In a variety of organisms, including plants and several eubacteria, isoprenoids are synthesized by the mevalonate-independent 2-C-methyl-d-erythritol 4-phosphate (MEP) pathway. Although different enzymes of this pathway have been described, the terminal biosynthetic steps of the MEP pathway have not been fully elucidated. In this work, we demonstrate that the gcpE gene of Escherichia coli is involved in this pathway. E. coli cells were genetically engineered to utilize exogenously provided mevalonate for isoprenoid biosynthesis by the mevalonate pathway. These cells were then deleted for the essential gcpE gene and were viable only if the medium was supplemented with mevalonate or the cells were complemented with an episomal copy of gcpE.
LytB and GcpE, because they are codistributed with other pathway enzymes, have been predicted to catalyze unknown steps in the nonmevalonate pathway for isoprenoid biosynthesis. We constructed a conditional Escherichia coli lytB mutant and found that LytB is essential for survival and that depletion of LytB results in cell lysis, which is consistent with a role for this protein in isoprenoid biosynthesis. Alcohols which can be converted to pathway intermediates beyond the hypothesized LytB step(s) support limited growth of E. coli lytB mutants. An informatic analysis of protein structure suggested that GcpE is a globular protein of the TIM barrel class and that LytB is also a globular protein. Possible biochemical roles for LytB and GcpE are suggested.
The photosynthetic cyanobacterium Synechocystis sp. strain PCC6803 possesses homologs of known genes of the non-mevalonate 2-C-methyl-d-erythritol 2-phosphate (MEP) pathway for synthesis of isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). Isoprenoid biosynthesis in extracts of this cyanobacterium, measured by incorporation of radiolabeled IPP, was not stimulated by pyruvate, an initial substrate of the MEP pathway in Escherichia coli, or by deoxyxylulose-5-phosphate, the first pathway intermediate in E. coli. However, high rates of IPP incorporation were obtained with addition of dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (GA3P), as well as a variety of pentose phosphate cycle compounds. Fosmidomycin (at 1 μM and 1 mM), an inhibitor of deoxyxylulose-5-phosphate reductoisomerase, did not significantly inhibit phototrophic growth of the cyanobacterium, nor did it affect [14C]IPP incorporation stimulated by DHAP plus GA3P. To date, it has not been possible to unequivocally demonstrate IPP isomerase activity in this cyanobacterium. The combined results suggest that the MEP pathway, as described for E. coli, is not the primary path by which isoprenoids are synthesized under photosynthetic conditions in Synechocystis sp. strain PCC6803. Our data support alternative routes of entry of pentose phosphate cycle substrates derived from photosynthesis.
Essential isoprenoid compounds are synthesized using the 2-C-methyl-d-erythritol 4-phosphate (MEP) pathway in many gram-negative bacteria, some gram-positive bacteria, some apicomplexan parasites, and plant chloroplasts. The alternative mevalonate pathway is found in archaea and eukaryotes, including cytosolic biosynthesis in plants. The existence of orthogonal essential pathways in eukaryotes and bacteria makes the MEP pathway an attractive target for the development of antimicrobial agents. A system is described for identifying mutations in the MEP pathway of Salmonella enterica serovar Typhimurium. Using this system, point mutations induced by diethyl sulfate were found in the all genes of the essential MEP pathway and also in genes involved in uptake of methylerythritol. Curiously, none of the MEP pathway genes could be identified in the same parent strain by transposon mutagenesis, despite extensive searches. The results complement the biochemical and bioinformatic approaches to the elucidation of the genes involved in the MEP pathway and also identify key residues for activity in the enzymes of the pathway.
