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

Potentiating Activity of GmhA Inhibitors on Gram-Negative Bacteria

Inhibition of the biosynthesis of bacterial heptoses opens novel perspectives for antimicrobial therapies. The enzyme GmhA responsible for the first committed biosynthetic step catalyzes the conversion of sedoheptulose 7-phosphate into d-glycero-d-manno-heptose 7-phosphate and harbors a Zn2+ ion in the active site. A series of phosphoryl- and phosphonyl-substituted derivatives featuring a hydroxamate moiety were designed and prepared from suitably protected ribose or hexose derivatives. High-resolution crystal structures of GmhA complexed to two N-formyl hydroxamate inhibitors confirmed the binding interactions to a central Zn2+ ion coordination site. Some of these compounds were found to be nanomolar inhibitors of GmhA. While devoid of HepG2 cytotoxicity and antibacterial activity of their own, they demonstrated in vitro lipopolysaccharide heptosylation inhibition in Enterobacteriaceae as well as the potentiation of erythromycin and rifampicin in a wild-type Escherichia coli strain. These inhibitors pave the way for a novel treatment of Gram-negative infections.

The Multifaceted MEP Pathway: Towards New Therapeutic Perspectives

Isoprenoids, a diverse class of natural products, are present in all living organisms. Their two universal building blocks are synthesized via two independent pathways: the mevalonate pathway and the 2-C-methyl-ᴅ-erythritol 4-phosphate (MEP) pathway. The presence of the latter in pathogenic bacteria and its absence in humans make all its enzymes suitable targets for the development of novel antibacterial drugs. (E)-4-Hydroxy-3-methyl-but-2-enyl diphosphate (HMBPP), the last intermediate of this pathway, is a natural ligand for the human Vγ9Vδ2 T cells and the most potent natural phosphoantigen known to date. Moreover, 5-hydroxypentane-2,3-dione, a metabolite produced by Escherichia coli 1-deoxy-ᴅ-xylulose 5-phosphate synthase (DXS), the first enzyme of the MEP pathway, structurally resembles (S)-4,5-dihydroxy-2,3-pentanedione, a signal molecule implied in bacterial cell communication. In this review, we shed light on the diversity of potential uses of the MEP pathway in antibacterial therapies, starting with an overview of the antibacterials developed for each of its enzymes. Then, we provide insight into HMBPP, its synthetic analogs, and their prodrugs. Finally, we discuss the potential contribution of the MEP pathway to quorum sensing mechanisms. The MEP pathway, providing simultaneously antibacterial drug targets and potent immunostimulants, coupled with its potential role in bacterial cell-cell communication, opens new therapeutic perspectives.

Over 40 Years of Fosmidomycin Drug Research: A Comprehensive Review and Future Opportunities

To address the continued rise of multi-drug-resistant microorganisms, the development of novel drugs with new modes of action is urgently required. While humans biosynthesize the essential isoprenoid precursors isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) via the established mevalonate pathway, pathogenic protozoa and certain pathogenic eubacteria use the less well-known methylerythritol phosphate pathway for this purpose. Important pathogens using the MEP pathway are, for example, Plasmodium falciparum, Mycobacterium tuberculosis, Pseudomonas aeruginosa and Escherichia coli. The enzymes of that pathway are targets for antiinfective drugs that are exempt from target-related toxicity. 2C-Methyl-D-erythritol 4-phosphate (MEP), the second enzyme of the non-mevalonate pathway, has been established as the molecular target of fosmidomycin, an antibiotic that has so far failed to be approved as an anti-infective drug. This review describes the development and anti-infective properties of a wide range of fosmidomycin derivatives synthesized over the last four decades. Here we discuss the DXR inhibitor pharmacophore, which comprises a metal-binding group, a phosphate or phosphonate moiety and a connecting linker. Furthermore, non-fosmidomycin-based DXRi, bisubstrate inhibitors and several prodrug concepts are described. A comprehensive structure-activity relationship (SAR) of nearly all inhibitor types is presented and some novel opportunities for further drug development of DXR inhibitors are discussed.

