Tools for discovery of inhibitors of the 1-deoxy-D-xylulose 5-phosphate (DXP) synthase and DXP reductoisomerase: an approach with enzymes from the pathogenic bacterium Pseudomonas aeruginosa

An update on the progress of microbial biotransformation of commercial monoterpenes

Monoterpenes, a class of isoprenoid compounds, are extensively used in flavor, fragrance, perfumery, and cosmetics. They display many astonishing bioactive properties of biological and pharmacological significance. All monoterpenes are derived from universal precursor geranyl diphosphate. The demand for new monoterpenoids has been increasing in flavor, fragrances, perfumery, and pharmaceuticals. Chemical methods, which are harmful for human and the environment, synthesize most of these products. Over the years, researchers have developed alternative methods for the production of newer monoterpenoids. Microbial biotransformation is one of them, which relied on microbes and their enzymes. It has produced many new desirable commercially important monoterpenoids. A growing number of reports reflect an ever-expanding scope of microbial biotransformation in food and aroma industries. Simultaneously, our knowledge of the enzymology of monoterpene biosynthetic pathways has been increasing, which facilitated the biotransformation of monoterpenes. In this article, we have covered the progress made on microbial biotransformation of commercial monoterpenes with a brief introduction to their biosynthesis. We have collected several reports from authentic web sources, including Google Scholar, Pubmed, Web of Science, and Scopus published in the past few years to extract information on the topic.

First crystal structures of 1-deoxy-D-xylulose 5-phosphate synthase (DXPS) from Mycobacterium tuberculosis indicate a distinct mechanism of intermediate stabilization

The development of drug resistance by Mycobacterium tuberculosis and other pathogenic bacteria emphasizes the need for new antibiotics. Unlike animals, most bacteria synthesize isoprenoid precursors through the MEP pathway. 1-Deoxy-d-xylulose 5-phosphate synthase (DXPS) catalyzes the first reaction of the MEP pathway and is an attractive target for the development of new antibiotics. We report here the successful use of a loop truncation to crystallize and solve the first DXPS structures of a pathogen, namely M. tuberculosis (MtDXPS). The main difference found to other DXPS structures is in the active site where a highly coordinated water was found, showing a new mechanism for the enamine-intermediate stabilization. Unlike other DXPS structures, a “fork-like” motif could be identified in the enamine structure, using a different residue for the interaction with the cofactor, potentially leading to a decrease in the stability of the intermediate. In addition, electron density suggesting a phosphate group could be found close to the active site, provides new evidence for the D-GAP binding site. These results provide the opportunity to improve or develop new inhibitors specific for MtDXPS through structure-based drug design.

Determination of the Activity of 1-Deoxy-D-Xylulose 5-Phosphate Synthase by Pre-column Derivatization-HPLC Using 1,2-Diamino-4,5-Methylenedioxybenzene as a Derivatizing Reagent

α-Ketoacids can be determined by HPLC through pre-column derivatization with 1,2-diamino-4,5-methylenedioxybenzene (DMB) as a derivatizing reagent. Using this method, the specific activity and the steady-state kinetic of 1-deoxy-D-xylulose-5-phosphate synthase (DXS) were measured. Firstly, DXS substrate pyruvate was derivatized with DMB in acidic solution; then the corresponding quinoxalinone was elucidated by LC-ESI-MS and quantified by HPLC-UV. The optimum derivatization conditions were as follows: aqueous medium at pH 1.0, reaction temperature 80 °C, reaction time 60 min, molar ratio of DMB to pyruvate 10:1. The HPLC was run with isocratic elution using the mixture of methanol and water (60:40, v/v) as a mobile phase. The detective limit and the linear correlation range of the method were 0.05 µM and 0.002-1.0 mM (R = 0.994), respectively. The relative standard deviation (RSD) of six determinations was 2.48%. The steady-state kinetic parameters of DXS for pyruvate determined with the method were identical to the reported data. The established method is a practical route for evaluation of DXS activity, especially in the research and development of DXS inhibitors.