The cytosolic mevalonate (MVA) pathway in Hevea brasiliensis latex is the conventionally accepted pathway which provides isopentenyl diphosphate (IPP) for cis-polyisoprene (rubber) biosynthesis. However, the plastidic 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway may be an alternative source of IPP since its more recent discovery in plants. Quantitative RT-PCR (qRT-PCR) expression profiles of genes from both pathways in latex showed that subcellular compartmentalization of IPP for cis-polyisoprene synthesis is related to the degree of plastidic carotenoid synthesis. From this, the occurrence of two schemes of IPP partitioning and utilization within one species is proposed whereby the supply of IPP for cis-polyisoprene from the MEP pathway is related to carotenoid production in latex. Subsequently, a set of latex unique gene transcripts was sequenced and assembled and they were then mapped to IPP-requiring pathways. Up to eight such pathways, including cis-polyisoprene biosynthesis, were identified. Our findings on pre- and post-IPP metabolic routes form an important aspect of a pathway knowledge-driven approach to enhancing cis-polyisoprene biosynthesis in transgenic rubber trees.
cis-polyisoprene; gene expression; Hevea brasiliensis; isopentenyl diphosphate; isoprenoids; latex pathways; rubber biosynthesis; transcriptome
It is proposed that the lytB gene encodes an enzyme of the deoxyxylulose-5-phosphate (DOXP) pathway that catalyzes a step at or subsequent to the point at which the pathway branches to form isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). A mutant of the cyanobacterium Synechocystis strain PCC 6803 with an insertion in the promoter region of lytB grew slowly and produced greenish-yellow, easily bleached colonies. Insertions in the coding region of lytB were lethal. Supplementation of the culture medium with the alcohol analogues of IPP and DMAPP (3-methyl-3-buten-1-ol and 3-methyl-2-buten-1-ol) completely alleviated the growth impairment of the mutant. The Synechocystis lytB gene and a lytB cDNA from the flowering plant Adonis aestivalis were each found to significantly enhance accumulation of carotenoids in Escherichia coli engineered to produce these colored isoprenoid compounds. When combined with a cDNA encoding deoxyxylulose-5-phosphate synthase (dxs), the initial enzyme of the DOXP pathway, the individual salutary effects of lytB and dxs were multiplied. In contrast, the combination of lytB and a cDNA encoding IPP isomerase (ipi) was no more effective in enhancing carotenoid accumulation than ipi alone, indicating that the ratio of IPP and DMAPP produced via the DOXP pathway is influenced by LytB.
Engineering biosynthetic pathways in heterologous microbial host organisms offers an elegant approach to pathway elucidation via the incorporation of putative biosynthetic enzymes and characterization of resulting novel metabolites. Our previous work in Escherichia coli demonstrated the feasibility of a facile modular approach to engineering the production of labdane-related diterpene (20 carbon) natural products. However, yield was limited (<0.1 mg/L), presumably due to reliance on endogenous production of the isoprenoid precursors dimethylallyl diphosphate and isopentenyl diphosphate. Here, we report incorporation of either a heterologous mevalonate pathway (MEV) or enhancement of the endogenous methyl erythritol phosphate pathway (MEP) with our modular metabolic engineering system. With MEP pathway enhancement, it was found that pyruvate supplementation of rich media and simultaneous overexpression of three genes (idi, dxs, and dxr) resulted in the greatest increase in diterpene yield, indicating distributed metabolic control within this pathway. Incorporation of a heterologous MEV pathway in bioreactor grown cultures resulted in significantly higher yields than MEP pathway enhancement. We have established suitable growth conditions for diterpene production levels ranging from 10 to >100 mg/L of E. coli culture. These amounts are sufficient for nuclear magnetic resonance analyses, enabling characterization of enzymatic products and hence, pathway elucidation. Furthermore, these results represent an up to >1,000-fold improvement in diterpene production from our facile, modular platform, with MEP pathway enhancement offering a cost effective alternative with reasonable yield. Finally, we reiterate here that this modular approach is expandable and should be easily adaptable to the production of any terpenoid natural product.
Electronic supplementary material
The online version of this article (doi:10.1007/s00253-009-2219-x) contains supplementary material, which is available to authorized users.
