Crystal structure of Mycobacterium tuberculosis MenB,a key enzyme in vitamin K2 biosynthesis

Bacterial enzymes of the menaquinone (Vitamin K2) pathway are potential drug targets because they lack human homologs. MenB, 1,4-dihydroxy-2-naphthoyl-CoA synthase, the fourth enzyme in the biosynthetic pathway leading from chorismate to menaquinone, catalyzes the conversion of O-succinylbenzoyl-CoA (OSB-CoA) to 1,4-dihydroxy-2-naphthoyl-CoA (DHNA-CoA). Based on our interest in developing novel tuberculosis chemotherapeutics, we have solved the structures of MenB from Mycobacterium tuberculosis and its complex with acetoacetyl-coenzyme A at 1.8 and 2.3 A resolution, respectively. Like other members of the crotonase superfamily, MenB folds as an (alpha3)2 hexamer, but its fold is distinct in that the C terminus crosses the trimer-trimer interface, forming a flexible part of the active site within the opposing trimer. The highly conserved active site of MenB contains a deep pocket lined by Asp-192, Tyr-287, and hydrophobic residues. Mutagenesis shows that Asp-192 and Tyr-287 are essential for enzymatic catalysis. We postulate a catalytic mechanism in which MenB enables proton transfer within the substrate to yield an oxyanion as the initial step in catalysis. Knowledge of the active site geometry and characterization of the catalytic mechanism of MenB will aid in identifying new inhibitors for this potential drug target.     

One-step purification of 5-enolpyruvylshikimate-3-phosphate synthase enzyme from Mycobacterium tuberculosis

Currently, there are 8 million new cases and 2 million deaths annually from tuberculosis, and it is expected that a total of 225 million new cases and 79 million deaths will occur between 1998 and 2030. The reemergence of tuberculosis as a public health threat, the high susceptibility of HIV-infected persons, and the proliferation of multi-drug-resistant strains have created a need to develop new antimycobacterial agents. The existence of homologues to the shikimate pathway enzymes has been predicted by the determination of the genome sequence of Mycobacterium tuberculosis. We have previously reported the cloning and overexpression of M. tuberculosis aroA-encoded EPSP synthase in both soluble and active forms, without IPTG induction. Here, we describe the purification of M. tuberculosis EPSP synthase (mtEPSPS) expressed in Escherichia coli BL21(DE3) host cells. Purification of mtEPSPS was achieved by a one-step purification protocol using an anion exchange column. The activity of the homogeneous enzyme was measured by a coupled assay using purified shikimate kinase and purine nucleoside phosphorylase proteins. A total of 53 mg of homogeneous enzyme could be obtained from 1L of LB cell culture, with a specific activity value of approximately 18 Umg(-1). The results presented here provide protein in quantities necessary for structural and kinetic studies, which are currently underway in our laboratory.     

Crystal structure of 1-deoxy-D-xylulose 5-phosphate synthase, a crucial enzyme for isoprenoids biosynthesis

Isopentenyl pyrophosphate (IPP) is a common precursor for the synthesis of all isoprenoids, which have important functions in living organisms. IPP is produced by the mevalonate pathway in archaea, fungi, and animals. In contrast, IPP is synthesized by a mevalonate-independent pathway in most bacteria, algae, and plant plastids. 1-Deoxy-D-xylulose 5-phosphate synthase (DXS) catalyzes the first and the rate-limiting step of the mevalonate-independent pathway and is an attractive target for the development of novel antibiotics, antimalarials, and herbicides. We report here the first structural information on DXS, from Escherichia coli and Deinococcus radiodurans, in complex with the coenzyme thiamine pyrophosphate (TPP). The structure contains three domains (I, II, and III), each of which bears homology to the equivalent domains in transketolase and the E1 subunit of pyruvate dehydrogenase. However, DXS has a novel arrangement of these domains as compared with the other enzymes, such that the active site of DXS is located at the interface of domains I and II in the same monomer, whereas that of transketolase is located at the interface of the dimer. The coenzyme TPP is mostly buried in the complex, but the C-2 atom of its thiazolium ring is exposed to a pocket that is the substrate-binding site. The structures identify residues that may have important roles in catalysis, which have been confirmed by our mutagenesis studies.     

