Many bacterial pathogens utilize the 2-C-methyl-D-erythritol 4-phosphate pathway for biosynthesizing isoprenoid precursors, a pathway that is vital for bacterial survival and absent from human cells, providing a potential source of drug targets. However, the characterization of 4-diphosphocytidyl-2-C-methyl-D-erythritol (CDP-ME) kinase (IspE) has been hindered due to a lack of enantiopure CDP-ME and difficulty in obtaining pure IspE. Here, enantiopure CDP-ME was chemically synthesized and recombinant IspE from bacterial pathogens were purified and characterized. Although gene disruption was not possible in Mycobacterium tuberculosis, IspE is essential in Mycobacterium smegmatis. The biochemical and kinetic characteristics of IspE provide the basis for development of a high throughput screen and structural characterization.
4-Diphosphocytidyl-2C-methyl-d-erythritol kinase (IspE) catalyses the ATP-dependent conversion of 4-diphosphocytidyl-2C-methyl-d-erythritol (CDPME) to 4-diphosphocytidyl-2C-methyl-d-erythritol 2-phosphate with the release of ADP. This reaction occurs in the non-mevalonate pathway of isoprenoid precursor biosynthesis and because it is essential in important microbial pathogens and absent from mammals it represents a potential target for anti-infective drugs. We set out to characterize the biochemical properties, determinants of molecular recognition and reactivity of IspE and report the cloning and purification of recombinant Aquifex aeolicus IspE (AaIspE), kinetic data, metal ion, temperature and pH dependence, crystallization and structure determination of the enzyme in complex with CDP, CDPME and ADP. In addition, 4-fluoro-3,5-dihydroxy-4-methylpent-1-enylphosphonic acid (compound 1) was designed to mimic a fragment of the substrate, a synthetic route to 1 was elucidated and the complex structure determined. Surprisingly, this ligand occupies the binding site for the ATP α-phosphate not the binding site for the methyl-d-erythritol moiety of CDPME. Gel filtration and analytical ultracentrifugation indicate that AaIspE is a monomer in solution. The enzyme displays the characteristic α/β galacto-homoserine-mevalonate-phosphomevalonate kinase fold, with the catalytic centre positioned in a deep cleft between the ATP- and CDPME-binding domains. Comparisons indicate a high degree of sequence conservation on the IspE active site across bacterial species, similarities in structure, specificity of substrate recognition and mechanism. The biochemical characterization, attainment of well-ordered and reproducible crystals and the models resulting from the analyses provide reagents and templates to support the structure-based design of broad-spectrum antimicrobial agents.
enzyme–ligand complex; GHMP kinase; isoprenoid biosynthesis; molecular recognition; non-mevalonate pathway
Enantiomerically pure 2-C-methyl-D-erythritol 4-phosphate 1 (MEP) is synthesized from 1,2-O-isopropylidene-α-D-xylofuranose via facile benzylation in good yield. Subsequently, 1 is used for enzymatic synthesis of 4-diphosphocytidyl-2-C-methyl-D-erythritol 2 (CDP-ME) using 4-diphosphocytidyl-2-C-methyl-D-erythritol synthase (IspD). The chemoenzymatically synthesized 2 can be used as substrate for assay of IspE and for high throughput screening to identify IspE inhibitors.
The nonmevalonate route to isoprenoid biosynthesis is essential in Gram-negative bacteria and apicomplexan parasites. The enzymes of this pathway are absent from mammals, contributing to their appeal as chemotherapeutic targets. One enzyme, 2C-methyl-d-erythritol-2,4-cyclodiphosphate synthase (IspF), has been validated as a target by genetic approaches in bacteria. Virtual screening against Escherichia coli IspF (EcIspF) was performed by combining a hierarchical filtering methodology with molecular docking. Docked compounds were inspected and 10 selected for experimental validation. A surface plasmon resonance assay was developed and two weak ligands identified. Crystal structures of EcIspF complexes were determined to support rational ligand development. Cytosine analogues and Zn2+-binding moieties were characterized. One of the putative Zn2+-binding compounds gave the lowest measured KD to date (1.92 ± 0.18 μM). These data provide a framework for the development of IspF inhibitors to generate lead compounds of therapeutic potential against microbial pathogens.