α,α-Difluorophosphonohydroxamic Acid Derivatives among the Best Antibacterial Fosmidomycin Analogues

Three α,α-difluorophosphonate derivatives of fosmidomycin were synthesized from diethyl 1,1-difluorobut-3-enylphosphonate and were evaluated on Escherichia coli. Two of them are among the best 1-deoxy-d-xylulose 5-phosphate reductoisomerase inhibitors, with IC₅₀ in the nM range, much better than fosmidomycin, the reference compound. They also showed an enhanced antimicrobial activity against E. coli on Petri dishes in comparison with the corresponding phosphates and the non-fluorinated phosphonate.

Synthesis and Antiplasmodial Activity of Novel Fosmidomycin Derivatives and Conjugates with Artemisinin and Aminochloroquinoline

Malaria, despite many efforts, remains among the most problematic infectious diseases worldwide, mainly due to the development of drug resistance by Plasmodium falciparum. The antibiotic fosmidomycin (FSM) is also known for its antimalarial activity by targeting the non-mevalonate isoprenoid synthesis pathway, which is essential for the malaria parasites but is absent in mammalians. In this study, we synthesized and evaluated against the chloroquine-resistant P. falciparum FcB1/Colombia strain, a series of FSM analogs, derivatives, and conjugates with other antimalarial agents, such as artemisinin (ART) and aminochloroquinoline (ACQ). The biological evaluation revealed four new compounds with higher antimalarial activity than FSM: two FSM-ACQ derivatives and two FSM-ART conjugates, with 3.5-5.4 and 41.5-23.1 times more potent activities than FSM, respectively.

Synthesis and biological evaluation of aryl phosphoramidate prodrugs of fosfoxacin and its derivatives

Aryl phosphoramidate prodrugs of fosfoxacin derivatives 15a-b and 8a-b were synthesized and investigated for their ability to target bacteria. No growth inhibition was observed neither for Mycobacterium smegmatis nor for Escherichia coli on solid medium, demonstrating the absence of release of the active compounds in the bacterial cells. Investigation of the stability of the prodrugs and their multienzymatic cleavage in abiotic and biotic conditions showed that the use of aryl phosphoramidate prodrug approach to deliver non-nucleotides compounds is not obvious and might not be appropriate for an antimicrobial drug.

Targeting Metalloenzymes for Therapeutic Intervention

Metalloenzymes are central to a wide range of essential biological activities, including nucleic acid modification, protein degradation, and many others. The role of metalloenzymes in these processes also makes them central for the progression of many diseases and, as such, makes metalloenzymes attractive targets for therapeutic intervention. Increasing awareness of the role metalloenzymes play in disease and their importance as a class of targets has amplified interest in the development of new strategies to develop inhibitors and ultimately useful drugs. In this Review, we provide a broad overview of several drug discovery efforts focused on metalloenzymes and attempt to map out the current landscape of high-value metalloenzyme targets.

MEPicides: α,β-Unsaturated Fosmidomycin Analogues as DXR Inhibitors against Malaria

Severe malaria due to Plasmodium falciparum remains a significant global health threat. DXR, the second enzyme in the MEP pathway, plays an important role to synthesize building blocks for isoprenoids. This enzyme is a promising drug target for malaria due to its essentiality as well as its absence in humans. In this study, we designed and synthesized a series of α,β-unsaturated analogues of fosmidomycin, a natural product that inhibits DXR in P. falciparum. All compounds were evaluated as inhibitors of P. falciparum. The most promising compound, 18a, displays on-target, potent inhibition against the growth of P. falciparum (IC₅₀ = 13 nM) without significant inhibition of HepG2 cells (IC₅₀ > 50 μM). 18a was also tested in a luciferase-based Plasmodium berghei mouse model of malaria and showed exceptional in vivo efficacy. Together, the data support MEPicide 18a as a novel, potent, and promising drug candidate for the treatment of malaria.

