundreds of isoprenoids have been analyzed to determine their biosynthetic origins. It can be concluded that in angiosperms, under standard growth conditions, C₂₀-phytyl moieties, C₃₀-triterpenes and C₄₀-carotenoids are made nearly exclusively within compartmentalized pathways, while mixed origins are widespread for other types of isoprenoid-derived molecules. It seems likely that this coexistence is essential for the interaction of plants with their environment. A major purpose of this review is to summarize such observations, especially within an ecological and functional context and with some emphasis on regulation. This latter aspect still requires more work and present conclusions are preliminary, although some general features seem to exist.

Antibacterial and antitubercular activity of fosmidomycin, FR900098, and their lipophilic analogs

The nonmevalonate pathway (NMP) of isoprene biosynthesis is an exciting new route toward novel antibiotic development. Inhibitors against several enzymes in this pathway are currently under examination. A significant liability of many of these agents is poor cell penetration. To overcome and improve our understanding of this problem, we have synthesized a series of lipophilic, prodrug analogs of fosmidomycin and FR900098, inhibitors of the NMP enzyme Dxr. Several of these compounds show improved antibacterial activity against a panel of organisms relative to the parent compound, including activity against Mycobacterium tuberculosis (Mtb). Our results show that this strategy can be an effective way for improving whole cell activity of NMP inhibitors.

N-O chemistry for antibiotics: discovery of N-alkyl-N-(pyridin-2-yl)hydroxylamine scaffolds as selective antibacterial agents using nitroso Diels-Alder and ene chemistry

The discovery, syntheses, and structure-activity relationships (SAR) of a new family of heterocyclic antibacterial compounds based on N-alkyl-N-(pyridin-2-yl)hydroxylamine scaffolds are described. A structurally diverse library of ∼100 heterocyclic molecules generated from Lewis acid-mediated nucleophilic ring-opening reactions with nitroso Diels-Alder cycloadducts and nitroso ene reactions with substituted alkenes was evaluated in whole cell antibacterial assays. Compounds containing the N-alkyl-N-(pyridin-2-yl)hydroxylamine structure demonstrated selective and potent antibacterial activity against the Gram-positive bacterium Micrococcus luteus ATCC 10240 (MIC(90) = 2.0 μM or 0.41 μg/mL) and moderate activity against other Gram-positive strains including antibiotic resistant strains of Staphylococcus aureus (MRSA) and Enterococcus faecalis (VRE). A new synthetic route to the active core was developed using palladium-catalyzed Buchwald-Hartwig amination reactions of N-alkyl-O-(4-methoxybenzyl)hydroxylamines with 2-halo-pyridines that facilitated SAR studies and revealed the simplest active structural fragment. This work shows the value of using a combination of diversity-oriented synthesis (DOS) and parallel synthesis for identifying new antibacterial scaffolds.

Identification of novel small molecule inhibitors of 4-diphosphocytidyl-2-C-methyl-D-erythritol (CDP-ME) kinase of Gram-negative bacteria

The biosyntheses of isoprenoids is essential for the survival in all living organisms, and requires one of the two biochemical pathways: (a) Mevalonate (MVA) Pathway or (b) Methylerythritol Phosphate (MEP) Pathway. The latter pathway, which is used by all Gram-negative bacteria, some Gram-positive bacteria and a few apicomplexan protozoa, provides an attractive target for the development of new antimicrobials because of its absence in humans. In this report, we describe two different approaches that we used to identify novel small molecule inhibitors of Escherichia coli and Yersinia pestis 4-diphosphocytidyl-2-C-methyl D-erythritol (CDP-ME) kinases, key enzymes of the MEP pathway encoded by the E. coli ispE and Y. pestisipk genes, respectively. In the first approach, we explored existing inhibitors of the GHMP kinases while in the second approach; we performed computational high-throughput screening of compound libraries by targeting the CDP-ME binding site of the two bacterial enzymes. From the first approach, we identified two compounds with 6-(benzylthio)-2-(2-hydroxyphenyl)-4-oxo-3,4-dihydro-2H-1,3-thiazine-5-carbonitrile and (Z)-3-methyl-4-((5-phenylfuran-2-yl)methylene)isoxazol-5(4H)-one scaffolds which inhibited E. coli CDP-ME kinase in vitro. We then performed substructure search and docking experiments based on these two scaffolds and identified twenty three analogs for structure-activity relationship (SAR) studies. Three new compounds from the isoxazol-5(4H)-one series have shown inhibitory activities against E. coli and Y. pestis CDP-ME kinases with the IC(50) values ranging from 7 to 13 μM. The second approach by computational high-throughput screening (HTS) of two million drug-like compounds yielded two compounds with benzenesulfonamide and acetamide moieties which, at a concentration of 20 μM, inhibited 80% and 65%, respectively, of control CDP-ME kinase activity.

