Dihydrodipicolinate synthase is not inhibited by its substrate, (S)-aspartate beta-semialdehyde

Enhancing allosteric inhibition of dihydrodipicolinate synthase through the design and synthesis of novel dimeric compounds

The synthesis of the first dimeric inhibitor of E. coli dihydrodipicolinate synthase (DHDPS) is reported herein. Inspired by 2,4-thiazolidinedione based ligands previously shown to inhibit DHDPS, a series of dimeric inhibitors were designed and synthesised, incorporating various alkyl chain bridges between two 2,4-thiazolidinedione moieties. Aiming to exploit the multimeric nature of this enzyme and enhance potency, a dimeric compound with a single methylene bridge achieved the desired outcome with low micromolar inhibition of E. coli DHDPS observed. This work highlights the continued importance of investigation into DHDPS as an antibacterial target. Furthermore, we demonstrate the design of dimeric ligands can provide a promising strategy to improve potency in the search for novel bioactive compounds.

Colorimetric ortho-aminobenzaldehyde assay developed for the high-throughput chemical screening of inhibitors against dihydrodipicolinate synthase from pathogenic bacteria

In search of a new class of antibacterial agents, compounds that target the essential bacterial enzyme, dihydrodipicolinate synthase (DHDPS), are of interest to drug discovery efforts. DHDPS catalyzes the first committed step in the diaminopimelate (DAP) pathway to the biosynthesis of lysine in bacteria and plants. The ortho-aminobenzaldehyde (o-ABA) assay is typically used as a qualitative tool for identifying fractions containing DHDPS during purification. This report is about the development of a high-throughput o-ABA assay format for the quantification of DHDPS enzyme activity using multi-well plates. The colorimetric assay is suitable for determining enzymatic parameters (K _M and V_max) and identifying inhibitors of DHDPS in a high-throughput screen.

The Cysteine Desulfurase IscS Is a Significant Target of 2-Aminoacrylate Damage in Pseudomonas aeruginosa

Pseudomonas aeruginosa encodes eight members of the Rid protein superfamily. PA5339, a member of the RidA subfamily, is required for full growth and motility of P. aeruginosa. Our understanding of RidA integration into the metabolic network of P. aeruginosa is at an early stage, with analyses largely guided by the well-established RidA paradigm in Salmonella enterica. A P. aeruginosa strain lacking RidA has a growth and motility defect in a minimal glucose medium, both of which are exacerbated by exogenous serine. All described ridA mutant phenotypes are rescued by supplementation with isoleucine, indicating the primary generator of the reactive metabolite 2-aminoacrylate (2AA) in ridA mutants is a threonine/serine dehydratase. However, the critical (i.e., phenotype determining) targets of 2AA leading to growth and motility defects in P. aeruginosa remained undefined. This study was initiated to probe the effects of 2AA stress on the metabolic network of P. aeruginosa by defining the target(s) of 2AA that contribute to physiological defects of a ridA mutant. Suppressor mutations that restored growth to a P. aeruginosa ridA mutant were isolated, including an allele of iscS (encoding cysteine desulfurase). Damage to IscS was identified as a significant cause of growth defects of P. aeruginosa during enamine stress. A suppressing allele encoded an IscS variant that was less sensitive to damage by 2AA, resulting in a novel mechanism of phenotypic suppression of a ridA mutant. IMPORTANCE 2-aminoacrylate (2AA) is a reactive metabolite formed as an intermediate in various enzymatic reactions. In the absence of RidA, this metabolite can persist in vivo where it attacks and inactivates specific PLP-dependent enzymes, causing metabolic defects and organism-specific phenotypes. This work identifies the cysteine desulfurase IscS as the critical target of 2AA in Pseudomonas aeruginosa. A single substitution in IscS decreased sensitivity to 2AA and suppressed growth phenotypes of a ridA mutant. Here, we provide the first report of suppression of a ridA mutant phenotype by altering the sensitivity of a target enzyme to 2AA.