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 Mg2+ ions and exhibits optimal activity between pH 7.5 and 7.9; the Km for 1-deoxyxylulose 5-phosphate was calculated to be 47.1 μM, and the Km for NADPH was 29.7 μM. The specificity constant of Rv2780c in the forward direction is 1.5 × 106 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.
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
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.
The nonmevalonate pathway is responsible for isoprenoid production in microbes, including H. pylori, M. tuberculosis and P. falciparum, but is nonexistent in humans, thus providing a desirable route for antibacterial and antimalarial drug discovery. We coordinate a structural study of IspH, a [4Fe-4S] protein responsible for converting HMBPP to IPP and DMAPP in the ultimate step in the nonmevalonate pathway. By performing accelerated molecular dynamics simulations on both substrate-free and HMBPP-bound [Fe4S4]2+ IspH, we elucidate how substrate binding alters the dynamics of the protein. Using principal component analysis, we note that while substrate-free IspH samples various open and closed conformations, the closed conformation observed experimentally for HMBPP-bound IspH is inaccessible in the absence of HMBPP. In contrast, simulations with HMBPP bound are restricted from accessing the open states sampled by the substrate-free simulations. Further investigation of the substrate-free simulations reveals large fluctuations in the HMBPP binding pocket, as well as allosteric pocket openings – both of which are achieved through the hinge motions of the individual domains in IspH. Coupling these findings with solvent mapping and various structural analyses reveals alternative druggable sites that may be exploited in future drug design efforts.
Drug resistance has recently entered into media conversations through the lens of MRSA (methicillin-resistant Staphylococcus aureus) infections, but conventional therapies are also failing to address resistance in cases of malaria and other bacterial infections, such as tuberculosis. To address these problems, we must develop new antibacterial and antimalarial medications. Our research focuses on understanding the structure and dynamics of IspH, an enzyme whose function is necessary for the survival of most bacteria and malaria-causing protozoans. Using computer simulations, we track how the structure of IspH changes in the presence and absence of its natural substrate. By inspecting the pockets that form in the normal motions of IspH, we propose a couple new routes by which new molecules may be developed to disrupt the function of IspH. It is our hope that these structural studies may be precursors to the development of novel therapies that may add to our current arsenal against bacterial and malarial infections.
We have recently reported the expression of murine Implantation Serine Proteinase genes in pre-implantation embryos (ISP1) and uterus (ISP1 and ISP2). These proteinases belong to the S1 proteinase family and are similar to mast cell tryptases, which function as multimers.
Here, we report the purification and initial characterization of ISP1 and 2 with respect to their physico-chemical properties and physiological function. In addition to being co-expressed in uterus, we show that ISP1 and ISP2 are also co-expressed in the pre-implantation embryo. Together, they form a heterodimer with an approximate molecular weight of 63 kD. This complex is the active form of the enzyme, which we have further characterized as being trypsin-like, based on substrate and inhibitor specificities. In addition to having a role in embryo hatching and outgrowth, we demonstrate that ISP enzyme is localized to the site of embryo invasion during implantation and that its activity is important for successful implantation in vivo.
On the basis of similarities in structural, chemical, and functional properties, we suggest that this ISP enzyme complex represents the classical hatching enzyme, strypsin. Our results demonstrate a critical role for ISP in embryo hatching and implantation.
The prevalence of tuberculosis, the prolonged and expensive treatment that this disease requires and an increase in drug resistance indicate an urgent need for new treatments. The 1-deoxy-D-xylulose 5-phosphate pathway of isoprenoid precursor biosynthesis is an attractive chemotherapeutic target because it occurs in many pathogens, including Mycobacterium tuberculosis, and is absent from humans. To underpin future drug development it is important to assess which enzymes in this biosynthetic pathway are essential in the actual pathogens and to characterize them.