Inhibition of 1-deoxy-D-xylulose-5-phosphate reductoisomerase by lipophilic phosphonates: SAR, QSAR, and crystallographic studies

1-Deoxy-D-xylulose-5-phosphate reductoisomerase (DXR) is a novel target for developing new antibacterial (including antituberculosis) and antimalaria drugs. Forty-one lipophilic phosphonates, representing a new class of DXR inhibitors, were synthesized, among which 5-phenylpyridin-2-ylmethylphosphonic acid possesses the most activity against E. coli DXR (EcDXR) with a K(i) of 420 nM. Structure-activity relationships (SAR) are discussed, which can be rationalized using our EcDXR:inhibitor structures, and a predictive quantitative SAR (QSAR) model is also developed. Since inhibition studies of DXR from Mycobacterium tuberculosis (MtDXR) have not been performed well, 48 EcDXR inhibitors with a broad chemical diversity were found, however, to generally exhibit considerably reduced activity against MtDXR. The crystal structure of a MtDXR:inhibitor complex reveals the flexible loop containing the residues 198-208 has no strong interactions with the 3,4-dichlorophenyl group of the inhibitor, representing a structural basis for the reduced activity. Overall, these results provide implications in the future design and development of potent DXR inhibitors.

Crystallization and preliminary X-ray analysis of 4-diphosphocytidyl-2-C-methyl-D-erythritol kinase (IspE) from Mycobacterium tuberculosis

The 4-diphosphocytidyl-2-C-methyl-D-erythritol kinase (IspE) from Mycobacterium tuberculosis, an enzyme from the 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway, is crucial and essential for the survival of this pathogenic bacterium. IspE catalyzes the conversion of 4-diphosphocytidyl-2-C-methyl-D-erythritol (CDP-ME) to 4-diphosphocytidyl-2-C-methyl-D-erythritol 2-phosphate (CDP-ME2P) in an ATP-dependent manner. Solving the crystal structure of M. tuberculosis IspE will shed light on its structural details and mechanism of action and may provide the basis for the future design of drugs for the treatment of multidrug-resistant and extremely drug-resistant M. tuberculosis strains. Recombinant M. tuberculosis IspE was crystallized at 291 K using NaCl or Li2SO4 as a precipitant. A 2.1 Å resolution native data set was collected from a single flash-cooled crystal (100 K) belonging to space group P2(1)2(1)2(1), with unit-cell parameters a=52.5, b=72.3, c=107.3 Å. One molecule was assumed per asymmetric unit, which gives a Matthews coefficient of 3.4 Å3 Da(-1) with 63% solvent content.

Structures of 1-Deoxy-D-Xylulose-5-Phosphate Reductoisomerase/Lipophilic Phosphonate Complexes

Fosmidomycin, a potent inhibitor of 1-deoxy-D-xylulose-5-phosphate reductoisomerase (DXR), has antibacterial and antimalaria activity. Due to its poor pharmacokinetics, more lipophilic DXR inhibitors are needed. However, the hydrophobic binding site(s) in DXR remains elusive. Here, pyridine/quinoline containing phosphonates are identified to be DXR inhibitors with IC(50) values as low as 840 nM. We also report three DXR:inhibitor structures, revealing a novel binding mode. The indole group of Trp211 is found to move ~4.6 Å to open up a mainly hydrophobic pocket, where the pyridine/quinoline rings of the inhibitors are located and have strong π-π stacking/charge-transfer interactions with the indole. Docking studies demonstrate our structures could be used to predict the binding modes of other lipophilic DXR inhibitors. Overall, this work shows an important role of Trp211 in inhibitor recognition and provides a structural basis for future drug design and development.