An ImmunoPEGliposome for Targeted Antimalarial Combination Therapy at the Nanoscale

Combination therapies, where two drugs acting through different mechanisms are administered simultaneously, are one of the most efficient approaches currently used to treat malaria infections. However, the different pharmacokinetic profiles often exhibited by the combined drugs tend to decrease treatment efficacy as the compounds are usually eliminated from the circulation at different rates. To circumvent this obstacle, we have engineered an immunoliposomal nanovector encapsulating hydrophilic and lipophilic compounds in its lumen and lipid bilayer, respectively. The antimalarial domiphen bromide has been encapsulated in the liposome membrane with good efficiency, although its high IC₅₀ of ca. 1 µM for living parasites complicates its use as immunoliposomal therapy due to erythrocyte agglutination. The conjugation of antibodies against glycophorin A targeted the nanocarriers to Plasmodium-infected red blood cells and to gametocytes, the sole malaria parasite stage responsible for the transmission from the human to the mosquito vector. The antimalarials pyronaridine and atovaquone, which block the development of gametocytes, have been co-encapsulated in glycophorin A-targeted immunoliposomes. The co-immunoliposomized drugs have activities significantly higher than their free forms when tested in in vitro Plasmodium falciparum cultures: Pyronaridine and atovaquone concentrations that, when encapsulated in immunoliposomes, resulted in a 50% inhibition of parasite growth had no effect on the viability of the pathogen when used as free drugs.

Phage Display on the Anti-infective Target 1-Deoxy-d-xylulose-5-phosphate Synthase Leads to an Acceptor-Substrate Competitive Peptidic Inhibitor

Enzymes of the 2-C-methyl-d-erythritol-4-phosphate pathway for the biosynthesis of isoprenoid precursors are validated drug targets. By performing phage display on 1-deoxy-d-xylulose-5-phosphate synthase (DXS), which catalyzes the first step of this pathway, we discovered several peptide hits and recognized false-positive hits. The enriched peptide binder P12 emerged as a substrate (d-glyceraldehyde-3-phosphate)-competitive inhibitor of Deinococcus radiodurans DXS. The results indicate possible overlap of the cofactor- and acceptor-substrate-binding pockets and provide inspiration for the design of inhibitors of DXS with a unique and novel mechanism of inhibition.

Mechanistic binding insights for 1-deoxy-D-Xylulose-5-Phosphate synthase, the enzyme catalyzing the first reaction of isoprenoid biosynthesis in the malaria-causing protists, Plasmodium falciparum and Plasmodium vivax

We have successfully truncated and recombinantly-expressed 1-deoxy-D-xylulose-5-phosphate synthase (DXS) from both Plasmodium vivax and Plasmodium falciparum. We elucidated the order of substrate binding for both of these ThDP-dependent enzymes using steady-state kinetic analyses, dead-end inhibition, and intrinsic tryptophan fluorescence titrations. Both enzymes adhere to a random sequential mechanism with respect to binding of both substrates: pyruvate and D-glyceraldehyde-3-phosphate. These findings are in contrast to other ThDP-dependent enzymes, which exhibit classical ordered and/or ping-pong kinetic mechanisms. A better understanding of the kinetic mechanism for these two Plasmodial enzymes could aid in the development of novel DXS-specific inhibitors that might prove useful in treatment of malaria.

Kinetic characterization and allosteric inhibition of the Yersinia pestis 1-deoxy-D-xylulose 5-phosphate reductoisomerase (MEP synthase)