Synthesis and structure-activity relationship studies of 2,4-thiazolidinediones and analogous heterocycles as inhibitors of dihydrodipicolinate synthase

Dihydrodipicolinate synthase (DHDPS), responsible for the first committed step of the diaminopimelate pathway for lysine biosynthesis, has become an attractive target for the development of new antibacterial and herbicidal agents. Herein, we report the discovery and exploration of the first inhibitors of E. coli DHDPS which have been identified from screening lead and are not based on substrates from the lysine biosynthesis pathway. Over 50 thiazolidinediones and related analogues have been prepared in order to thoroughly evaluate the structure-activity relationships against this enzyme of significant interest.

Differential lysine-mediated allosteric regulation of plant dihydrodipicolinate synthase isoforms

Lysine biosynthesis in plants occurs via the diaminopimelate pathway. The first committed and rate-limiting step of this pathway is catalysed by dihydrodipicolinate synthase (DHDPS), which is allosterically regulated by the end product, l-lysine (lysine). Given that lysine is a common nutritionally limiting amino acid in cereal crops, there has been much interest in probing the regulation of DHDPS. Interestingly, knockouts in Arabidopsis thaliana of each isoform (AtDHDPS1 and AtDHDPS2) result in different phenotypes, despite the enzymes sharing > 85% protein sequence identity. Accordingly, in this study, we compared the catalytic activity, lysine-mediated inhibition and structures of both A. thaliana DHDPS isoforms. We found that although the recombinantly produced enzymes have similar kinetic properties, AtDHDPS1 is 10-fold more sensitive to lysine. We subsequently used X-ray crystallography to probe for structural differences between the apo- and lysine-bound isoforms that could account for the differential allosteric inhibition. Despite no significant changes in the overall structures of the active or allosteric sites, we noted differences in the rotamer conformation of a key allosteric site residue (Trp116) and proposed that this could result in differences in lysine dissociation. Microscale thermophoresis studies supported our hypothesis, with AtDHDPS1 having a ~ 6-fold tighter lysine dissociation constant compared to AtDHDPS2, which agrees with the lower half minimal inhibitory concentration for lysine observed. Thus, we highlight that subtle differences in protein structures, which could not have been predicted from the primary sequences, can have profound effects on the allostery of a key enzyme involved in lysine biosynthesis in plants. DATABASES: Structures described are available in the Protein Data Bank under the accession numbers 6VVH and 6VVI.

Identification of 2, 3-dihydrodipicolinate as the product of the dihydrodipicolinate synthase reaction from Escherichia coli

Dihydrodipicolinate synthase (DHDPS) catalyzes the first step in the pathway for the biosynthesis of L-lysine in most bacteria and plants. The substrates for the enzyme are pyruvate and L-aspartate-β-semialdehyde (ASA). The product of the reaction was originally proposed to be 2,3-dihydrodipicolinate (DHDP), but has now generally been assumed to be (4S)-4-hydroxy-2,3,4,5-tetrahydro-(2S)-dipicolinate (HTPA). ASA is unstable at high pH and it is proposed that ASA reacts with itself. At high pH ASA also reacts with Tris buffer and both reactions are largely reversible at low pH. It is proposed that the basic un-protonated form of the amine of Tris or the α-amine of ASA reacts with the aldehyde functional group of ASA to generate an imine product. Proton NMR spectra of ASA done at different pH values shows new NMR peaks at high pH, but not at low pH, confirming the presence of reaction products for ASA at high pH. The enzymatic product of the DHDPS reaction was examined at low pH by proton NMR starting with either 3 h-pyruvate or 3 d-pyruvate and identical NMR spectra were obtained with four new NMR peaks observed at 1.5, 2.3, 3.9 and 4.1 ppm in both cases. The NMR results were most consistent with DHDP as the reaction product. The UV-spectral studies of the DHDPS reaction shows the formation of an initial product with a broad spectral peak at 254 nM. The DHDPS reaction product was further examined by reduction of the enzymatic reaction components with borohydride followed by GC-MS analysis of the mixture. Three peaks were found at 88, 119 and 169 m/z, consistent with pyruvate, homoserine (reduction product of ASA), and the reduction product of DHDP (1,2,3,6-tetrahydropyridine-2,6-dicarboxylate). There was no indication for a peak associated with the reduced form of HTPA.