Cloning and functional characterization of an enzyme from Helicobacter pylori that catalyzes two steps of the methylerythritol phosphate pathway for isoprenoid biosynthesis

BACKGROUND

The methylerythritol phosphate pathway for isoprenoid biosynthesis is an attractive target for the design of new specific antibiotics for the treatment of gastrointestinal diseases associated with the presence of the bacterium Helicobacter pylori since this pathway which is essential to the bacterium is absent in humans.

RESULTS

This work reports the molecular cloning of one of the genes of the methylerythritol phosphate pathway form H. pylori (ispDF; HP_1440) its expression in Escherichia coli and the functional characterization of the recombinant enzyme. As shown by genetic complementation and in vitro functional assays the product of the ispDF gene form H. pylori is a bifunctional enzyme which can replace both CDP-methylerythritol synthase and methylerythritol cyclodiphosphate synthase from E. coli.

GENERAL SIGNIFICANCE

Designing inhibitors that affect at the same time both enzyme activities of the H. pylori bifunctional enzyme (i.e. by disrupting protein oligomerization) would result in more effective antibiotics which would be able to continue their action even if the bacterium acquired a resistance to another antibiotic directed against one of the individual activities.

CONCLUSION

The bifunctional enzyme would be an excellent target for the design of new, selective antibiotics for the treatment of H. pylori associated diseases.

Expression and characterization of soluble 4-diphosphocytidyl-2-C-methyl-D-erythritol kinase from bacterial pathogens

Many bacterial pathogens utilize the 2-C-methyl-D-erythritol 4-phosphate pathway for biosynthesizing isoprenoid precursors, a pathway that is vital for bacterial survival and absent from human cells, providing a potential source of drug targets. However, the characterization of 4-diphosphocytidyl-2-C-methyl-D-erythritol (CDP-ME) kinase (IspE) has been hindered due to a lack of enantiopure CDP-ME and difficulty in obtaining pure IspE. Here, enantiopure CDP-ME was chemically synthesized and recombinant IspE from bacterial pathogens were purified and characterized. Although gene disruption was not possible in Mycobacterium tuberculosis, IspE is essential in Mycobacterium smegmatis. The biochemical and kinetic characteristics of IspE provide the basis for development of a high throughput screen and structural characterization.

Revealing substrate promiscuity of 1-deoxy-D-xylulose 5-phosphate synthase

A study of DXP synthase has revealed flexibility in the acceptor substrate binding pocket for nonpolar substrates and has uncovered new details of the catalytic mechanism to show that pyruvate can act as both donor and acceptor substrate.

Homology modeling of Mycobacterium tuberculosis 2C-methyl-D-erythritol-4-phosphate cytidylyltransferase, the third enzyme in the MEP pathway for isoprenoid biosynthesis

Tuberculosis is one of the leading infectious diseases in humans. Discovering new treatments for this disease is urgently required, especially in view of the emergence of multiple drug resistant organisms and to reduce the total duration of current treatments. The synthesis of isoprenoids in Mycobacterium tuberculosis has been reported as an interesting pathway to target, and particular attention has been focused on the methylerythritol phosphate (MEP) pathway comprising the early steps of isoprenoid biosynthesis. In this context we have studied the enzyme 2C-methyl-D-erythritol-4-phosphate cytidylyltransferase (CMS), the third enzyme in the MEP pathway, since the lack of a resolved structure of this protein in M. tuberculosis has seriously limited its use as a drug target. We performed homology modeling of M. tuberculosis CMS in order to provide a reliable model for use in structure-based drug design. After evaluating the quality of the model, we performed a thorough study of the catalytic site and the dimerization interface of the model, which suggested the most important sites (conserved and non-conserved) that could be useful for drug discovery and mutagenesis studies. We found that the metal coordination of CDP-methylerythritol in M. tuberculosis CMS differs substantially with respect to the Escherichia coli variant, consistent with the fact that the former is able to utilize several metal ions for catalysis. Moreover, we propose that electrostatic interactions could explain the higher affinity of the MEP substrate compared with the cytosine 5'-triphosphate substrate in the M. tuberculosis enzyme as reported previously.