The fifth enzyme of this pathway, encoded by ispF, is 2C-methyl-D-erythritol-2,4-cyclodiphosphate synthase (IspF). A two-step recombination strategy was used to construct ispF deletion mutants in M. tuberculosis but only wild-type double crossover strains were isolated. The chromosomal copy could be deleted when a second functional copy was provided on an integrating plasmid, demonstrating that ispF is an essential gene under the conditions tested thereby confirming its potential as a drug target. We attempted structure determination of the M. tuberculosis enzyme (MtIspF), but failed to obtain crystals. We instead analyzed the orthologue M. smegmatis IspF (MsIspF), sharing 73% amino acid sequence identity, at 2.2 Å resolution. The high level of sequence conservation is particularly pronounced in and around the active site. MsIspF is a trimer with a hydrophobic cavity at its center that contains density consistent with diphosphate-containing isoprenoids. The active site, created by two subunits, comprises a rigid CDP-Zn2+ binding pocket with a flexible loop to position the 2C-methyl-D-erythritol moiety of substrate. Sequence-structure comparisons indicate that the active site and interactions with ligands are highly conserved.
Our study genetically validates MtIspF as a therapeutic target and provides a model system for structure-based ligand design.
The MEP pathway, which is absent in animals but present in most pathogenic bacteria, in the parasite responsible for malaria and in plant plastids, is a target for the development of antimicrobial drugs. IspH, an oxygen-sensitive [4Fe-4S] enzyme, catalyzes the last step of this pathway and converts (E)-4-hydroxy-2-methylbut-2-enyl 1-diphosphate (HMBPP) into the two isoprenoid precursors: isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). A crucial step in the mechanism of this enzyme is the binding of the C4 hydroxyl of HMBPP to the unique fourth iron site in the [4Fe-4S]2+ moiety. Here we report the synthesis and the kinetic investigations of two new extremely potent inhibitors of E. coli IspH where the OH group of HMBPP is replaced by an amino and a thiol group. (E)-4-Mercapto-3-methyl-but-2-en-1-yl diphosphate is a reversible tight-binding inhibitor of IspH with Ki = 20 ± 2 nM. A detailed kinetic analysis revealed that (E)-4-amino-3-methylbut-2-en-1-yl diphosphate is a reversible slow-binding inhibitor of IspH with Ki = 54 ± 19 nM. The slow binding behavior of this inhibitor is best described by a one-step mechanism with the slow step consisting in the formation of the enzyme-inhibitor (EI) complex.
Leishmania ISPs are ecotin-like natural peptide inhibitors of trypsin-family serine peptidases, enzymes that are absent from the Leishmania genome. This led to the proposal that ISPs inhibit host serine peptidases and we have recently shown that ISP2 inhibits neutrophil elastase, thereby enhancing parasite survival in murine macrophages. In this study we show that ISP1 has less serine peptidase inhibitory activity than ISP2, and in promastigotes both are generally located in the cytosol and along the flagellum. However, in haptomonad promastigotes there is a prominent accumulation of ISP1 and ISP2 in the hemidesmosome and for ISP2 on the cell surface. An L. major mutant deficient in all three ISP genes (Δisp1/2/3) was generated and compared with Δisp2/3 mutants to elucidate the physiological role of ISP1. In in vitro cultures, the Δisp1/2/3 mutant contained more haptomonad, nectomonad and leptomonad promastigotes with elongated flagella and reduced motility compared with Δisp2/3 populations, moreover it was characterized by very high levels of release of exosome-like vesicles from the flagellar pocket. These data suggest that ISP1 has a primary role in flagellar homeostasis, disruption of which affects differentiation and flagellar pocket dynamics.
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
In disease-associated genes, the understanding of the functional significance of deep intronic nucleotide variants may represent a difficult challenge. We have previously reported a new disease-causing mechanism that involves an intronic splicing processing element (ISPE) in ATM, composed of adjacent consensus 5′ and 3′ splice sites. A GTAA deletion within ISPE maintains potential adjacent splice sites, disrupts a non-canonical U1 snRNP interaction and activates an aberrant exon. In this paper, we demonstrate that binding of U1 snRNA through complementarity within a ∼40 nt window downstream of the ISPE prevents aberrant splicing. By selective mutagenesis at the adjacent consensus ISPE splice sites, we show that this effect is not due to a resplicing process occurring at the ISPE. Functional comparison of the ATM mouse counterpart and evaluation of the pre-mRNA splicing intermediates derived from affected cell lines and hybrid minigene assays indicate that U1 snRNP binding at the ISPE interferes with the cryptic acceptor site. Activation of this site results in a stringent 5′–3′ order of intron sequence removal around the cryptic exon. Artificial U1 snRNA loading by complementarity to heterologous exonic sequences represents a potential therapeutic method to prevent the usage of an aberrant CFTR cryptic exon. Our results suggest that ISPE-like intronic elements binding U1 snRNPs may regulate correct intron processing.