Comparison of untagged and his-tagged dihydrodipicolinate synthase from the enteric pathogen Vibrio cholerae

Given the emergence of multi drug resistant Vibrio cholerae strains, there is an urgent need to characterize new anti-cholera targets. One such target is the enzyme dihydrodipicolinate synthase (DHDPS; EC 4.3.3.7), which catalyzes the first committed step in the diaminopimelate pathway. This pathway is responsible for the production of two key metabolites in bacteria and plants, namely meso-2,6-diaminopimelate and L-lysine. Here, we report the cloning, expression and purification of untagged and His-tagged recombinant DHDPS from V. cholerae (Vc-DHDPS) and provide comparative structural and kinetic analyses. Structural studies employing circular dichroism spectroscopy and analytical ultracentrifugation demonstrate that the recombinant enzymes are folded and exist as dimers in solution. Kinetic analyses of untagged and His-tagged Vc-DHDPS show that the enzymes are functional with specific activities of 75.6 U/mg and 112 U/mg, K_M (pyruvate) of 0.14 mM and 0.15 mM, K_M (L-aspartate-4-semialdehyde) of 0.08 mM and 0.09 mM, and k_cat of 34 and 46 s^-1, respectively. These results demonstrate there are no significant changes in the structure and function of Vc-DHDPS upon the addition of an N-terminal His tag and, hence, the tagged recombinant product is suitable for future studies, including screening for new inhibitors as potential anti-cholera agents. Additionally, a polyclonal antibody raised against untagged Vc-DHDPS is validated for specifically detecting recombinant and native forms of the enzyme.

Dihydrodipicolinate Synthase: Structure, Dynamics, Function, and Evolution

Enzymes are usually comprised of multiple subunits and more often than not they are made up of identical subunits. In this review we examine lysine biosynthesis and focus on the enzyme dihydrodipicolinate synthase in terms of its structure, function and the evolution of its varied number of subunits (quaternary structure). Dihydrodipicolinate synthase is the first committed step in the biosynthesis of lysine, which occurs naturally in plants, bacteria, archaea and fungi, but is not synthesized in mammals. In bacteria, there have been four separate pathways identified from tetrahydrodipicolinate to meso-diaminopimelate, which is the immediate precursor to lysine. Dihydrodipicolinate synthases from many bacterial and plant species have been structurally characterised and the results show considerable variability with respect to their quaternary structure, hinting at their evolution. The oligomeric state of the enzyme plays a key role, both in catalysis and in the allosteric regulation of the enzyme by lysine. While most bacteria and plants have tetrameric enzymes, where the structure of the dimeric building blocks is conserved, the arrangement of the dimers differs. We also review a key development in the field, namely the discovery of a human dihydrodipicolinate synthase-like enzyme, now known as 4-hydroxy-2-oxoglutarate aldolase . This discovery complicates the rationale underpinning drug development against bacterial dihydrodipicolinate synthases, since genetic errors in 4-hydroxy-2-oxoglutarate aldolase cause the disease Primary Hyperoxaluria Type 3 and therefore compounds that are geared towards the inhibition of bacterial dihydrodipicolinate synthase may be toxic to mammalian cells.

Synthesis and folding of a mirror-image enzyme reveals ambidextrous chaperone activity

Mirror-image proteins (composed of D-amino acids) are promising therapeutic agents and drug discovery tools, but as synthesis of larger D-proteins becomes feasible, a major anticipated challenge is the folding of these proteins into their active conformations. In vivo, many large and/or complex proteins require chaperones like GroEL/ES to prevent misfolding and produce functional protein. The ability of chaperones to fold D-proteins is unknown. Here we examine the ability of GroEL/ES to fold a synthetic d-protein. We report the total chemical synthesis of a 312-residue GroEL/ES-dependent protein, DapA, in both L- and D-chiralities, the longest fully synthetic proteins yet reported. Impressively, GroEL/ES folds both L- and D-DapA. This work extends the limits of chemical protein synthesis, reveals ambidextrous GroEL/ES folding activity, and provides a valuable tool to fold d-proteins for drug development and mirror-image synthetic biology applications.