Structure-activity relationship studies on acremomannolipin A, the potent calcium signal modulator with a novel glycolipid structure 3: role of the length of alditol side chain

Five homologs of a novel glycolipid acremomannolipin A (1a), the potential Ca(2+) signal modulator isolated from Acremonium strictum, bearing alditols of different length (1g-1k) were synthesized by a stereoselective β-mannosylation of appropriately protected mannosyl sulfoxide (2) with five alditols (1g: C2, 1h: C3, 1i: C4, 1j: C5 and 1k: C7 units), and their potential in modulating Ca(2+) signaling were evaluated. Homologs with alditols of more than 4 carbons (1i, 1j and 1k) were equally or more potent than the parent compound (1a) regardless of the length of the alditol chain. Whereas activities of two homologs with shorter chains (1g and 1h) decreased to a considerable extent. The results indicated that the length of the alditol side chain was a crucial determinant for the potent calcium signal modulating activity.

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.

Synthetic Fosmidomycin analogues with altered chelating moieties do not inhibit 1-deoxy-d-xylulose 5-phosphate Reductoisomerase or Plasmodium falciparum growth in vitro

Fourteen new fosmidomycin analogues with altered metal chelating groups were prepared and evaluated for inhibition of E. coli Dxr, M. tuberculosis Dxr and the growth of P. falciparum K1 in human erythrocytes. None of the synthesized compounds showed activity against either enzyme or the Plasmodia. This study further underlines the importance of the hydroxamate functionality and illustrates that identifying effective alternative bidentate ligands for this target enzyme is challenging.

Metabolic plasticity for isoprenoid biosynthesis in bacteria

Isoprenoids are a large family of compounds synthesized by all free-living organisms. In most bacteria, the common precursors of all isoprenoids are produced by the MEP (methylerythritol 4-phosphate) pathway. The MEP pathway is absent from archaea, fungi and animals (including humans), which synthesize their isoprenoid precursors using the completely unrelated MVA (mevalonate) pathway. Because the MEP pathway is essential in most bacterial pathogens (as well as in the malaria parasites), it has been proposed as a promising new target for the development of novel anti-infective agents. However, bacteria show a remarkable plasticity for isoprenoid biosynthesis that should be taken into account when targeting this metabolic pathway for the development of new antibiotics. For example, a few bacteria use the MVA pathway instead of the MEP pathway, whereas others possess the two full pathways, and some parasitic strains lack both the MVA and the MEP pathways (probably because they obtain their isoprenoids from host cells). Moreover, alternative enzymes and metabolic intermediates to those of the canonical MVA or MEP pathways exist in some organisms. Recent work has also shown that resistance to a block of the first steps of the MEP pathway can easily be developed because several enzymes unrelated to isoprenoid biosynthesis can produce pathway intermediates upon spontaneous mutations. In the present review, we discuss the major advances in our knowledge of the biochemical toolbox exploited by bacteria to synthesize the universal precursors for their essential isoprenoids.

Antimalarial and Structural Studies of Pyridine-containing Inhibitors of 1-Deoxyxylulose-5-phosphate Reductoisomerase

1-Deoxy-D-xylulose-5-phosphate reductoisomerase (DXR) in the non-mevalonate isoprene biosynthesis pathway is a target for developing antimalarial drugs. Fosmidomycin, a potent DXR inhibitor, showed safety as well as efficacy against P. falciparum malaria in clinical trials. Based on our previous quantitative structure activity relationship (QSAR) and crystallographic studies, several novel pyridine-containing fosmidomycin derivatives were designed, synthesized and found to be highly potent inhibitors of P. falciparum DXR (PfDXR) having Ki values of 1.9 - 13 nM, with the best one being ~11× more active than fosmidomycin. These compounds also potently block the proliferation of multi-drug resistant P. falciparum with EC50 values as low as 170 nM. A 2.3 Å crystal structure of PfDXR in complex with one of the inhibitors is reported, showing the flexible loop of the protein undergoes conformational changes upon ligand binding and a hydrogen bond and favorable hydrophobic interactions between the pyridine group and PfDXR account for the enhanced activity.

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.

