Structure and catalytic mechanism of 2-C-methyl-D-erythritol 2,4-cyclodiphosphate (MECDP) synthase, an enzyme in the non-mevalonate pathway of isoprenoid synthesis

Crystal structure of IspF from Bacillus subtilis and absence of protein complex assembly amongst IspD/IspE/IspF enzymes in the MEP pathway

2-C-Methyl-d-erythritol 2,4-cyclodiphosphate synthase (IspF) is a key enzyme in the 2-C-Methyl-d-erythritol-4-phosphate (MEP) pathway of isoprenoid biosynthesis. This enzyme catalyzes the 4-diphosphocytidyl-2-C-methyl-d-erythritol 2-phosphate (CDPME2P) to 2-C-methyl-d-erythritol 2,4-cyclodiphosphate (MEcDP) with concomitant release of cytidine 5'-diphospate (CMP). Bacillus subtilis is a potential host cell for the production of isoprenoids, but few studies are performed on the key enzymes of MEP pathway in B. subtilis In this work, the high-resolution crystal structures of IspF in native and complex with CMP from B. subtilis have been determined. Structural comparisons indicate that there is a looser packing of the subunits of IspF in B. subtilis, whereas the solvent accessible surface of its active pockets is smaller than that in Escherichia coli. Meanwhile, the protein-protein associations of 2-C-Methyl-d-erythritol-4-phosphatecytidyltransferase (IspD), CDPME kinase (IspE) and IspF from B. subtilis and E. coli, which catalyze three consecutive steps in the MEP pathway, are analyzed by native gel shift and size exclusion chromatography methods. The data here show that protein complex assembly is not detectable. These results will be useful for isoprenoid biosynthesis by metabolic engineering.

Identification of functional candidates amongst hypothetical proteins of Treponema pallidum ssp. pallidum

Syphilis is a globally occurring venereal disease, and its infection is propagated through sexual contact. The causative agent of syphilis, Treponema pallidum ssp. pallidum, a Gram-negative sphirochaete, is an obligate human parasite. Genome of T. pallidum ssp. pallidum SS14 strain (RefSeq NC_010741.1) encodes 1,027 proteins, of which 444 proteins are known as hypothetical proteins (HPs), i.e., proteins of unknown functions. Here, we performed functional annotation of HPs of T. pallidum ssp. pallidum using various database, domain architecture predictors, protein function annotators and clustering tools. We have analyzed the sequences of 444 HPs of T. pallidum ssp. pallidum and subsequently predicted the function of 207 HPs with a high level of confidence. However, functions of 237 HPs are predicted with less accuracy. We found various enzymes, transporters, binding proteins in the annotated group of HPs that may be possible molecular targets, facilitating for the survival of pathogen. Our comprehensive analysis helps to understand the mechanism of pathogenesis to provide many novel potential therapeutic interventions.

Crystal structures of IspF from Plasmodium falciparum and Burkholderia cenocepacia: comparisons inform antimicrobial drug target assessment

Background

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.

Results

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.

Conclusions

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.

Isoprenoid precursor biosynthesis offers potential targets for drug discovery against diseases caused by apicomplexan parasites

Two, simple, C5 compounds, dimethylally diphosphate and isopentenyl diphosphate, are the universal precursors of isoprenoids, a large family of natural products involved in numerous important biological processes. Two distinct biosynthetic pathways have evolved to supply these precursors. Humans use the mevalonate route whilst many species of bacteria including important pathogens, plant chloroplasts and apicomplexan parasites exploit the non-mevalonate pathway. The absence from humans, combined with genetic and chemical validation suggests that the non-mevalonate pathway holds the potential to support new drug discovery programmes targeting Gram-negative bacteria and the apicomplexan parasites responsible for causing serious human diseases, and also infections of veterinary importance. The non-mevalonate pathway relies on eight enzyme-catalyzed stages exploiting a range of cofactors and metal ions. A wealth of structural and mechanistic data, mainly derived from studies of bacterial enzymes, now exists for most components of the pathway and these will be described. Particular attention will be paid to how these data inform on the apicomplexan orthologues concentrating on the enzymes from Plasmodium spp. these cause malaria, one the most important parasitic diseases in the world today.