Crystal structure and kinetic study of dihydrodipicolinate synthase from Mycobacterium tuberculosis

The three-dimensional structure of the enzyme dihydrodipicolinate synthase (KEGG entry Rv2753c, EC 4.2.1.52) from Mycobacterium tuberculosis (Mtb-DHDPS) was determined and refined at 2.28 A (1 A=0.1 nm) resolution. The asymmetric unit of the crystal contains two tetramers, each of which we propose to be the functional enzyme unit. This is supported by analytical ultracentrifugation studies, which show the enzyme to be tetrameric in solution. The structure of each subunit consists of an N-terminal (beta/alpha)(8)-barrel followed by a C-terminal alpha-helical domain. The active site comprises residues from two adjacent subunits, across an interface, and is located at the C-terminal side of the (beta/alpha)(8)-barrel domain. A comparison with the other known DHDPS structures shows that the overall architecture of the active site is largely conserved, albeit the proton relay motif comprising Tyr(143), Thr(54) and Tyr(117) appears to be disrupted. The kinetic parameters of the enzyme are reported: K(M)(ASA)=0.43+/-0.02 mM, K(M)(pyruvate)=0.17+/-0.01 mM and V(max)=4.42+/-0.08 micromol x s(-1) x mg(-1). Interestingly, the V(max) of Mtb-DHDPS is 6-fold higher than the corresponding value for Escherichia coli DHDPS, and the enzyme is insensitive to feedback inhibition by (S)-lysine. This can be explained by the three-dimensional structure, which shows that the (S)-lysine-binding site is not conserved in Mtb-DHDPS, when compared with DHDPS enzymes that are known to be inhibited by (S)-lysine. A selection of metabolites from the aspartate family of amino acids do not inhibit this enzyme. A comprehensive understanding of the structure and function of this important enzyme from the (S)-lysine biosynthesis pathway may provide the key for the design of new antibiotics to combat tuberculosis.     

Crystal structure of chalcone synthase,a key enzyme for isoflavonoid biosynthesis in soybean

Isoflavonoid is one of the groups of flavonoids that play pivotal roles in the survival of land plants. Chalcone synthase (CHS), the first enzyme of the isoflavonoid biosynthetic pathway, catalyzes the formation of a common isoflavonoid precursor. We have previously reported that an isozyme of soybean CHS (termed GmCHS1) is a key component of the isoflavonoid metabolon, a protein complex to enhance efficiency of isoflavonoid production. Here, we determined the crystal structure of GmCHS1 as a first step of understanding the metabolon structure, as well as to better understand the catalytic mechanism of GmCHS1.     

Crystal structure of Archaeoglobus fulgidus CTP:inositol-1-phosphate cytidylyltransferase, a key enzyme for di-myo-inositol-phosphate synthesis in (hyper)thermophiles

Many Archaea and Bacteria isolated from hot, marine environments accumulate di-myo-inositol-phosphate (DIP), primarily in response to heat stress. The biosynthesis of this compatible solute involves the activation of inositol to CDP-inositol via the action of a recently discovered CTP:inositol-1-phosphate cytidylyltransferase (IPCT) activity. In most cases, IPCT is part of a bifunctional enzyme comprising two domains: a cytoplasmic domain with IPCT activity and a membrane domain catalyzing the synthesis of di-myo-inositol-1,3′-phosphate-1′-phosphate from CDP-inositol and l-myo-inositol phosphate. Herein, we describe the first X-ray structure of the IPCT domain of the bifunctional enzyme from the hyperthermophilic archaeon Archaeoglobus fulgidus DSMZ 7324. The structure of the enzyme in the apo form was solved to a 1.9-Å resolution. The enzyme exhibited apparent K_m values of 0.9 and 0.6 mM for inositol-1-phosphate and CTP, respectively. The optimal temperature for catalysis was in the range 90 to 95°C, and the V_max determined at 90°C was 62.9 μmol · min⁻¹ · mg of protein⁻¹. The structure of IPCT is composed of a central seven-stranded mixed β-sheet, of which six β-strands are parallel, surrounded by six α-helices, a fold reminiscent of the dinucleotide-binding Rossmann fold. The enzyme shares structural homology with other pyrophosphorylases showing the canonical motif G-X-G-T-(R/S)-X₄-P-K. CTP, l-myo-inositol-1-phosphate, and CDP-inositol were docked into the catalytic site, which provided insights into the binding mode and high specificity of the enzyme for CTP. This work is an important step toward the final goal of understanding the full catalytic route for DIP synthesis in the native, bifunctional enzyme.     