Synthesis of functionalized cinnamaldehyde derivatives by an oxidative Heck reaction and their use as starting materials for preparation of Mycobacterium tuberculosis 1-deoxy-D-xylulose-5-phosphate reductoisomerase inhibitors

Cinnamaldehyde derivatives were synthesized in good to excellent yields in one step by a mild and selective, base-free palladium(II)-catalyzed oxidative Heck reaction starting from acrolein and various arylboronic acids. Prepared α,β-unsaturated aldehydes were used for synthesis of novel α-aryl substituted fosmidomycin analogues, which were evaluated for their inhibition of Mycobacterium tuberculosis 1-deoxy-d-xylulose 5-phosphate reductoisomerase. IC₅₀ values between 0.8 and 27.3 μM were measured. The best compound showed activity comparable to that of the most potent previously reported α-aryl substituted fosmidomycin-class inhibitor.

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.

Inhibition Studies on Enzymes Involved in Isoprenoid Biosynthesis: Focus on Two Potential Drug Targets: DXR and IDI-2 Enzymes

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.

Molecular basis of fosmidomycin's action on the human malaria parasite Plasmodium falciparum

The human malaria parasite Plasmodium falciparum is responsible for the deaths of more than a million people each year. Fosmidomycin has been proven to be efficient in the treatment of P. falciparum malaria by inhibiting 1-deoxy-D-xylulose 5-phosphate reductoisomerase (DXR), an enzyme of the non-mevalonate pathway, which is absent in humans. However, the structural details of DXR inhibition by fosmidomycin in P. falciparum are unknown. Here, we report the crystal structures of fosmidomycin-bound complete quaternary complexes of PfDXR. Our study revealed that (i) an intrinsic flexibility of the PfDXR molecule accounts for an induced-fit movement to accommodate the bound inhibitor in the active site and (ii) a cis arrangement of the oxygen atoms of the hydroxamate group of the bound inhibitor is essential for tight binding of the inhibitor to the active site metal. We expect the present structures to be useful guides for the design of more effective antimalarial compounds.

Fosmidomycin uptake into Plasmodium and Babesia-infected erythrocytes is facilitated by parasite-induced new permeability pathways

Background

Highly charged compounds typically suffer from low membrane permeability and thus are generally regarded as sub-optimal drug candidates. Nonetheless, the highly charged drug fosmidomycin and its more active methyl-derivative FR900098 have proven parasiticidal activity against erythrocytic stages of the malaria parasite Plasmodium falciparum. Both compounds target the isoprenoid biosynthesis pathway present in bacteria and plastid-bearing organisms, like apicomplexan parasites. Surprisingly, the compounds are inactive against a range of apicomplexans replicating in nucleated cells, including Toxoplasma gondii.

Methodology/Principal Findings

Since non-infected erythrocytes are impermeable for FR90098, we hypothesized that these drugs are taken up only by erythrocytes infected with Plasmodium. We provide evidence that radiolabeled FR900098 accumulates in theses cells as a consequence of parasite-induced new properties of the host cell, which coincide with an increased permeability of the erythrocyte membrane. Babesia divergens, a related parasite that also infects human erythrocytes and is also known to induce an increase in membrane permeability, displays a similar susceptibility and uptake behavior with regard to the drug. In contrast, Toxoplasma gondii-infected cells do apparently not take up the compounds, and the drugs are inactive against the liver stages of Plasmodium berghei, a mouse malaria parasite.

Conclusions/Significance

Our findings provide an explanation for the observed differences in activity of fosmidomycin and FR900098 against different Apicomplexa. These results have important implications for future screens aimed at finding new and safe molecular entities active against P. falciparum and related parasites. Our data provide further evidence that parasite-induced new permeability pathways may be exploited as routes for drug delivery.

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.

NMR studies of DOXP reductoisomerase and its inhibitor complex

1-Deoxy-D-xylulose 5-phosphate (DOXP) reductoisomerase (EC1.1.1.267) catalyses the second step of the 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway of isoprenoid biosynthesis. The enzyme is used by most bacteria, apicomplexan parasites and the plastids of plants, but not by humans, and therefore represents an attractive target for antibacterial, antiparasitic and herbicidal compounds. Fosmidomycin, an inhibitor of DXR, has been found to be active against bacterial infections and malaria in early clinical studies. Here, we report sample optimisation, partial backbone assignment and secondary-structure prediction of E. coli DXR by heteronuclear NMR analysis for further NMR-aided drug discovery. Perdeuterated (15)N,(13)C-labelled samples were prepared under oxygen exclusion in the presence of Mg(2+), NADPH and the inhibitor FR-900098, a close derivative of fosmidomycin. (1)H and (15)N backbone assignment was achieved for 44 % of the primary structure, and (13)C backbone assignment was achieved for 50 % of the primary structure. Comparison with previously solved crystal structures revealed that the assigned fragments were located mainly in helical regions on the solvent-exposed surface of the enzyme. Torsion angle likelihood obtained from shift and sequence similarity (TALOS) was used for secondary structure prediction, resulting in agreement with eight available crystal structures; deviations could be observed for the catalytic loop region.