Polyisoprenoid alcohols--recent results of structural studies

Polyisoprenoid alcohols (polyprenols and dolichols) are linear polymers of from several up to more than 100 isoprene units identified in almost all living organisms. Studies of their chemical structures have resulted in the discovery of new variants such as the recently described alloprenols with reversed configuration of the double bond in the alpha-isoprene unit. In parallel, structural elucidation of metabolically labeled plant dolichols has indicated that both the mevalonate and methylerythritol phosphate pathways are involved in the biosynthesis of dolichols in roots, leading to the construction of a spatial model of their biosynthesis. According to this model, in root cells, synthesis of the dolichol molecule is initiated in the plastids, and the resulting intermediates, oligoprenyl diphosphates, are exported to the cytoplasm and are elongated up to the desired chain length. The metabolic consequences of this putative model are discussed in the context of the enzymatic machinery involved.

A survey of synthetic and natural phytotoxic compounds and phytoalexins as potential antimalarial compounds

The apicomplexan parasites pathogens such as Plasmodium spp. possess an apicoplast, a plastid organelle similar to those of plants. The apicoplast has some essential plant-like metabolic pathways and processes, making these parasites susceptible to inhibitors of these functions. The main objective of this paper is to determine if phytotoxins with plastid target sites are more likely to be good antiplasmodial compounds than are those with other modes of action. The antiplasmodial activities of some compounds with established phytotoxic action were determined in vitro on a chloroquine (CQ) sensitive (D6, Sierra Leone) strain of Plasmodium falciparum. In this study, we provide in vitro activities of almost 50 such compounds, as well as a few phytoalexins against P. falciparum. Endothall, anisomycin, and cerulenin had sufficient antiplasmodial action to be considered as new lead antimalarial structures. Some derivatives of fusicoccin possessed markedly improved antiplasmodial action than the parent compound. Our results suggest that phytotoxins with plastid targets may not necessarily be better antiplasmodials than those that act at other molecular sites. The herbicides, phytotoxins and the phytoalexins reported here with significant antiplasmodial activity may be useful probes for identification of new antimalarial drug targets and may also be used as new lead structures for new antiplasmodial drug discovery.

Characterization of the Mycobacterium tuberculosis 4-diphosphocytidyl-2-C-methyl-D-erythritol synthase: potential for drug development

Mycobacterium tuberculosis utilizes the methylerythritol phosphate (MEP) pathway for biosynthesis of isopentenyl diphosphate and its isomer, dimethylallyl diphosphate, precursors of all isoprenoid compounds. This pathway is of interest as a source of new drug targets, as it is absent from humans and disruption of the responsible genes has shown a lethal phenotype for Escherichia coli. In the MEP pathway, 4-diphosphocytidyl-2-C-methyl-D-erythritol is formed from 2-C-methyl-D-erythritol 4-phosphate (MEP) and CTP in a reaction catalyzed by a 4-diphosphocytidyl-2-C-methyl-D-erythritol synthase (IspD). In the present work, we demonstrate that Rv3582c is essential for M. tuberculosis: Rv3582c has been cloned and expressed, and the encoded protein has been purified. The purified M. tuberculosis IspD protein was capable of catalyzing the formation of 4-diphosphocytidyl-2-C-methyl-D-erythritol in the presence of MEP and CTP. The enzyme was active over a broad pH range (pH 6.0 to 9.0), with peak activity at pH 8.0. The activity was absolutely dependent upon divalent cations, with 20 mM Mg2+ being optimal, and replacement of CTP with other nucleotide 5'-triphosphates did not support activity. Under the conditions tested, M. tuberculosis IspD had Km values of 58.5 microM for MEP and 53.2 microM for CTP. Calculated kcat and kcat/Km values were 0.72 min(-1) and 12.3 mM(-1) min(-1) for MEP and 1.0 min(-1) and 18.8 mM(-1) min(-1) for CTP, respectively.

Biosynthesis of isoprenoids in plants: structure of the 2C-methyl-D-erithrytol 2,4-cyclodiphosphate synthase from Arabidopsis thaliana. Comparison with the bacterial enzymes

The X-ray crystal structure of the 2C-methyl-D-erythritol 2,4-cyclodiphosphate synthase (MCS) from Arabidopsis thaliana has been solved at 2.3 A resolution in complex with a cytidine-5-monophosphate (CMP) molecule. This is the first structure determined of an MCS enzyme from a plant. Major differences between the A. thaliana and bacterial MCS structures are found in the large molecular cavity that forms between subunits and involve residues that are highly conserved among plants. In some bacterial enzymes, the corresponding cavity has been shown to be an isoprenoid diphosphate-like binding pocket, with a proposed feedback-regulatory role. Instead, in the structure from A. thaliana the cavity is unsuited for binding a diphosphate moiety, which suggests a different regulatory mechanism of MCS enzymes between bacteria and plants.