Crystal structure of the pantothenate synthetase from Mycobacterium tuberculosis, snapshots of the enzyme in action

Pantothenate synthetase (PS) from Mycobacterium tuberculosis represents a potential target for antituberculosis drugs. PS catalyzes the ATP-dependent condensation of pantoate and beta-alanine to form pantothenate. Previously, we determined the crystal structure of PS from M. tuberculosis and its complexes with AMPCPP, pantoate, and pantoyl adenylate. Here, we describe the crystal structure of this enzyme complexed with AMP and its last substrate, beta-alanine, and show that the phosphate group of AMP serves as an anchor for the binding of beta-alanine. This structure confirms that binding of beta-alanine in the active site cavity can occur only after formation of the pantoyl adenylate intermediate. A new crystal form was also obtained; it displays the flexible wall of the active site cavity in a conformation incapable of binding pantoate. Soaking of this crystal form with ATP and pantoate gives a fully occupied complex of PS with ATP. Crystal structures of these complexes with substrates, the reaction intermediate, and the reaction product AMP provide a step-by-step view of the PS-catalyzed reaction. A detailed reaction mechanism and its implications for inhibitor design are discussed.     

Crystal structure of the Mycobacterium tuberculosis beta-ketoacyl-acyl carrier protein synthase III

Mycolic acids (alpha-alkyl-beta-hydroxy long chain fatty acids) cover the surface of mycobacteria, and inhibition of their biosynthesis is an established mechanism of action for several key front-line anti-tuberculosis drugs. In mycobacteria, long chain acyl-CoA products (C(14)-C(26)) generated by a type I fatty-acid synthase can be used directly for the alpha-branch of mycolic acid or can be extended by a type II fatty-acid synthase to make the meromycolic acid (C(50)-C(56)))-derived component. An unusual Mycobacterium tuberculosis beta-ketoacyl-acyl carrier protein (ACP) synthase III (mtFabH) has been identified, purified, and shown to catalyze a Claisen-type condensation between long chain acyl-CoA substrates such as myristoyl-CoA (C(14)) and malonyl-ACP. This enzyme, presumed to play a key role in initiating meromycolic acid biosynthesis, was crystallized, and its structure was determined at 2.1-A resolution. The mtFabH homodimer is closely similar in topology and active-site structure to Escherichia coli FabH (ecFabH), with a CoA/malonyl-ACP-binding channel leading from the enzyme surface to the buried active-site cysteine residue. Unlike ecFabH, mtFabH contains a second hydrophobic channel leading from the active site. In the ecFabH structure, this channel is blocked by a phenylalanine residue, which constrains specificity to acetyl-CoA, whereas in mtFabH, this residue is a threonine, which permits binding of longer acyl chains. This same channel in mtFabH is capped by an alpha-helix formed adjacent to a 4-amino acid sequence insertion, which limits bound acyl chain length to 16 carbons. These observations offer a molecular basis for understanding the unusual substrate specificity of mtFabH and its probable role in regulating the biosynthesis of the two different length acyl chains required for generation of mycolic acids. This mtFabH presents a new target for structure-based design of novel antimycobacterial agents.     