Isoprenoid biosynthesis via the methylerythritol phosphate pathway: structural variations around phosphonate anchor and spacer of fosmidomycin, a potent inhibitor of deoxyxylulose phosphate reductoisomerase

Fosmidomycin and its analogue FR-900098 are potent inhibitors of 1-deoxy-d-xylulose 5-phosphate reducto-isomerase (DXR), the second enzyme of the MEP pathway for the biosynthesis of isoprenoids. This paper describes the synthesis of analogues of the two reverse phosphonohydroxamic acids 3 and 4, in which the length of the carbon spacer is modified, the N-methyl group of 3 is replaced by an ethyl group, and the phosphate group is replaced by potential isosteric moieties, i.e., sulfonate or carboxylate functionalities. The potential of the synthesized analogues to inhibit the E. coli DXR was evaluated.

Kinetic characterization and phosphoregulation of the Francisella tularensis 1-deoxy-D-xylulose 5-phosphate reductoisomerase (MEP synthase)

Deliberate and natural outbreaks of infectious disease underscore the necessity of effective vaccines and antimicrobial/antiviral therapeutics. The prevalence of antibiotic resistant strains and the ease by which antibiotic resistant bacteria can be intentionally engineered further highlights the need for continued development of novel antibiotics against new bacterial targets. Isoprenes are a class of molecules fundamentally involved in a variety of crucial biological functions. Mammalian cells utilize the mevalonic acid pathway for isoprene biosynthesis, whereas many bacteria utilize the methylerythritol phosphate (MEP) pathway, making the latter an attractive target for antibiotic development. In this report we describe the cloning and characterization of Francisella tularensis MEP synthase, a MEP pathway enzyme and potential target for antibiotic development. In vitro growth-inhibition assays using fosmidomycin, an inhibitor of MEP synthase, illustrates the effectiveness of MEP pathway inhibition with F. tularensis. To facilitate drug development, F. tularensis MEP synthase was cloned, expressed, purified, and characterized. Enzyme assays produced apparent kinetic constants (K_M^DXP = 104 µM, K_M^NADPH = 13 µM, k_cat^DXP = 2 s⁻¹, k_cat^NADPH = 1.3 s⁻¹), an IC₅₀ for fosmidomycin of 247 nM, and a K_i for fosmidomycin of 99 nM. The enzyme exhibits a preference for Mg⁺² as a divalent cation. Titanium dioxide chromatography-tandem mass spectrometry identified Ser177 as a site of phosphorylation. S177D and S177E site-directed mutants are inactive, suggesting a mechanism for post-translational control of metabolic flux through the F. tularensis MEP pathway. Overall, our study suggests that MEP synthase is an excellent target for the development of novel antibiotics against F. tularensis.

Synthesis and evaluation of alpha,beta-unsaturated alpha-aryl-substituted fosmidomycin analogues as DXR inhibitors

Fosmidomycin, which acts through inhibition of 1-deoxy-D-xylulose phosphate reductoisomerase (DXR) in the non-mevalonate pathway, represents a valuable recent addition to the armamentarium against uncomplicated malaria. In this paper, we describe the synthesis and biological evaluation of E- and Z-alpha,beta-unsaturated alpha-aryl-substituted analogues of FR900098, a fosmidomycin congener, utilizing a Stille or a Suzuki coupling to introduce the aryl group. In contrast with our expectations based on the promising activity earlier observed for several alpha-substituted fosmidomycin analogues, all synthesized analogues exhibited much lower binding affinity for DXR than fosmidomycin.

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).