Terpenoid; Natural products biosynthesis; Metabolic engineering; Isoprenoid
2-C-methyl-D-erythritol-4-phosphate (MEP) is a key chemical intermediate of the non-mevalonate pathway for isoprenoid biosynthesis employed by many pathogenic microbes. MEP is also the precursor for the synthesis of 4-diphosphocytidyl-2-C-methyl D-erythritol (CDP-ME), another key intermediate of the non-mevalonate pathway. As this pathway is non-existent in higher animals, including humans, it represents great opportunities for novel antimicrobial development. To facilitate the in-depth studies of this pathway, we reported here a formal synthesis of CDP-ME through a new synthesis of 2-C-Methyl-D-erythritol-4-phosphoric acid from D-(+)-arabitol.
MEP; CDP-ME; selective phosphorylation; dioxanone; monophosphate
Isoprenoids, which are a large group of natural and chemical compounds with a variety of applications as e.g. fragrances, pharmaceuticals and potential biofuels, are produced via two different metabolic pathways, the mevalonate (MVA) pathway and the 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway. Here, we attempted to replace the endogenous MVA pathway in Saccharomyces cerevisiae by a synthetic bacterial MEP pathway integrated into the genome to benefit from its superior properties in terms of energy consumption and productivity at defined growth conditions. It was shown that the growth of a MVA pathway deficient S. cerevisiae strain could not be restored by the heterologous MEP pathway even when accompanied by the co-expression of genes erpA, hISCA1 and CpIscA involved in the Fe-S trafficking routes leading to maturation of IspG and IspH and E. coli genes fldA and fpr encoding flavodoxin and flavodoxin reductase believed to be responsible for electron transfer to IspG and IspH.
Vanda Mimi Palmer (VMP), an orchid hybrid of Vanda tesselata and Vanda Tan Chay Yan is a highly scented tropical orchid which blooms all year round. Previous studies revealed that VMP produces a variety of isoprenoid volatiles during daylight. Isoprenoids are well known to contribute significantly to the scent of most fragrant plants. They are a large group of secondary metabolites which may possess valuable characteristics such as flavor, fragrance and toxicity and are produced via two pathways, the mevalonate (MVA) pathway or/and the 2-C-methyl-D-erythritol-4-phosphate (MEP) pathway. In this study, a sesquiterpene synthase gene denoted VMPSTS, previously isolated from a floral cDNA library of VMP was cloned and expressed in Lactococcus lactis to characterize the functionality of the protein. L. lactis, a food grade bacterium which utilizes the mevalonate pathway for isoprenoid production was found to be a suitable host for the characterization of plant terpene synthases. Through recombinant expression of VMPSTS, it was revealed that VMPSTS produced multiple sesquiterpenes and germacrene D dominates its profile.
Vanda Mimi Palmer; Lactococcus lactis; isoprenoids; sesquiterpene synthase; orchid; fragrance
Isoprenoids are a large and diverse group of metabolites with interesting properties such as flavour, fragrance and therapeutic properties. They are produced via two pathways, the mevalonate pathway or the 2-C-methyl-D-erythritol-4-phosphate (MEP) pathway. While plants are the richest source of isoprenoids, they are not the most efficient producers. Escherichia coli and yeasts have been extensively studied as heterologous hosts for plant isoprenoids production. In the current study, we describe the usage of the food grade Lactococcus lactis as a potential heterologous host for the production of sesquiterpenes from a local herbaceous Malaysian plant, Persicaria minor (synonym Polygonum minus). A sesquiterpene synthase gene from P. minor was successfully cloned and expressed in L. lactis. The expressed protein was identified to be a β-sesquiphellandrene synthase as it was demonstrated to be functional in producing β-sesquiphellandrene at 85.4% of the total sesquiterpenes produced based on in vitro enzymatic assays. The recombinant L. lactis strain developed in this study was also capable of producing β-sesquiphellandrene in vivo without exogenous substrates supplementation. In addition, overexpression of the strain’s endogenous 3-hydroxy-3-methylglutaryl coenzyme-A reductase (HMGR), an established rate-limiting enzyme in the eukaryotic mevalonate pathway, increased the production level of β-sesquiphellandrene by 1.25–1.60 fold. The highest amount achieved was 33 nM at 2 h post-induction.