Crystal structure of ATP phosphoribosyltransferase from Mycobacterium tuberculosis

The N-1-(5'-phosphoribosyl)-ATP transferase catalyzes the first step of the histidine biosynthetic pathway and is regulated by a feedback mechanism by the product histidine. The crystal structures of the N-1-(5'-phosphoribosyl)-ATP transferase from Mycobacterium tuberculosis in complex with inhibitor histidine and AMP has been determined to 1.8 A resolution and without ligands to 2.7 A resolution. The active enzyme exists primarily as a dimer, and the histidine-inhibited form is a hexamer. The structure represents a new fold for a phosphoribosyltransferase, consisting of three continuous domains. The inhibitor AMP binds in the active site cavity formed between the two catalytic domains. A model for the mechanism of allosteric inhibition has been derived from conformational differences between the AMP:His-bound and apo structures.     

Crystal structure of uncleaved L-aspartate-alpha-decarboxylase from Mycobacterium tuberculosis

L-aspartate-alpha-decarboxylase (ADC) is a critical regulatory enzyme in the pantothenate biosynthetic pathway and belongs to a small class of self-cleaving and pyruvoyl-dependent amino acid decarboxylases. The expression level of ADC in Mycobacterium tuberculosis (Mtb) was confirmed by cDNA analysis, immunoblotting with an anti-ADC polyclonal antibody using whole cell lysate and immunoelectron microscopy. The recombinant ADC proenzyme from Mycobacterium tuberculosis (MtbADC) was overexpressed in E. coli and the protein structure was determined at 2.99 A resolution. The proteins fold into the double-psi beta-barrel structure. The subunits of the two tetramers (there are eight ADC molecules in the asymmetric unit) form pseudo fourfold rotational symmetry, similar to the E. coli ADC proenzyme structure. As pantothenate is synthesized in microorganisms, plants, and fungi but not in animals, structure elucidation of Mtb ADC is of substantial interest for structure-based drug development.     

Crystal structure of PapA5, a phthiocerol dimycocerosyl transferase from Mycobacterium tuberculosis

Polyketide-associated protein A5 (PapA5) is an acyltransferase that is involved in production of phthiocerol and phthiodiolone dimycocerosate esters, a class of virulence-enhancing lipids produced by Mycobacterium tuberculosis. Structural analysis of PapA5 at 2.75-A resolution reveals a two-domain structure that shares unexpected similarity to structures of chloramphenicol acetyltransferase, dihydrolipoyl transacetylase, carnitine acetyltransferase, and VibH, a non-ribosomal peptide synthesis condensation enzyme. The PapA5 active site includes conserved histidine and aspartic acid residues that are critical to PapA5 acyltransferase activity. PapA5 catalyzes acyl transfer reactions on model substrates that contain long aliphatic carbon chains, and two hydrophobic channels were observed linking the PapA5 surface to the active site with properties consistent with these biochemical activities and substrate preferences. An additional alpha helix not observed in other acyltransferase structures blocks the putative entrance into the PapA5 active site, indicating that conformational changes may be associated with PapA5 activity. PapA5 represents the first structure solved for a protein involved in polyketide synthesis in Mycobacteria.     

Crystal structure of MshB from Mycobacterium tuberculosis, a deacetylase involved in mycothiol biosynthesis

All living species require protection against the damaging effects of the reactive oxygen species that are a natural by-product of aerobic life. In most organisms, glutathione is a critical component of these defences, maintaining a reducing environment inside cells. Some bacteria, however, including pathogenic mycobacteria, use an alternative low molecular mass thiol compound called mycothiol (MSH) for this purpose. Enzymes that synthesize MSH are attractive candidates for the design of novel anti-TB drugs because of the importance of MSH for mycobacterial life and the absence of such enzymes in humans. We have determined the three-dimensional structure of MshB (Rv1170), a metal-dependent deacetylase from Mycobacterium tuberculosis that catalyses the second step in MSH biosynthesis. The structure, determined at 1.9A resolution by X-ray crystallography (R=19.0%, R(free)=21.4%), reveals an alpha/beta fold in which helices pack against a seven-stranded mostly parallel beta-sheet. Large loops emanating from the C termini of the beta-strands enclose a deep cavity, which is the location of the putative active site. At the bottom of this cavity is a metal-binding site associated with a sequence motif AHPDDE that is invariant in all homologues. An adventitiously bound beta-octylglucoside molecule, used in crystallization, enables us to model the binding of the true substrate and propose a metal-dependent mechanistic model for deacetylation. Sequence comparisons indicate that MshB is representative of a wider family of enzymes that act on substituted N-acetylglucosamine residues, including a deacetylase involved in the biosynthesis of glycosylphosphatidylinositol (GPI) anchors in eukaryotes.     