Early in T-cell development, cells proceed through stages that are critically dependent on signaling through the Notch receptor. As cells mature, thymocytes transition from being Notch dependent to being Notch independent, but the stage of development during which this transition occurs is unknown. We used an in vitro differentiation system in which thymocytes can be cultured in the presence or absence of a Notch ligand to identify the stage of development in which thymocytes transition from being Notch responsive to Notch non-responsive. We identified the immature single-positive (ISP) CD8+ stage of T-cell development as being this transition point. ISP thymocytes were responsive to Notch, but ISP cells responded to Notch ligation in a manner that was distinct from the response by double-negative (DN) thymocytes. Fewer ISP thymocytes proliferated and more ISP cells died in culture than DN thymocytes. Further, fewer double-positive (DP) thymocytes generated by culturing ISP thymocytes were in the S, G2 or M phase of the cell cycle as compared with DP thymocytes derived from DN thymocytes. These data indicate that the DP population created varied depending on the input population. In summary, the data presented here indicate that ISP thymocytes responded to Notch differently than DN thymocytes and ISP thymocytes represent the transition stage from Notch-dependent survival and proliferation to Notch-independent survival and proliferation.
signaling pathways; thymocytes
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.
Implantation S1 family serine proteinases (ISPs) are tryptases involved in embryo hatching and uterine implantation in the mouse. The two different ISP proteins (ISP1 and ISP2) have been detected in both pre- and post-implantation embryo tissue. To date, native ISP obtained from uterus and blastocyst tissues has been isolated only as an active hetero-dimer that exhibits trypsin-like substrate specificity. We hypothesised that in isolation, ISP1 might have a unique substrate specificity that could relate to its role when expressed alone in individual tissues. Thus, we isolated recombinant ISP1 expressed in Pichia pastoris and evaluated its substrate specificity. Using several chromogenic substrates and serine proteinase inhibitors, we demonstrate that ISP1 exhibits trypsin-like substrate specificity, having a preference for lysine over arginine at the P1 position. Phage display peptide mimetics revealed an expanded but mixed substrate specificity of ISP1, including chymotryptic and elastase activity. Based upon targets observed using phage display, we hypothesised that ISP1 might signal to cells by cleaving and activating proteinase-activated receptors (PARs) and therefore assessed PARs 1, 2 and 4 as potential ISP1 targets. We observed that ISP1 silenced enzyme-triggered PAR signaling by receptor-disarming. This PAR-disarming action of ISP1 may be important for embryo development and implantation.
2C-methyl-D-erythritol-2,4-cyclodiphosphate synthase (IspF) catalyzes the conversion of 4-diphosphocytidyl-2C-methyl-D-erythritol-2-phosphate to 2C-methyl-D-erythritol-2,4-cyclodiphosphate and cytidine monophosphate in production of isoprenoid-precursors via the methylerythritol phosphate biosynthetic pathway. IspF is found in the protozoan Plasmodium falciparum, a parasite that causes cerebral malaria, as well as in many Gram-negative bacteria such as Burkholderia cenocepacia. IspF represents a potential target for development of broad-spectrum antimicrobial drugs since it is proven or inferred as essential in these pathogens and absent from mammals. Structural studies of IspF from these two important yet distinct pathogens, and comparisons with orthologues have been carried out to generate reagents, to support and inform a structure-based approach to early stage drug discovery.