Biochemistry of the non-mevalonate isoprenoid pathway

The non-mevalonate pathway of isoprenoid (terpenoid) biosynthesis is essential in many eubacteria including the major human pathogen, Mycobacterium tuberculosis, in apicomplexan protozoa including the Plasmodium spp. causing malaria, and in the plastids of plants. The metabolic route is absent in humans and is therefore qualified as a promising target for new anti-infective drugs and herbicides. Biochemical and structural knowledge about all enzymes involved in the pathway established the basis for discovery and development of inhibitors by high-throughput screening of compound libraries and/or structure-based rational design.

Convergent strategies in biosynthesis

This review article focuses on how nature sometimes solves the same problem in the biosynthesis of small molecules but using very different approaches. Four examples, involving isopentenyl diphosphate, menaquinone, lysine, and aromatic polyketides, are highlighted that represent different strategies in convergent metabolism.

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

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

A structure-based approach to ligand discovery for 2C-methyl-D-erythritol-2,4-cyclodiphosphate synthase: a target for antimicrobial therapy

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 K_D 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.

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.

The structure of Mycobacteria 2C-methyl-D-erythritol-2,4-cyclodiphosphate synthase, an essential enzyme, provides a platform for drug discovery

Background

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.

Results

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.

Conclusion

Our study genetically validates MtIspF as a therapeutic target and provides a model system for structure-based ligand design.

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

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

The non-mevalonate pathway of isoprenoid precursor biosynthesis

The recently discovered non-mevalonate biosynthetic route to isoprenoid precursors is an essential metabolic pathway in plants, apicomplexan parasites, and many species of bacteria. The pathway relies on eight enzymes exploiting different cofactors and metal ions. Structural and mechanistic data now exist for most components of the pathway though there remain some gaps in our knowledge. The individual enzymes represent new, validated targets for broad spectrum antimicrobial drug and herbicide development. Detailed knowledge of the pathway may also be exploited to genetically modify microorganisms and plants to produce compounds of agricultural and medical interest.

Cloning and characterization of 2-C-methyl-D: -erythritol 2,4-cyclodiphosphate synthase (MECS) gene from Ginkgo biloba

Ginkgo biloba contains secondary metabolites with interesting pharmacological properties, including highly modified diterpenoid ginkgolide, potent and selective antagonist of platelet-activating factor. 2-C-Methyl-D: -erythritol 2,4-cyclodiphosphate synthase gene (GbMECS) involved in ginkgolide biosynthesis pathway was cloned and characterized from G. biloba embryonic roots, and the full open reading frame was deduced as protein consisting of 238 amino acid residues. Putative mature protein with a 179 residue-long sequence, obtained by deleting N-terminal chloroplast transit peptide region composed of 59 amino acid residues, rescued Esherichia coli NMW26, an E. coli knock-out mutant of ygbB (EcMECS). Transcription levels of GbMECS were two-fold higher in embryo roots compared to leaves. When full-length GbMECS with chloroplast transit peptide sequence was fused to green fluorescent protein gene (GFP), and transiently expressed in Arabidopsis thaliana protoplast, green fluorescence was found in chloroplast, indication of protein transportation into plastid.