Crystal structure of human squalene synthase. A key enzyme in cholesterol biosynthesis

Squalene synthase catalyzes the biosynthesis of squalene, a key cholesterol precursor, through a reductive dimerization of two farnesyl diphosphate (FPP) molecules. The reaction is unique when compared with those of other FPP-utilizing enzymes and proceeds in two distinct steps, both of which involve the formation of carbocationic reaction intermediates. Because FPP is located at the final branch point in the isoprenoid biosynthesis pathway, its conversion to squalene through the action of squalene synthase represents the first committed step in the formation of cholesterol, making it an attractive target for therapeutic intervention. We have determined, for the first time, the crystal structures of recombinant human squalene synthase complexed with several different inhibitors. The structure shows that SQS is folded as a single domain, with a large channel in the middle of one face. The active sites of the two half-reactions catalyzed by the enzyme are located in the central channel, which is lined on both sides by conserved aspartate and arginine residues, which are known from mutagenesis experiments to be involved in FPP binding. One end of this channel is exposed to solvent, whereas the other end leads to a completely enclosed pocket surrounded by conserved hydrophobic residues. These observations, along with mutagenesis data identifying residues that affect substrate binding and activity, suggest that two molecules of FPP bind at one end of the channel, where the active center of the first half-reaction is located, and then the stable reaction intermediate moves into the deep pocket, where it is sequestered from solvent and the second half-reaction occurs. Five alpha helices surrounding the active center are structurally homologous to the active core in the three other isoprenoid biosynthetic enzymes whose crystal structures are known, even though there is no detectable sequence homology.     

Crystal Structure of AhpE from Mycobacterium tuberculosis, a 1-Cys peroxiredoxin

All living systems require protection against the damaging effects of reactive oxygen species. The genome of Mycobacterium tuberculosis, the cause of TB, encodes a number of peroxidases that are thought to be active against organic and inorganic peroxides, and are likely to play a key role in the ability of this organism to survive within the phagosomes of macrophages. The open reading frame Rv2238c in M.tuberculosis encodes a 153-residue protein AhpE, which is a peroxidase of the 1-Cys peroxiredoxin (Prx) family. The crystal structure of AhpE, determined at 1.87 A resolution (R(cryst)=0.179, R(free)=0.210), reveals a compact single-domain protein with a thioredoxin fold. AhpE forms both dimers and octamers; a tightly-associated dimer and a ring-like octamer, generated by crystallographic 4-fold symmetry. In this native structure, the active site Cys45 is in its oxidized, sulfenic acid (S-O-H) state. A second crystal form of AhpE, obtained after soaking in sodium bromide and refined at 1.90 A resolution (R(cryst)=0.242, R(free)=0.286), reveals the reduced structure. In this structure, a conformational change in an external loop, in two of the four molecules in the asymmetric unit, allows Arg116 to stabilise the Cys45 thiolate ion, and concomitantly closes a surface channel. This channel is identified as the likely binding site for a physiological reductant, and the conformational change is inferred to be important for the reaction cycle of AhpE.     