There is significant progress toward understanding catalysis throughout the essential MEP pathway to isoprenoids in human pathogens; however, little is known about pathway regulation. The present study begins by testing the hypothesis that isoprenoid biosynthesis is regulated via feedback inhibition of the fifth enzyme cyclodiphosphate IspF by downstream isoprenoid diphosphates. Here, we demonstrate recombinant E. coli IspF is not inhibited by downstream metabolites and isopentenyl diphosphate (IDP), dimethylallyl diphosphate (DMADP), geranyl diphosphate (GDP) and farnesyl diphosphate (FDP) under standard assay conditions. However, 2C-methyl-d-erythritol 4-phosphate (MEP), the product of reductoisomerase IspC and first committed MEP pathway intermediate, activates and sustains this enhanced IspF activity, and the IspF-MEP complex is inhibited by FDP. We further show that the methylerythritol scaffold itself, which is unique to this pathway, drives the activation and stabilization of active IspF. Our results suggest a novel feed-forward regulatory mechanism for 2Cmethyl-d-erythritol 2,4-cyclodiphosphate (MEcDP) production and support an isoprenoid biosynthesis regulatory mechanism via feedback inhibition of the IspF-MEP complex by FDP. The results have important implications for development of inhibitors against the IspF-MEP complex, which may be the physiologically relevant form of the enzyme.
cyclodiphosphate synthase; IspF; methylerythritol phosphate; MEP pathway regulation
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 μM for MEP and 53.2 μM 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.
Human Vγ2Vδ2 T cells monitor isoprenoid metabolism by recognizing (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMBPP), an intermediate in the 2-C-methyl-D-erythritol-4-phosphate pathway used by microbes, and isopentenyl pyrophosphate (IPP), an intermediate in the mevalonate pathway used by humans. Aminobisphosphonates and alkylamines indirectly stimulate Vγ2Vδ2 cells by inhibiting farnesyl diphosphate synthase (FDPS) in the mevalonate pathway, thereby increasing IPP/ApppI that directly stimulate. In this study, we further characterize stimulation by these compounds, and define pathways used by new classes of compounds. Consistent with FDPS inhibition, stimulation of Vγ2Vδ2 cells by aminobisphosphonates and alkylamines was much more sensitive to statin inhibition than stimulation by prenyl pyrophosphates. However, the continuous presence of aminobisphosphonates was toxic for T cells, and blocked their proliferation. Aminobisphosphonate stimulation was rapid and prolonged, independent of known antigen presenting molecules, and resistant to fixation. New classes of stimulatory compounds–mevalonate, the alcohol of HMBPP, and alkenyl phosphonates–likely stimulate differently. Mevalonate, a rate-limiting metabolite, appears to enter cells to increase IPP levels whereas the alcohol of HMBPP and alkenyl phosphonates are directly recognized. The critical chemical feature of bisphosphonates is the amino moiety, because its loss switched aminobisphosphonates to direct antigens. Transfection of APC with siRNA downregulating FDPS rendered them stimulatory for Vγ2Vδ2 cells, and increased cellular IPP. siRNAs for isopentenyl diphosphate isomerase functioned similarly. Our results show that a variety of manipulations affecting isoprenoid metabolism lead to stimulation of Vγ2Vδ2 T cells and that pulsing aminobisphosphonates would be more effective for the ex vivo expansion of Vγ2Vδ2 T cells for adoptive cancer immunotherapy.
gamma delta T cell; Vgamma2Vdelta2 T cells; human; bisphosphonate; antigen presentation; prenyl pyrophosphates; isopentenyl pyrophosphate; isoprenoid metabolism; farnesyl diphosphate synthase; siRNA
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.