Evaluation of fosmidomycin analogs as inhibitors of the Synechocystis sp. PCC6803 1-deoxy-d-xylulose 5-phosphate reductoisomerase

Analogs of the antibiotic fosmidomycin, an inhibitor of the methylerythritol phosphate pathway to isoprenoids, were synthesized and evaluated against the recombinant Synechocystis sp. PCC6803 1-deoxy-d-xylulose 5-phosphate reductoisomerase (DXR). Fosfoxacin, the phosphate analog of fosmidomycin, and its acetyl congener were found to be more potent inhibitors of DXR than fosmidomycin.

Kinetic characterization of Synechocystis sp. PCC6803 1-deoxy-d-xylulose 5-phosphate reductoisomerase mutants

The methylerythritol phosphate pathway to isoprenoids has been firmly established as an alternate to the mevalonate pathway in many bacteria, plants, algae, and the malaria parasite Plasmodium falciparum. The second enzyme in this pathway, deoxy-D-xylulose 5-phosphate reductoisomerase (DXR; E.C. 1.1.1.267), has been the focus of many investigations since it was found to be the target of the antibacterial and antimalarial compound, fosmidomycin. Several x-ray crystal structures of the Escherichia coli and Zymomonas mobilis DXR enzymes have provided important structural information about the residues potentially involved in substrate binding and catalysis. Site-directed mutagenesis studies can be used to complement the structural studies, providing kinetic data for specific changes of active site residues. Active site mutants were prepared of the recombinant Synechocystis sp. PCC6803 DXR, targeting residues D152, S153, E154, H155, M206, and E223. Alteration of the three acidic residues had major effects on catalysis, changes to S153 and M206 had variable effects on binding and catalysis, and a H155A mutation had only minimal effects on the kinetic parameters.

Mutation in the flexible loop of 1-deoxy-D-xylulose 5-phosphate reductoisomerase broadens substrate utilization

The second enzyme in the methylerythritol phosphate pathway to isoprenoids, 1-deoxy-D-xylulose 5-phosphate reductoisomerase (DXR; EC 1.1.1.267) mediates the transformation of 1-deoxy-D-xylulose 5-phosphate (DXP) into 2-C-methyl-D-erythritol 4-phosphate. Several DXR mutants have been prepared to study amino acid residues important in binding or catalysis, but in-depth studies of many conserved residues in the flexible loop portion of the enzyme have not been conducted. In the course of our studies of this enzyme, an analog of DXP, 1,2-dideoxy-D-threo-3-hexulose 6-phosphate (1-methyl-DXP), was found to be a weak competitive inhibitor. Using the X-ray crystal structures of DXR as a guide, a highly conserved tryptophan residue in the flexible loop was identified that potentially blocks the use of this analog as a substrate. To test this hypothesis, four mutants of the Synechocystis sp. PCC6803 DXR were prepared and a W204F mutant was found to utilize the analog as a substrate.

Bisphosphonate Inhibitors ofToxoplasmagondiGrowth: In Vitro,QSAR, and In Vivo Investigations