A double mutation of Escherichia coli2C-methyl-D-erythritol-2,4-cyclodiphosphate synthase disrupts six hydrogen bonds with, yet fails to prevent binding of, an isoprenoid diphosphate

The essential enzyme 2C-methyl-D-erythritol-2,4-cyclodiphosphate (MECP) synthase, found in most eubacteria and the apicomplexan parasites, participates in isoprenoid-precursor biosynthesis and is a validated target for the development of broad-spectrum antimicrobial drugs. The structure and mechanism of the enzyme have been elucidated and the recent exciting finding that the enzyme actually binds diphosphate-containing isoprenoids at the interface formed by the three subunits that constitute the active protein suggests the possibility of feedback regulation of MECP synthase. To investigate such a possibility, a form of the enzyme was sought that did not bind these ligands but which would retain the quaternary structure necessary to create the active site. Two amino acids, Arg142 and Glu144, in Escherichia coli MECP synthase were identified as contributing to ligand binding. Glu144 interacts directly with Arg142 and positions the basic residue to form two hydrogen bonds with the terminal phosphate group of the isoprenoid diphosphate ligand. This association occurs at the trimer interface and three of these arginines interact with the ligand phosphate group. A dual mutation was designed (Arg142 to methionine and Glu144 to leucine) to disrupt the electrostatic attractions between the enzyme and the phosphate group to investigate whether an enzyme without isoprenoid diphosphate could be obtained. A low-resolution crystal structure of the mutated MECP synthase Met142/Leu144 revealed that geranyl diphosphate was retained despite the removal of six hydrogen bonds normally formed with the enzyme. This indicates that these two hydrophilic residues on the surface of the enzyme are not major determinants of isoprenoid binding at the trimer interface but rather that hydrophobic interactions between the hydrocarbon tail and the core of the enzyme trimer dominate ligand binding.

The plastid-derived organelle ofprotozoan human parasites asa target of established and emerging drugs

Human diseases like malaria, toxoplasmosis or cryptosporidiosis are caused by intracellular protozoan parasites of the phylum Apicomplexa and are still a major health problem worldwide. In the case of Plasmodium falciparum, the causative agent of tropical malaria, resistance against previously highly effective drugs is widespread and requires the continued development of new and affordable drugs. Most apicomplexan parasites possess a single plastid-derived organelle called apicoplast, which offers the great opportunity to tailor highly specific inhibitors against vital metabolic pathways resident in this compartment. This is due to the fact that several of these pathways, being of bacterial or algal origin, are absent in the mammalian host. In fact, the targets of several antibiotics already in use for years against some of these diseases can now be traced to the apicoplast and by knowing the molecular entities which are affected by these substances, improved drugs or drug combinations can be envisaged to emerge from this knowledge. Likewise, apicoplast-resident pathways like fatty acid or isoprenoid biosynthesis have already been proven to be the likely targets of the next drug generation. In this review the current knowledge on the different targets and available inhibitors (both established and experimental) will be summarised and an overview of the clinical efficacy of drugs that inhibit functions in the apicoplast and which have been tested in humans so far will be given.

Structural classification of thioredoxin-like fold proteins

Protein structure classification is necessary to comprehend the rapidly growing structural data for better understanding of protein evolution and sequence-structure-function relationships. Thioredoxins are important proteins that ubiquitously regulate cellular redox status and various other crucial functions. We define the thioredoxin-like fold using the structure consensus of thioredoxin homologs and consider all circular permutations of the fold. The search for thioredoxin-like fold proteins in the PDB database identified 723 protein domains. These domains are grouped into eleven evolutionary families based on combined sequence, structural, and functional evidence. Analysis of the protein-ligand structure complexes reveals two major active site locations for the thioredoxin-like proteins. Comparison to existing structure classifications reveals that our thioredoxin-like fold group is broader and more inclusive, unifying proteins from five SCOP folds, five CATH topologies and seven DALI domain dictionary globular folding topologies. Considering these structurally similar domains together sheds new light on the relationships between sequence, structure, function and evolution of thioredoxins.