Mycobacterium tuberculosis synthesizes isoprenoids via the nonmevalonate or DOXP pathway. Previous work demonstrated that three enzymes in the pathway (Dxr, IspD, and IspF) are all required for growth in vitro. We demonstrate the essentiality of the key genes dxs1 and gcpE, confirming that the pathway is of central importance and that the second homolog of the synthase (dxs2) cannot compensate for the loss of dxs1. We looked at the effect of overexpression of Dxr, Dxs1, Dxs2, and GcpE on viability and on growth in M. tuberculosis. Overexpression of dxs1 or dxs2 was inhibitory to growth, whereas overexpression of dxr or gcpE was not. Toxicity is likely to be, at least partially, due to depletion of pyruvate from the cells. Overexpression of dxs1 or gcpE resulted in increased flux through the pathway, as measured by accumulation of the metabolite 4-hydroxy-3-methyl-but-2-enyl pyrophosphate. We identified the functional translational start site and promoter region for dxr and demonstrated that it is expressed as part of a polycistronic mRNA with gcpE and two other genes. Increased expression of this operon was seen in cells overexpressing Dxs1, indicating that transcriptional control is effected by the first enzyme of the pathway via an unknown regulator.
Many pathogenic bacteria utilize the 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway for the biosynthesis of isopentenyl diphosphate and dimethylallyl diphosphate, two major building blocks of isoprenoid compounds. The fifth enzyme in the MEP pathway, 2-C-methyl-D-erythritol 2,4-cyclodiphosphate (ME-CPP) synthase (IspF), catalyzes the conversion of 4-diphosphocytidyl-2-C-methyl-D-erythritol 2-phosphate (CDP-ME2P) to ME-CPP with a corresponding release of cytidine 5-monophosphate (CMP). Since there is no ortholog of IspF in human cells IspF is of interest as a potential drug target. However, study of IspF has been hindered by a lack of enantiopure CDP-ME2P. Herein, we report the first synthesis of enantiomerically pure CDP-ME2P from commercially available D-arabinose. Cloned, expressed, and purified M. tuberculosis IspF was able to utilize the synthetic CDP-ME2P as a substrate, a result confirmed by mass spectrometry. A convenient, sensitive, in vitro IspF assay was developed by coupling the CMP released during production of ME-CPP to mononucleotide kinase, which can be used for high throughput screening.
Isoprenoid compounds constitute an immensely diverse group of acyclic, monocyclic and polycyclic compounds that play important roles in all living organisms. Despite the diversity of their structures, this plethora of natural products arises from only two 5-carbon precursors, isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). This review will discuss the enzymes in the mevalonate (MVA) and methylerythritol phosphate (MEP) biosynthetic pathways leading to IPP and DMAPP with a particular focus on MEP synthase (DXR) and IPP isomerase (IDI), which are potential targets for the development of antibiotic compounds. DXR is the second enzyme in the MEP pathway and the only one for which inhibitors with antimicrobial activity at pharmaceutically relevant concentrations are known. All of the published DXR inhibitors are fosmidomycin analogues, except for a few bisphosphonates with moderate inhibitory activity. These far, there are no other candidates that target DXR. IDI was first identified and characterised over 40 years ago (IDI-1) and a second convergently evolved isoform (IDI-2) was discovered in 2001. IDI-1 is a metalloprotein found in Eukarya and many species of Bacteria. Its mechanism has been extensively studied. In contrast, IDI-2 requires reduced flavin mononucleotide as a cofactor. The mechanism of action for IDI-2 is less well defined. This review will describe how lead inhibitors are being improved by structure-based drug design and enzymatic assays against DXR to lead to new drug families and how mechanistic probes are being used to address questions about the mechanisms of the isomerases.
DXR; IDI; isomerase; isopentenyl; isoprenoid; MEP; mevalonate; MVA; reductoisomerase
Plants have two isoprenoid biosynthetic pathways: the cytosolic mevalonate (MVA) pathway and the plastidic 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway. Since the discovery of the MEP pathway, possible metabolic cross-talk between these pathways has prompted intense research. Although many studies have shown the existence of such cross-talk using feeding experiments, it remains to be determined if native cross-talk, rather than exogenously applied metabolites, can compensate for complete blockage of the MVA pathway. Previously, Arabidopsis mutants for HMG1 and HMG2 encoding HMG-CoA reductase (HMGR) were isolated. Although it was shown that HMGR1 is a functional HMGR, the enzyme activity of HMGR2 has not been confirmed. It is demonstrated here that HMG2 encodes a functional reductase with similar activity to HMGR1, using enzyme assays and complementation experiments. To estimate the contribution of native cross-talk, an attempt was made to block the MVA pathway by making double mutants lacking both HMG1 and HMG2, but no double homozygotes were detected in the progeny of self-pollinated HMG1/hmg1 hmg2/hmg2 plants. hmg1 hmg2 male gametophytes appeared to be lethal based on crossing experiments, and microscopy indicated that ∼50% of the microspores from the HMG1/hmg1 hmg2/hmg2 plant appeared shrunken and exhibited poorly defined endoplasmic reticulum membranes. In situ hybridization showed that HMG1 transcripts were expressed in both the tapetum and microspores, while HMG2 mRNA appeared only in microspores. It is concluded that native cross-talk from the plastid cannot compensate for complete blockage of the MVA pathway, at least during male gametophyte development, because either HMG1 or HMG2 is required for male gametophyte development.