The methylerythritol phosphate (MEP) pathway found in many bacteria governs the synthesis of isoprenoids, which are crucial lipid precursors for vital cell components such as ubiquinone. Because mammals synthesize isoprenoids via an alternate pathway, the bacterial MEP pathway is an attractive target for novel antibiotic development, necessitated by emerging antibiotic resistance as well as biodefense concerns. The first committed step in the MEP pathway is the reduction and isomerization of 1-deoxy-D-xylulose-5-phosphate (DXP) to methylerythritol phosphate (MEP), catalyzed by MEP synthase. To facilitate drug development, we cloned, expressed, purified, and characterized MEP synthase from Yersinia pestis. Enzyme assays indicate apparent kinetic constants of KMDXP = 252 µM and KMNADPH = 13 µM, IC50 values for fosmidomycin and FR900098 of 710 nM and 231 nM respectively, and Ki values for fosmidomycin and FR900098 of 251 nM and 101 nM respectively. To ascertain if the Y. pestis MEP synthase was amenable to a high-throughput screening campaign, the Z-factor was determined (0.9) then the purified enzyme was screened against a pilot scale library containing rationally designed fosmidomycin analogs and natural product extracts. Several hit molecules were obtained, most notably a natural product allosteric affector of MEP synthase and a rationally designed bisubstrate derivative of FR900098 (able to associate with both the NADPH and DXP binding sites in MEP synthase). It is particularly noteworthy that allosteric regulation of MEP synthase has not been described previously. Thus, our discovery implicates an alternative site (and new chemical space) for rational drug development.

Defining critical residues for substrate binding to 1-deoxy-D-xylulose 5-phosphate synthase--active site substitutions stabilize the predecarboxylation intermediate C2α-lactylthiamin diphosphate

1-Deoxy-D-xylulose 5-phosphate (DXP) synthase catalyzes the formation of DXP from pyruvate and D-glyceraldehyde 3-phosphate (GraP) in a thiamin diphosphate-dependent manner, and is the first step in the essential pathway to isoprenoids in human pathogens. Understanding the mechanism of this unique enzyme is critical for developing new anti-infective agents that selectively target isoprenoid biosynthesis. The present study used mutagenesis and a combination of protein fluorescence, CD and kinetics experiments to investigate the roles of Arg420, Arg478 and Tyr392 in substrate binding and catalysis. The results support a random sequential, preferred order mechanism, and predict that Arg420 and Arg478 are involved in binding of the acceptor substrate, GraP. D-Glyceraldehyde, an alternative acceptor substrate lacking the phosphoryl group predicted to interact with Arg420 and Arg478, also accelerates decarboxylation of the predecarboxylation intermediate C2α-lactylthiamin diphosphate (LThDP) on DXP synthase, indicating that this binding interaction is not absolutely required, and that the hydroxyaldehyde sufficiently triggers decarboxylation. Unexpectedly, Tyr392 contributes to GraP affinity, and is not required for LThDP formation or its GraP-promoted decarboxylation. Time-resolved CD spectroscopy and NMR experiments indicate that LThDP is significantly stabilized on R420A and Y392F variants as compared with wild-type DXP synthase in the absence of acceptor substrate, but these substitutions do not appear to affect the rate of GraP-promoted LThDP decarboxylation in the presence of high levels of GraP, and LThDP formation remains the rate-limiting step. These results suggest a role of these residues in promoting GraP binding, which in turn facilitates decarboxylation, and also highlight interesting differences between DXP synthase and other thiamin diphosphate-dependent enzymes.

Targeting DXP synthase in human pathogens: enzyme inhibition and antimicrobial activity of butylacetylphosphonate

The unique methylerythritol phosphate pathway for isoprenoid biosynthesis is essential in most bacterial pathogens. The first enzyme in this pathway, 1-deoxy-D-xylulose 5-phosphate (DXP) synthase, catalyzes a distinct thiamin diphosphate (ThDP)-dependent reaction to form DXP from D-glyceraldehyde 3-phosphate (D-GAP) and pyruvate and represents a potential anti-infective drug target. We have previously demonstrated that the unnatural bisubstrate analog, butylacetylphosphonate (BAP), exhibits selective inhibition of Escherichia coli DXP synthase over mammalian ThDP-dependent enzymes. Here, we report the selective inhibition by BAP against recombinant DXP synthase homologs from Mycobacterium tuberculosis, Yersinia pestis and Salmonella enterica. We also demonstrate antimicrobial activity of BAP against both Gram-negative and Gram-positive strains (including E. coli, S. enterica and Bacillus anthracis), and several clinically isolated pathogens. Our results suggest a mechanism of action involving inhibition of DXP synthase and show that BAP acts synergistically with established antimicrobial agents, highlighting a potential strategy to combat emerging resistance in bacterial pathogens.