Two Arabidopsis thaliana dihydrodipicolinate synthases, DHDPS1 and DHDPS2, are unequally redundant

In Arabidopsis thalinana (L.) Heynh., DHDPS1 and DHDPS2 encode orthologous dihydrodipicolinate synthases (DHDPS), the first enzyme of the lysine (Lys) biosynthesis pathway. A TDNA insertion mutant of dhdps2 was previously reported to be viable and to accumulate free threonine (Thr). Analysis of additional TDNA insertion lines showed that dhdps1 and dhdps2 mutants are both viable and that whereas dhdps2 mutants accumulate Thr, dhdps1 plants do not. Thr-accumulation was complemented by heterologous expression of Escherichia coli DapA, indicating that the phenotype is due to reduced DHDPS activity in dhdps2. DHDPS1 contributes ~30% towards the total DHDPS activity in leaves of young plants and DHDPS2 contributes 70%; therefore, the threshold of activity resulting in Thr accumulation lies within this narrow range. dhdps1-dhdps2 double mutants could not be isolated, even after exogenous feeding with Lys. Segregation analysis indicated that gametes lacking functional DHDPS genes are defective, as are embryos. Plants carrying only a single DHDPS2 gene do not accumulate Thr, but they show a gametophytic defect that is partially rescued by Lys application. Despite the accumulation of Thr, dhdps2 seedlings are no more sensitive than wild-type plants to growth inhibition by Lys or the Lys precursor diaminopimelate. They also are not rescued by methionine at growth-inhibitory Lys concentrations. Exogenous application of Lys and methionine to dhdps2 mutants did not reduce the accumulation of Thr.

Cloning, expression, purification and crystallization of dihydrodipicolinate synthase from Agrobacterium tumefaciens

Dihydrodipicolinate synthase (DHDPS) catalyzes the first committed step of the lysine-biosynthesis pathway in bacteria, plants and some fungi. This study describes the cloning, expression, purification and crystallization of DHDPS (NP_354047.1) from the plant pathogen Agrobacterium tumefaciens (AgT-DHDPS). Enzyme-kinetics studies demonstrate that AgT-DHDPS possesses DHDPS activity in vitro. Crystals of AgT-DHDPS were grown in the unliganded form and in forms with substrate bound and with substrate plus allosteric inhibitor (lysine) bound. X-ray diffraction data sets were subsequently collected to a maximum resolution of 1.40 Å. Determination of the structure with and without substrate and inhibitor will offer insight into the design of novel pesticide agents.

Cloning, expression, purification and crystallization of dihydrodipicolinate synthase from the grapevine Vitis vinifera

Dihydrodipicolinate synthase (DHDPS) catalyses the first committed step of the lysine-biosynthesis pathway in bacteria, plants and some fungi. This study describes the cloning, expression, purification and crystallization of DHDPS from the grapevine Vitis vinifera (Vv-DHDPS). Following in-drop cleavage of the hexahistidine tag, cocrystals of Vv-DHDPS with the substrate pyruvate were grown in 0.1 M Bis-Tris propane pH 8.2, 0.2 M sodium bromide, 20%(w/v) PEG 3350. X-ray diffraction data in space group P1 at a resolution of 2.2 Å are presented. Preliminary diffraction data analysis indicated the presence of eight molecules per asymmetric unit (V(M) = 2.55 Å(3) Da(-1), 52% solvent content). The pending crystal structure of Vv-DHDPS will provide insight into the molecular evolution in quaternary structure of DHDPS enzymes.

Biochemical studies and crystal structure determination of dihydrodipicolinate synthase from Pseudomonas aeruginosa

The intracellular enzyme dihydrodipicolinate synthase (DHDPS, E.C. 4.2.1.52) from Pseudomonas aeruginosa is a potential drug target because it is essential for the growth of bacteria while it is absent in humans. Therefore, in order to design new compounds using structure based approach for inhibiting the function of DHDPS from P. aeruginosa (Ps), we have cloned, characterized biochemically and biophysically and have determined its three-dimensional structure. The gene encoding DHDPS (dapA) was cloned in a vector pET-28c(+) and the recombinant protein was overexpressed in the Escherichia coli host. The K(m) values of the recombinant enzyme estimated for the substrates, pyruvate and (S)-aspartate-β-semialdehyde [(S)-ASA] were found to be 0.90±0.13 mM and 0.17±0.02 mM, respectively. The circular dichroism studies showed that the enzyme adopts a characteristic β/α conformation which is retained up to 65°C. The fluorescence data indicated the presence of exposed tryptophan residues in the enzyme. The three-dimensional structure determination showed that DHDPS forms a homodimer which is stabilized by several hydrogen bonds and van der Waals forces at the interface. The active site formed with residues Thr44, Tyr107 and Tyr133 is found to be stereochemically suitable for catalytic function. It may be noted that Tyr107 of the catalytic triad belongs to the partner molecule in the dimer. The structure of the complex of PsDHDPS with (S)-lysine determined at 2.65 Å resolution revealed the positions of three lysine molecules bound to the protein.