Anther; cross-talk; HMG-CoA reductase; isoprenoid; male gametophyte; MEP pathway; MVA pathway; pollen; sterol; tapetum
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. pestis ipk 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 Escherichia 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 IC50 values ranging from 7μM to 13μM. 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 20μM, inhibited 80% and 65%, respectively, of control CDP-ME kinase activity.
In mycobacteria, the biosynthesis of the precursors to the essential isoprenoids, isopentenyl diphosphate and dimethylallyl pyrophosphate is carried out by the methylerythritol phosphate (MEP) pathway. This route of synthesis is absent in humans, who utilize the alternative mevalonate acid (MVA) route, thus making the enzymes of the MEP pathway of chemotherapeutic interest. One such identified target is the second enzyme of the pathway, 1-Deoxy-D-xylulose 5-phosphate reductoisomerase (DXR). Only limited information is currently available concerning the catalytic mechanism and structural dynamics of this enzyme, and only recently has a crystal structure of Mycobacterium tuberculosis species of this enzyme been resolved including all factors required for binding. Here, the dynamics of the enzyme is studied in complex with NADPH, Mn2+, in the presence and absence of the fosmidomycin inhibitor using conventional molecular dynamics and an enhanced sampling technique, Reversible Digitally Filtered Molecular Dynamics. The simulations reveal significant differences in the conformational dynamics of the vital catalytic loop between the inhibitor-free and inhibitor-bound enzyme complexes and highlight the contributions of conserved residues in this region. The substantial fluctuations observed suggest that DXR may be a promising target for computer-aided drug discovery through the relaxed complex method.
The structure of a triclinic crystal form of 4-diphosphocytidyl-2C-methyl-d-erythritol kinase has been determined. Comparisons with a previously reported monoclinic crystal form raise questions about our knowledge of the quaternary structure of this enzyme.
4-Diphosphocytidyl-2C-methyl-d-erythritol kinase (IspE; EC 18.104.22.168) contributes to the 1-deoxy-d-xylulose 5-phosphate or mevalonate-independent biosynthetic pathway that produces the isomers isopentenyl diphosphate and dimethylallyl diphosphate. These five-carbon compounds are the fundamental building blocks for the biosynthesis of isoprenoids. The mevalonate-independent pathway does not occur in humans, but is present and has been shown to be essential in many dangerous pathogens, i.e. Plasmodium species, which cause malaria, and Gram-negative bacteria. Thus, the enzymes involved in this pathway have attracted attention as potential drug targets. IspE produces 4-diphosphosphocytidyl-2C-methyl-d-erythritol 2-phosphate by ATP-dependent phosphorylation of 4-diphosphocytidyl-2C-methyl-d-erythritol. A triclinic crystal structure of the Escherichia coli IspE–ADP complex with two molecules in the asymmetric unit was determined at 2 Å resolution and compared with a monoclinic crystal form of a ternary complex of E. coli IspE also with two molecules in the asymmetric unit. The molecular packing is different in the two forms. In the asymmetric unit of the triclinic crystal form the substrate-binding sites of IspE are occluded by structural elements of the partner, suggesting that the ‘triclinic dimer’ is an artefact of the crystal lattice. The surface area of interaction in the triclinic form is almost double that observed in the monoclinic form, implying that the dimeric assembly in the monoclinic form may also be an artifact of crystallization.