Feedback inhibition of deoxy-D-xylulose-5-phosphate synthase regulates the methylerythritol 4-phosphate pathway

The 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway leads to the biosynthesis of isopentenyl diphosphate (IDP) and dimethylallyl diphosphate (DMADP), the precursors for isoprene and higher isoprenoids. Isoprene has significant effects on atmospheric chemistry, whereas other isoprenoids have diverse roles ranging from various biological processes to applications in commercial uses. Understanding the metabolic regulation of the MEP pathway is important considering the numerous applications of this pathway. The 1-deoxy-D-xylulose-5-phosphate synthase (DXS) enzyme was cloned from Populus trichocarpa, and the recombinant protein (PtDXS) was purified from Escherichia coli. The steady-state kinetic parameters were measured by a coupled enzyme assay. An LC-MS/MS-based assay involving the direct quantification of the end product of the enzymatic reaction, 1-deoxy-D-xylulose 5-phosphate (DXP), was developed. The effect of different metabolites of the MEP pathway on PtDXS activity was tested. PtDXS was inhibited by IDP and DMADP. Both of these metabolites compete with thiamine pyrophosphate for binding with the enzyme. An atomic structural model of PtDXS in complex with thiamine pyrophosphate and Mg(2+) was built by homology modeling and refined by molecular dynamics simulations. The refined structure was used to model the binding of IDP and DMADP and indicated that IDP and DMADP might bind with the enzyme in a manner very similar to the binding of thiamine pyrophosphate. The feedback inhibition of PtDXS by IDP and DMADP constitutes an important mechanism of metabolic regulation of the MEP pathway and indicates that thiamine pyrophosphate-dependent enzymes may often be affected by IDP and DMADP.

1-Deoxy-D-xylulose 5-phosphate synthase catalyzes a novel random sequential mechanism

Emerging resistance of human pathogens to anti-infective agents make it necessary to develop new agents to treat infection. The methylerythritol phosphate pathway has been identified as an anti-infective target, as this essential isoprenoid biosynthetic pathway is widespread in human pathogens but absent in humans. The first enzyme of the pathway, 1-deoxy-D-xylulose 5-phosphate (DXP) synthase, catalyzes the formation of DXP via condensation of D-glyceraldehyde 3-phosphate (D-GAP) and pyruvate in a thiamine diphosphate-dependent manner. Structural analysis has revealed a unique domain arrangement suggesting opportunities for the selective targeting of DXP synthase; however, reports on the kinetic mechanism are conflicting. Here, we present the results of tryptophan fluorescence binding and kinetic analyses of DXP synthase and propose a new model for substrate binding and mechanism. Our results are consistent with a random sequential kinetic mechanism, which is unprecedented in this enzyme class.

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.

Thiamin (vitamin B1) biosynthesis and regulation: a rich source of antimicrobial drug targets?

Drug resistance of pathogens has necessitated the identification of novel targets for antibiotics. Thiamin (vitamin B1) is an essential cofactor for all organisms in its active form thiamin diphosphate (ThDP). Therefore, its metabolic pathways might be one largely untapped source of antibiotics targets. This review describes bacterial thiamin biosynthetic, salvage, and transport pathways. Essential thiamin synthetic enzymes such as Dxs and ThiE are proposed as promising drug targets. The regulation mechanism of thiamin biosynthesis by ThDP riboswitch is also discussed. As drug targets of existing antimicrobial compound pyrithiamin, the ThDP riboswitch might serves as alternative targets for more antibiotics.