Efficient recombinant protein production and crystallization protocols were developed, and high-resolution crystal structures of IspF from P. falciparum (Emphasis/Emphasis>IspF) and B. cenocepacia (BcIspF) in complex with cytidine nucleotides determined. Comparisons with orthologues, indicate a high degree of order and conservation in parts of the active site where Zn2+ is bound and where recognition of the cytidine moiety of substrate occurs. However, conformational flexibility is noted in that area of the active site responsible for binding the methylerythritol component of substrate. Unexpectedly, one structure of BcIspF revealed two molecules of cytidine monophosphate in the active site, and another identified citrate coordinating to the catalytic Zn2+. In both cases interactions with ligands appear to help order a flexible loop at one side of the active site. Difficulties were encountered when attempting to derive complex structures with other ligands.
High-resolution crystal structures of IspF from two important human pathogens have been obtained and compared to orthologues. The studies reveal new data on ligand binding, with citrate coordinating to the active site Zn2+ and when present in high concentrations cytidine monophosphate displays two binding modes in the active site. Ligand binding appears to order a part of the active site involved in substrate recognition. The high degree of structural conservation in and around the IspF active site suggests that any structural model might be suitable to support a program of structure-based drug discovery.
Antimicrobial drug target; Isoprenoid biosynthesis; X-ray crystallography; Zn2+-dependent enzyme
Several lines of evidence indicate that platelets protect against endovascular infections such as infective endocarditis (IE). It is highly likely that a principal mechanism of this platelet host defense role is the release of platelet microbicidal proteins (PMPs) in response to agonists generated at sites of endovascular infection. We studied the ability of platelets to limit the colonization and proliferation of Staphylococcus aureus in an in vitro model of IE. Three isogenic S. aureus strains, differing in their in vitro susceptibility to thrombin-induced platelet microbicidal protein-1 (tPMP), were used: ISP479C (parental strain; highly susceptible to tPMP [tPMPs]); ISP479R (transposon mutant derived from ISP479; tPMP resistant [tPMPr]); or 757-5 (tPMPr transductant of the ISP479R genotype in the ISP479 parental background). Time-kill assays and in vitro IE models were used to examine the temporal relationship between thrombin-induced platelet activation and S. aureus killing. In time-kill studies, early platelet activation (30 min prior to bacterial exposure) correlated with a significant bactericidal effect against tPMPs ISP479C (r2 > 0.90, P < 0.02) but not against tPMPr strains, ISP479R or 757-5. In the IE model, thrombin activation significantly inhibited proliferation of ISP479C within simulated vegetations compared to strains ISP479R or 757-5 (P < 0.05). The latter differences were observed despite there being no detectable differences among the three S. aureus strains in initial colonization of simulated vegetations. Collectively, these data indicate that platelets limit intravegetation proliferation of tPMPs but not tPMPr S. aureus. These findings underscore the likelihood that platelets play an important antimicrobial host defense role in preventing and/or limiting endovascular infections due to tPMPs pathogens.
IspH is a 4Fe-4S protein that carries out an essential reduction step in isoprenoid biosynthesis. Using hyperfine sublevel correlation (HYSCORE) spectroscopy, we show that pyridine inhibitors of IspH directly bind to the unique 4th Fe in the 4Fe-4S cluster, opening up new routes to inhibitor design, of interest in the context of both anti-bacterial as well as anti-malarial drug discovery.
Isoprenoid; non-mevalonate; IspH (LytB); inhibitors; HYSCORE
The [4Fe-4S] protein IspH in the methylerythritol phosphate isoprenoid biosynthesis pathway is an important anti-infective drug target, but its mechanism of action is still the subject of debate. Here, by using electron paramagnetic resonance (EPR) spectroscopy and 2H, 17O, and 57Fe isotopic labeling, we have characterized and assigned two key reaction intermediates in IspH catalysis. The results are consistent with the bioorganometallic mechanism proposed earlier, and the mechanism is proposed to have similarities to that of ferredoxin: thioredoxin reductase, in that one electron is transferred to the [4Fe-4S]2+ cluster, which then performs a formally two-electron reduction of its substrate, generating an oxidized high potential iron-sulfur protein (HiPIP)-like intermediate. The two paramagnetic reaction intermediates observed correspond to the two intermediates proposed in the bioorganometallic mechanism: the early π-complex in which the substrate’s 3-CH2OH group has rotated away from the reduced iron-sulfur cluster, and the next, η3-allyl complex formed after dehydroxylation. No free radical intermediates are observed, and the two paramagnetic intermediates observed do not fit in a Birch reduction-like or ferraoxetane mechanism. Additionally, we show by using EPR spectroscopy and X-ray crystallography that two substrate analogs (4 and 5) follow the same reaction mechanism.