mevalonate-independent pathway; isoprenoid biosynthesis; kinases
Embden-Meyerhof pathway (EMP) in tandem with 2-C-methyl-D-erythritol 4-phosphate pathway (MEP) is commonly used for isoprenoid biosynthesis in E. coli. However, this combination has limitations as EMP generates an imbalanced distribution of pyruvate and glyceraldehyde-3-phosphate (G3P). Herein, four glycolytic pathways—EMP, Entner-Doudoroff Pathway (EDP), Pentose Phosphate Pathway (PPP) and Dahms pathway were tested as MEP feeding modules for isoprene production. Results revealed the highest isoprene production from EDP containing modules, wherein pyruvate and G3P were generated simultaneously; isoprene titer and yield were more than three and six times higher than those of the EMP module, respectively. Additionally, the PPP module that generates G3P prior to pyruvate was significantly more effective than the Dahms pathway, in which pyruvate production precedes G3P. In terms of precursor generation and energy/reducing-equivalent supply, EDP+PPP was found to be the ideal feeding module for MEP. These findings may launch a new direction for the optimization of MEP-dependent isoprenoid biosynthesis pathways.
The conversion of 2C-methyl-d-erythritol 4-phosphate (MEP) to 2C-methyl-d-erythritol 2,4-cyclodiphosphate (cMEDP) in the MEP entry into the isoprenoid biosynthetic pathway occurs in three consecutive steps catalyzed by the IspD, IspE, and IspF enzymes, respectively. In Agrobacterium tumefaciens the ispD and ispF genes are fused to encode a bifunctional enzyme that catalyzes the first (synthesis of 4-diphosphocytidyl-2-C-methyl d-erythritol) and third (synthesis of 2C-methyl-d-erythritol 2,4-cyclodiphosphate) steps. Sedimentation velocity experiments indicate that the bifunctional IspDF enzyme and the IspE protein associate in solution raising the possibility of substrate channeling among the active sites in these two proteins. Kinetic evidence for substrate channeling was sought by measuring the time courses for product formation during incubations of MEP, CTP, and ATP with the IspDF and IspE proteins with and without an excess of the inactive IspE (D152A) mutant in presence or absence of 30% (v/v) glycerol. The time dependencies indicate that the enzyme-generated intermediates are not transferred from the IspD active site in IspDF to the active site of IspE or from the active site in IspE to the active site in the IspF module of IspDF.
bifunctional; IspDF; IspE; non-channeling
2-C-methyl-d-erythritol 4-phosphate is the first committed intermediate in the biosynthesis of the isoprenoid precursors isopentenyl diphosphate and dimethylallyl diphosphate. Supplementation of the growth medium with 2-C-methyl-d-erythritol has been shown to complement disruptions in the Escherichia coli gene for 1-deoxy-d-xylulose 5-phosphate synthase, the enzyme that synthesizes the immediate precursor of 2-C-methyl-d-erythritol 4-phosphate. In order to be utilized in isoprenoid biosynthesis, 2-C-methyl-d-erythritol must be phosphorylated. We describe the construction of Salmonella enterica serovar Typhimurium strain RMC26, in which the essential gene encoding 1-deoxy-d-xylulose 5-phosphate synthase has been disrupted by insertion of a synthetic mevalonate operon consisting of the yeast ERG8, ERG12, and ERG19 genes, responsible for converting mevalonate to isopentenyl diphosphate under the control of an arabinose-inducible promoter. Random mutagenesis of RMC26 produced defects in the sorbitol phosphotransferase system that prevented the transport of 2-C-methyl-d-erythritol into the cell. RMC26 and mutant strains of RMC26 unable to grow on 2-C-methyl-d-erythritol were incubated in buffer containing mevalonate and deuterium-labeled 2-C-methyl-d-erythritol. Ubiquinone-8 was isolated from these cells and analyzed for deuterium content. Efficient incorporation of deuterium was observed for RMC26. However, there was no evidence of deuterium incorporation into the isoprenoid side chain of ubiquinone Q8 in the RMC26 mutants.