We have investigated the activity of 60 bisphosphonates against the replication of Toxoplasma gondii in vitro and of three of the most active compounds, in vivo. The two most active compounds found were n-alkyl bisphosphonates containing long (n = 9 or 10) hydrocarbon chains, not the nitrogen-containing species used in bone resorption therapy. The target of all of the most active bisphosphonates appears to be the isoprene biosynthesis pathway enzyme farnesyl pyrophosphate synthase (FPPS), as indicated by the correlations between T. gondii growth inhibition and FPPS (human and Leishmania major) enzyme inhibition and by the fact that a T. gondii strain engineered to overexpress FPPS required considerably higher levels of bisphosphonates to achieve 50% growth inhibition, while the IC(50) for atovaquone (which does not inhibit FPPS) remained the same in the overexpressing strain. The phosphonate inhibitor of the non-mevalonate pathway, fosmidomycin, which inhibits the enzyme 1-deoxyxylulose-5-phosphate reductoisomerase, had no effect on T. gondii growth. To investigate structure-activity relationships (SARs) in more detail, we used two three-dimensional quantitative SAR methods: comparative molecular field analysis (CoMFA) and comparative molecular similarity indices analysis (CoMSIA), to investigate all 60 bisphosphonates. Both the CoMFA and CoMSIA models indicated a 60-70% contribution from steric interactions and a 30-40% contribution from electrostatic interactions and using four N = 55 training sets for each method, we found on average between a factor of 2 and 3 error in IC(50) prediction. The three most active compounds found in vitro were tested in vivo in a Smith-Webster mouse model and the two most active bisphosphonates were found to provide up to an 80% protection from death, a considerable improvement over that found previously with nitrogen-containing bisphosphonates. This effect may originate in the much higher therapeutic indices of these alkyl bisphosphonates, as deduced from in vitro assays using LD(50) values for growth inhibition of a human cell line. Overall, these results indicate that alkyl bisphosphonates are promising compounds for further development as agents against Toxoplasma gondii growth, in vivo.

1-Deoxy-d-xylulose 5-phosphate reductoisomerase: an overview

The methylerythritol phosphate pathway to isoprenoids, an alternate biosynthetic route present in many bacteria, algae, plants, and the malarial parasite Plasmodium falciparum, has become an attractive target for the development of new antimalarial and antibacterial compounds. The second enzyme in this pathway, 1-deoxy-D-xylulose 5-phosphate reductoisomerase (DXR; EC 1.1.1.267), has been shown to be the molecular target for fosmidomycin, a promising antimalarial drug. This enzyme converts 1-deoxy-D-xylulose 5-phosphate (DXP) into the branched compound 2-C-methyl-D-erythritol 4-phosphate (MEP). The transformation of DXP into MEP requires an isomerization, followed by a NADPH-dependent reduction. The discovery of DXR, its subsequent characterization, and the identification of inhibitors will be presented.

Substrate analogs for the investigation of deoxyxylulose 5-phosphate reductoisomerase inhibition: synthesis and evaluation

Deoxyxylulose 5-phosphate (DXP) analogs were synthesized and evaluated as alternative substrates and inhibitors of recombinant Synechocystis PCC6803 DXP reductoisomerase (DXR; EC 1.1.1.267). Five of the compounds tested (1,2-dideoxy-D-threo-3-hexulose 6-phosphate, 1-deoxy-l-ribulose 5-phosphate, 2S,3R-dihydroxybutyramide 4-phosphate, 4S-hydroxypentan-2-one 5-phosphate, and 3S-hydroxypentan-2-one 5-phosphate) acted as relatively weak competitive inhibitors when compared to fosmidomycin. A sixth compound, 3R,4S-dihydroxy-5-oxohexylphosphonic acid, served as an alternate substrate, as has recently been reported for the same compound with Escherichia coli DXR.

Falciparum malaria: current therapeutic challenges

PURPOSE OF REVIEW

Malaria remains a major cause of death in much of the world. The routine treatment of malaria is currently threatened by rising rates of drug resistance. Moreover, mortality among children with severe and complicated malaria remains unacceptably high. Here we review trends in antimalarial drug resistance and report on the progress of newer drugs and drug combinations. We then review some recent literature regarding the pathological processes involved in the aetiology of severe malaria that may lead to improvements in the management of children with severe disease.

RECENT FINDINGS

Resistance to first line therapies, including chloroquine and sulphadoxine/pyramethamine, continues to rise in many parts of the world. The availability of newer and more effective drugs and fixed drug combinations is hampered by financial and political considerations. Nevertheless, a number of promising drugs and supportive treatments for both mild and severe malaria are at various stages of development.

SUMMARY

A range of newer drugs and fixed drug combinations are now available that are safe and effective. However, these drugs remain expensive and their introduction will require political and financial support at every level. Considerable work is still required to achieve a better understanding of the processes involved in the pathogenesis of severe and complicated malaria. Only then will it be possible to develop new and appropriate therapies that will be widely applicable.