Mycobacterium tuberculosis reprograms waves of phosphatidylinositol 3-phosphate on phagosomal organelles

The potent human pathogen Mycobacterium tuberculosis persists in macrophages within a specialized, immature phagosome by interfering with the pathway of phagolysosome biogenesis. The molecular mechanisms underlying this process remain to be fully elucidated. Here, using four-dimensional microscopy, we detected on model phagosomes, which normally mature into phagolysosomes, the existence of cyclical waves of phosphatidylinositol 3-phosphate (PI3P), a membrane trafficking regulatory lipid essential for phagosomal acquisition of lysosomal characteristics. We show that mycobacteria interfere with the dynamics of PI3P on phagosomal organelles by altering the timing and characteristics of the PI3P waves on phagosomes. The default program of cyclical PI3P waves on model phagosomes is composed of an initial stage (phase I), represented by a strong PI3P burst occurring only upon the completion of phagosome formation, and a subsequent stage (phase II) of recurring PI3P waves on maturing phagosomes with the average periodicity of 20 min. Mycobacteria alter this program in two ways: (i) by inducing, in a cholesterol-dependent fashion, a neophase I* of premature PI3P production, coinciding with the process of mycobacterial entry into the macrophage, and (ii) by inhibiting the calmodulin-dependent phase II responsible for the acquisition of lysosomal characteristics. We conclude that the default pathway of phagosomal maturation into the phagolysosome includes temporally organized cyclical waves of PI3P on phagosomal membranes and that this process is targeted for reprogramming by mycobacteria as they prevent phagolysosome formation.     

Crystal structure of Mycobacterium tuberculosis diaminopimelate decarboxylase,an essential enzyme in bacterial lysine biosynthesis

The Mycobacterium tuberculosis lysA gene encodes the enzyme meso-diaminopimelate decarboxylase (DAPDC), a pyridoxal-5'-phosphate (PLP)-dependent enzyme. The enzyme catalyzes the final step in the lysine biosynthetic pathway converting meso-diaminopimelic acid (DAP) to l-lysine. The lysA gene of M. tuberculosis H37Rv has been established as essential for bacterial survival in immunocompromised mice, demonstrating that de novo biosynthesis of lysine is essential for in vivo viability. Drugs targeted against DAPDC could be efficient anti-tuberculosis drugs, and the three-dimensional structure of DAPDC from M. tuberculosis complexed with reaction product lysine and the ternary complex with PLP and lysine in the active site has been determined. The first structure of a DAPDC confirms its classification as a fold type III PLP-dependent enzyme. The structure shows a stable 2-fold dimer in head-to-tail arrangement of a triose-phosphate isomerase (TIM) barrel-like alpha/beta domain and a C-terminal beta sheet domain, similar to the ornithine decarboxylase (ODC) fold family. PLP is covalently bound via an internal aldimine, and residues from both domains and both subunits contribute to the binding pocket. Comparison of the structure with eukaryotic ODCs, in particular with a di-fluoromethyl ornithine (DMFO)-bound ODC from Trypanosoma bruceii, indicates that corresponding DAP-analogues might be potential inhibitors for mycobacterial DAPDCs.     

Crystal structure of Mycobacterium tuberculosis catalase-peroxidase

The Mycobacterium tuberculosis catalase-peroxidase is a multifunctional heme-dependent enzyme that activates the core anti-tuberculosis drug isoniazid. Numerous studies have been undertaken to elucidate the enzyme-dependent mechanism of isoniazid activation, and it is well documented that mutations that reduce activity or inactivate the catalase-peroxidase lead to increased levels of isoniazid resistance in M. tuberculosis. Interpretation of the catalytic activities and the effects of mutations upon the action of the enzyme to date have been limited due to the lack of a three-dimensional structure for this enzyme. In order to provide a more accurate model of the three-dimensional structure of the M. tuberculosis catalase-peroxidase, we have crystallized the enzyme and now report its crystal structure refined to 2.4-A resolution. The structure reveals new information about dimer assembly and provides information about the location of residues that may play a role in catalysis including candidates for protein-based radical formation. Modeling and computational studies suggest that the binding site for isoniazid is located near the delta-meso heme edge rather than in a surface loop structure as currently proposed. The availability of a crystal structure for the M. tuberculosis catalase-peroxidase also permits structural and functional effects of mutations implicated in causing elevated levels of isoniazid resistance in clinical isolates to be interpreted with improved confidence.