Crystallization of dihydrodipicolinate synthase from a clinical isolate of Streptococcus pneumoniae

Dihydrodipicolinate synthase (DHDPS; EC 4.2.1.52) catalyzes the rate-limiting step in the (S)-lysine biosynthesis pathway of bacteria and plants. Here, the cloning of the DHDPS gene from a clinical isolate of Streptococcus pneumoniae (OXC141 strain) and the strategy used to express, purify and crystallize the recombinant enzyme are described. Diffracting crystals were grown in high-molecular-weight PEG precipitants using the hanging-drop vapour-diffusion method. The best crystal, from which data were collected, diffracted to beyond 2.0 A resolution. Initially, the crystals were thought to belong to space group P4(2)2(1)2, with unit-cell parameters a = 105.5, b = 105.5, c = 62.4 A. However, the R factors remained high following initial processing of the data. It was subsequently shown that the data set was twinned and it was thus reprocessed in space group P2, resulting in a significant reduction in the R factors. Determination of the structure will provide insight into the design of novel antimicrobial agents targeting this important enzyme from S. pneumoniae.

Structure of dihydrodipicolinate synthase from Methanocaldococcus jannaschii

In bacteria and plants, dihydrodipicolinate synthase (DHDPS) plays a key role in the (S)-lysine biosynthesis pathway. DHDPS catalyzes the first step of the condensation of (S)-aspartate-beta-semialdehyde and pyruvate to form an unstable compound, (4S)-4-hydroxy-2,3,4,5-tetrahydro-(2S)-dipicolinic acid. The activity of DHDPS is allosterically regulated by (S)-lysine, a feedback inhibitor. The crystal structure of DHDPS from Methanocaldococcus jannaschii (MjDHDPS) was solved by the molecular-replacement method and was refined to 2.2 A resolution. The structure revealed that MjDHDPS forms a functional homotetramer, as also observed in Escherichia coli DHDPS, Thermotoga maritima DHDPS and Bacillus anthracis DHDPS. The binding-site region of MjDHDPS is essentially similar to those found in other known DHDPS structures.

Cloning and characterisation of dihydrodipicolinate synthase from the pathogen Neisseria meningitidis

Neisseria meningitidis is an obligate commensal bacterium of humans, and also an important human pathogen. To facilitate future drug studies, we report here the biochemical and structural characterisation of a key enzyme in (S)-lysine biosynthesis, dihydrodipicolinate synthase (DHDPS), from N. meningitidis (NmeDHDPS). X-ray crystallography revealed only minor structural differences between NmeDHDPS and the enzyme from E. coli at the active and allosteric effector sites. The catalytic capabilities of NmeDHDPS are similar to those of the enzyme from E. coli, but intriguingly NmeDHDPS is subject to substrate inhibition by high concentrations of the second substrate, (S)-aspartate semialdehyde, and is also significantly more sensitive to feedback inhibition by (S)-lysine. This heightened sensitivity to inhibition at both active and allosteric sites suggests that it may be possible to target DHDPS from N. meningitidis for antibiotic development.

Mutating the tight-dimer interface of dihydrodipicolinate synthase disrupts the enzyme quaternary structure: toward a monomeric enzyme

Dihydrodipicolinate synthase (DHDPS) is a tetrameric enzyme that is the first enzyme unique to the ( S)-lysine biosynthetic pathway in plants and bacteria. Previous studies have looked at the important role of Tyr107, an amino acid residue located at the tight-dimer interface between two monomers, in participating in a catalytic triad of residues during catalysis. In this study, we examine the importance of this residue in determining the quaternary structure of the DHDPS enzyme. The Tyr107 residue was mutated to tryptophan, and structural, biophysical, and kinetic studies were carried out on the mutant enzyme. These revealed that while the solid-state structure of the mutant enzyme was largely unchanged, as judged by X-ray crystallography, it exists as a mixture of primarily monomer and tetramer in solution, as determined by analytical ultracentrifugation, size-exclusion chromatography, and mass spectrometry. The catalytic ability of the DHDPS enzyme was reduced by the mutation, which also allowed the adventitious binding of alpha-ketoglutarate to the active site. A reduction in the apparent melting temperature of the mutant enzyme was observed. Thus, the tetrameric quaternary structure of DHDPS is critical to controlling specificity, heat stability, and intrinsic activity.