A single-molecule force spectroscopy nanosensor for the identification of new antibiotics and antimalarials

An important goal of nanotechnology is the application of individual molecule handling techniques to the discovery of potential new therapeutic agents. Of particular interest is the search for new inhibitors of metabolic routes exclusive of human pathogens, such as the 2-C-methyl-D-erythritol-4-phosphate (MEP) pathway essential for the viability of most human pathogenic bacteria and of the malaria parasite. Using atomic force microscopy single-molecule force spectroscopy (SMFS), we have probed at the single-molecule level the interaction of 1-deoxy-D-xylulose 5-phosphate synthase (DXS), which catalyzes the first step of the MEP pathway, with its two substrates, pyruvate and glyceraldehyde-3-phosphate. The data obtained in this pioneering SMFS analysis of a bisubstrate enzymatic reaction illustrate the substrate sequentiality in DXS activity and allow for the calculation of catalytic parameters with single-molecule resolution. The DXS inhibitor fluoropyruvate has been detected in our SMFS competition experiments at a concentration of 10 μM, improving by 2 orders of magnitude the sensitivity of conventional enzyme activity assays. The binding of DXS to pyruvate is a 2-step process with dissociation constants of k(off) = 6.1 × 10(-4) ± 7.5 × 10(-3) and 1.3 × 10(-2) ± 1.0 × 10(-2) s(-1), and reaction lengths of x(β) = 3.98 ± 0.33 and 0.52 ± 0.23 Å. These results constitute the first quantitative report on the use of nanotechnology for the biodiscovery of new antimalarial enzyme inhibitors and open the field for the identification of compounds represented only by a few dozens of molecules in the sensor chamber.

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.

The Mycobacterium tuberculosis MEP (2C-methyl-d-erythritol 4-phosphate) pathway as a new drug target

Tuberculosis (TB) is still a major public health problem, compounded by the human immunodeficiency virus (HIV)-TB co-infection and recent emergence of multidrug-resistant (MDR) and extensively drug resistant (XDR)-TB. Novel anti-TB drugs are urgently required. In this context, the 2C-methyl-d-erythritol 4-phosphate (MEP) pathway of Mycobacterium tuberculosis has drawn attention; it is one of several pathways vital for M. tuberculosis viability and the human host lacks homologous enzymes. Thus, the MEP pathway promises bacterium-specific drug targets and the potential for identification of lead compounds unencumbered by target-based toxicity. Indeed, fosmidomycin is now known to inhibit the second step in the MEP pathway. This review describes the cardinal features of the main enzymes of the MEP pathway in M. tuberculosis and how these can be manipulated in high throughput screening campaigns in the search for new anti-infectives against TB.

Targeting the formation of the cell wall core of M. tuberculosis

Mycobacteria have a unique cell wall, which is rich in drug targets. The cell wall core consists of a peptidoglycan layer, a mycolic acid layer, and an arabinogalactan polysaccharide connecting them. The detailed structure of the cell wall core is largely, although not completely, understood and will be presented. The biosynthetic pathways of all three components reveal significant drug targets that are the basis of present drugs and/or have potential for new drugs. These pathways will be reviewed and include enzymes involved in polyisoprene biosynthesis, soluble arabinogalactan precursor production, arabinogalactan polymerization, fatty acid synthesis, mycolate maturation, and soluble peptidoglycan precursor formation. Information relevant to targeting all these enzymes will be presented in tabular form. Selected enzymes will then be discussed in more detail. It is thus hoped this chapter will aid in the selection of targets for new drugs to combat tuberculosis.

Isoprenoid biosynthesis in plants - 2C-methyl-D-erythritol-4-phosphate synthase (IspC protein) of Arabidopsis thaliana

The ispC gene of Arabidopsis thaliana was expressed in pseudomature form without the putative plastid-targeting sequence in a recombinant Escherichia coli strain. The recombinant protein was purified by affinity chromatography and was shown to catalyze the formation of 2C-methyl-D-erythritol 4-phosphate from 1-deoxy-D-xylulose 5-phosphate at a rate of 5.6 micromol x min(-1) x mg(-1) (k(cat) 4.4 s(-1)). The Michaelis constants for 1-deoxy-D-xylulose 5-phosphate and the cosubstrate NADPH are 132 and 30 microm, respectively. The enzyme has an absolute requirement for divalent metal ions, preferably Mn2+ and Mg2+, and is inhibited by fosmidomycin with a Ki of 85 nm. The pH optimum is 8.0. NADH can substitute for NADPH, albeit at a low rate (14% as compared to NADPH). The enzyme catalyzes the reverse reaction at a rate of 2.1 micromol x min(-1) x mg(-1).