Hexameric assembly of the bifunctional methylerythritol 2,4-cyclodiphosphate synthase and protein-protein associations in the deoxy-xylulose-dependent pathway of isoprenoid precursor biosynthesis

The bifunctional methylerythritol 4-phosphate cytidylyltransferase methylerythritol 2,4-cyclodiphosphate synthase (IspDF) is unusual in that it catalyzes nonconsecutive reactions in the 1-deoxy-D-xylulose 5-phosphate (DOXP) pathway of isoprenoid precursor biosynthesis. The crystal structure of IspDF from the bacterial pathogen Campylobacter jejuni reveals an elongated hexamer with D3 symmetry compatible with the dimeric 2C-methyl-D-erythritol-4-phosphate cytidylyltransferase and trimeric 2C-methyl-D-erythritol-2,4-cyclodiphosphate synthase monofunctional enzymes. Complex formation of IspDF with 4-diphosphocytidyl-2C-methyl-D-erythritol kinase (IspE), the intervening enzyme activity in the pathway, has been observed in solution for the enzymes from C. jejuni and Agrobacterium tumefaciens. The monofunctional enzymes (2C-methyl-D-erythritol-4-phosphate cytidylyltransferase, IspE, and 2C-methyl-D-erythritol-2,4-cyclodiphosphate synthase) involved in the DOXP biosynthetic pathway of Escherichia coli also show physical associations. We propose that complex formation of the three enzymes at the core of the DOXP pathway can produce an assembly localizing 18 catalytic centers for the early stages of isoprenoid biosynthesis.

Crystal structure of 1-deoxy-d-xylulose-5-phosphate reductoisomerase from Zymomonas mobilis at 1.9-A resolution

1-Deoxy-d-xylulose-5-phosphate reductoisomerase (DXR) is the second enzyme in the non-mevalonate pathway of isoprenoid biosynthesis. The structure of the apo-form of this enzyme from Zymomonas mobilis has been solved and refined to 1.9-A resolution, and that of a binary complex with the co-substrate NADPH to 2.7-A resolution. The subunit of DXR consists of three domains. Residues 1-150 form the NADPH binding domain, which is a variant of the typical dinucleotide-binding fold. The second domain comprises a four-stranded mixed beta-sheet, with three helices flanking the sheet. Most of the putative active site residues are located on this domain. The C-terminal domain (residues 300-386) folds into a four-helix bundle. In solution and in the crystal, the enzyme forms a homo-dimer. The interface between the two monomers is formed predominantly by extension of the sheet in the second domain. The adenosine phosphate moiety of NADPH binds to the nucleotide-binding fold in the canonical way. The adenine ring interacts with the loop after beta1 and with the loops between alpha2 and beta2 and alpha5 and beta5. The nicotinamide ring is disordered in crystals of this binary complex. Comparisons to Escherichia coli DXR show that the two enzymes are very similar in structure, and that the active site architecture is highly conserved. However, there are differences in the recognition of the adenine ring of NADPH in the two enzymes.

Biosynthesis of isoprenoids: crystal structure of 4-diphosphocytidyl-2C-methyl-D-erythritol kinase

4-Diphosphocytidyl-2C-methyl-d-erythritol kinase, an essential enzyme in the nonmevalonate pathway of isopentenyl diphosphate and dimethylallyl diphosphate biosynthesis, catalyzes the single ATP-dependent phosphorylation stage affording 4-diphosphocytidyl-2C-methyl-d-erythritol-2-phosphate. The 2-A resolution crystal structure of the Escherichia coli enzyme in a ternary complex with substrate and a nonhydrolyzable ATP analogue reveals the molecular determinants of specificity and catalysis. The enzyme subunit displays the alpha/beta fold characteristic of the galactose kinase/homoserine kinase/mevalonate kinase/phosphomevalonate kinase superfamily, arranged into cofactor and substrate-binding domains with the catalytic center positioned in a deep cleft between domains. Comparisons with related members of this superfamily indicate that the core regions of each domain are conserved, whereas there are significant differences in the substrate-binding pockets. The nonmevalonate pathway is essential in many microbial pathogens and distinct from the mevalonate pathway used by mammals. The high degree of sequence conservation of the enzyme across bacterial species suggests similarities in structure, specificity, and mechanism. Our model therefore provides an accurate template to facilitate the structure-based design of broad-spectrum antimicrobial agents.