The final step of the methylerythritol phosphate isoprenoid biosynthesis pathway is catalysed by the iron–sulphur enzyme IspH, producing the universal precursors of terpenes: isopentenyl diphosphate and dimethylallyl diphosphate. Here we report an unforeseen reaction discovered during the investigation of the interaction of IspH with acetylene inhibitors by X-ray crystallography, Mößbauer, and nuclear magnetic resonance spectroscopy. In addition to its role as a 2H+/2e− reductase, IspH can hydrate acetylenes to aldehydes and ketones via anti-Markovnikov/Markovnikov addition. The reactions only occur with the oxidised protein and proceed via η1-O-enolate intermediates. One of these is characterized crystallographically and contains a C4 ligand oxygen bound to the unique, fourth iron in the 4Fe-4S cluster: this intermediate subsequently hydrolyzes to produce an aldehyde product. This unexpected side to IspH reactivity is of interest in the context of the mechanism of action of other acetylene hydratases, as well as in the design of antiinfectives targeting IspH.
The screening of a collection of highly mutagenized strains of Escherichia coli for defects in isoprenoid synthesis led to the isolation of a mutant that had temperature-sensitive farnesyl diphosphate synthase. The defective gene, named ispA, was mapped at about min 10 on the E. coli chromosome, and the gene order was shown to be tsx-ispA-lon. The mutant ispA gene was transferred to the E. coli strain with a defined genetic background by P1 transduction for investigation of its function. The in vitro activity of farnesyl diphosphate synthase of the mutant was 21% of that of the wild-type strain at 30 degrees C and 5% of that at 40 degrees C. At 42 degrees C the ubiquinone level was lower (66% of normal) in the mutant than in the wild-type strain, whereas at 30 degrees C the level in the mutant was almost equal to that in the wild-type strain. The polyprenyl phosphate level was slightly higher in the mutant than in the wild-type strain at 30 degrees C and almost the same in both strains at 42 degrees C. The mutant had no obvious phenotype regarding its growth properties.
The terminal oxygenase component of toluene dioxygenase from Pseudomonas putida F1 is an iron-sulfur protein (ISP(TOL)) that requires mononuclear iron for enzyme activity. Alignment of all available predicted amino acid sequences for the large (alpha) subunits of terminal oxygenases showed a conserved cluster of potential mononuclear iron-binding residues. These were between amino acids 210 and 230 in the alpha subunit (TodC1) of ISP(TOL). The conserved amino acids, Glu-214, Asp-219, Tyr-221, His-222, and His-228, were each independently replaced with an alanine residue by site-directed mutagenesis. Tyr-266 in TodC1, which has been suggested as an iron ligand, was treated in an identical manner. To assay toluene dioxygenase activity in the presence of TodC1 and its mutant forms, conditions for the reconstitution of wild-type ISP(TOL) activity from TodC1 and purified TodC2 (beta subunit) were developed and optimized. A mutation at Glu-214, Asp-219, His-222, or His-228 completely abolished toluene dioxygenase activity. TodC1 with an alanine substitution at either Tyr-221 or Tyr-266 retained partial enzyme activity (42 and 12%, respectively). In experiments with [14C]toluene, the two Tyr-->Ala mutations caused a reduction in the amount of Cis-[14C]-toluene dihydrodiol formed, whereas a mutation at Glu-214, Asp-219, His-222, or His-228 eliminated cis-toluene dihydrodiol formation. The expression level of all of the mutated TWO proteins was equivalent to that of wild-type TodC1 as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blot (immunoblot) analyses. These results, in conjunction with the predicted amino acid sequences of 22 oxygenase components, suggest that the conserved motif Glu-X3-4,-Asp-X2-His-X4-5-His is critical for catalytic function and the glutamate, aspartate, and histidine residues may act as mononuclear iron ligands at the site of oxygen activation.
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.