1-Deoxy-D-xylulose 5-phosphate reductoisomerase (IspC) from Mycobacterium tuberculosis: towards understanding mycobacterial resistance to fosmidomycin

1-Deoxy-d-xylulose 5-phosphate reductoisomerase (IspC) catalyzes the first committed step in the mevalonate-independent isopentenyl diphosphate biosynthetic pathway and is a potential drug target in some pathogenic bacteria. The antibiotic fosmidomycin has been shown to inhibit IspC in a number of organisms and is active against most gram-negative bacteria but not gram positives, including Mycobacterium tuberculosis, even though the mevalonate-independent pathway is the sole isopentenyl diphosphate biosynthetic pathway in this organism. Therefore, the enzymatic properties of recombinant IspC from M. tuberculosis were characterized. Rv2870c from M. tuberculosis converts 1-deoxy-d-xylulose 5-phosphate to 2-C-methyl-d-erythritol 4-phosphate in the presence of NADPH. The enzymatic activity is dependent on the presence of Mg(2+) ions and exhibits optimal activity between pH 7.5 and 7.9; the K(m) for 1-deoxyxylulose 5-phosphate was calculated to be 47.1 microM, and the K(m) for NADPH was 29.7 microM. The specificity constant of Rv2780c in the forward direction is 1.5 x 10(6) M(-1) min(-1), and the reaction is inhibited by fosmidomycin, with a 50% inhibitory concentration of 310 nM. In addition, Rv2870c complements an inactivated chromosomal copy of IspC in Salmonella enterica, and the complemented strain is sensitive to fosmidomycin. Thus, M. tuberculosis resistance to fosmidomycin is not due to intrinsic properties of Rv2870c, and the enzyme appears to be a valid drug target in this pathogen.

Isoprenoid biosynthesis in Synechocystis sp. strain PCC6803 is stimulated by compounds of the pentose phosphate cycle but not by pyruvate or deoxyxylulose-5-phosphate

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 micro M and 1 mM), an inhibitor of deoxyxylulose-5-phosphate reductoisomerase, did not significantly inhibit phototrophic growth of the cyanobacterium, nor did it affect [(14)C]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.

Accumulation of a potent gammadelta T-cell stimulator after deletion of the lytB gene in Escherichia coli

Activation of human Vgamma9/Vdelta2 T cells by many pathogens depends on the presence of small phosphorylated non-peptide compounds derived from the 2-C-methyl-d-erythritol 4-phosphate (MEP) pathway of isoprenoid biosynthesis. We here demonstrate that in Escherichia coli mutants deficient in lytB, an essential gene of the MEP pathway, a potent Vgamma9/Vdelta2 T-cell activator accumulates by a factor of approximately 150 compared to wild-type E. coli. The compound responsible for the strong immunogenicity of this E. coli mutant was subsequently characterized and identified as a small pyrophosphorylated metabolite, with a molecular mass of 262 Da, derived from the MEP pathway. Stimulation of human peripheral blood mononuclear cells (PBMC) with extracts prepared from the lytB-deficient E. coli mutant led to upregulation of T-cell activation markers on the surface of Vgamma9/Vdelta2 T cells as well as proliferation and expansion of Vgamma9/Vdelta2 T cells. This response was dependent on costimulatory growth factors, such as interleukin (IL)-2, IL-15 and IL-21. Significant levels of interferon-gamma (IFN-gamma) and tumour necrosis factor-alpha (TNF-alpha) were secreted in the presence of IL-2 and IL-15, but not in the presence of IL-21, demonstrating that proliferating phosphoantigen-reactive Vgamma9/Vdelta2 T cells do not necessarily produce proinflammatory cytokines.

GcpE is involved in the 2-C-methyl-D-erythritol 4-phosphate pathway of isoprenoid biosynthesis in Escherichia coli

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.