Characterization and crystal structure of lysine insensitive Corynebacterium glutamicum dihydrodipicolinate synthase (cDHDPS) protein

The lysine insensitive Corynebacterium glutamicum dihydrodipicolinate synthase enzyme (cDHDPS) was recently successfully introduced into maize plants to enhance the level of lysine in the grain. To better understand lysine insensitivity of the cDHDPS, we expressed, purified, kinetically characterized the protein, and solved its X-ray crystal structure. The cDHDPS enzyme has a fold and overall structure that is highly similar to other DHDPS proteins. A noteworthy feature of the active site is the evidence that the catalytic lysine residue forms a Schiff base adduct with pyruvate. Analyses of the cDHDPS structure in the vicinity of the putative binding site for S-lysine revealed that the allosteric binding site in the Escherichia coli DHDPS protein does not exist in cDHDPS due to three non-conservative amino acids substitutions, and this is likely why cDHDPS is not feedback inhibited by lysine.

Purification, crystallization and preliminary X-ray diffraction studies to near-atomic resolution of dihydrodipicolinate synthase from methicillin-resistant Staphylococcus aureus

In recent years, dihydrodipicolinate synthase (DHDPS; EC 4.2.1.52) has received considerable attention from both mechanistic and structural viewpoints. DHDPS is part of the diaminopimelate pathway leading to lysine, coupling (S)-aspartate-beta-semialdehyde with pyruvate via a Schiff base to a conserved active-site lysine. In this paper, the cloning, expression, purification, crystallization and preliminary X-ray diffraction analysis of DHDPS from methicillin-resistant Staphylococcus aureus, an important bacterial pathogen, are reported. The enzyme was crystallized in a number of forms, predominantly from PEG precipitants, with the best crystal diffracting to beyond 1.45 A resolution. The space group was P1 and the unit-cell parameters were a = 65.4, b = 67.6, c = 78.0 A, alpha = 90.1, beta = 68.9, gamma = 72.3 degrees . The crystal volume per protein weight (V(M)) was 2.34 A(3) Da(-1), with an estimated solvent content of 47% for four monomers per asymmetric unit. The structure of the enzyme will help to guide the design of novel therapeutics against the methicillin-resistant S. aureus pathogen.

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.

The purification, crystallization and preliminary X-ray diffraction analysis of dihydrodipicolinate synthase from Clostridium botulinum

In recent years, dihydrodipicolinate synthase (DHDPS; EC 4.2.1.52) has received considerable attention from both mechanistic and structural viewpoints. This enzyme, which is part of the diaminopimelate pathway leading to lysine, couples (S)-aspartate-beta-semialdehyde with pyruvate via a Schiff base to a conserved active-site lysine. In this paper, the expression, purification, crystallization and preliminary X-ray diffraction analysis of DHDPS from Clostridium botulinum, an important bacterial pathogen, are presented. The enzyme was crystallized in a number of forms, predominantly using PEG precipitants, with the best crystal diffracting to beyond 1.9 A resolution and displaying P4(2)2(1)2 symmetry. The unit-cell parameters were a = b = 92.9, c = 60.4 A. The crystal volume per protein weight (V(M)) was 2.07 A(3) Da(-1), with an estimated solvent content of 41%. The structure of the enzyme will help guide the design of novel therapeutics against the C. botulinum pathogen.

Inhibition of lysine biosynthesis: an evolving antibiotic strategy

Bacterial biosynthesis of lysine has come under increased scrutiny as a target for novel antibacterial agents as it provides lysine for protein synthesis and both lysine and meso-diaminopimelate for construction of the bacterial peptidoglycan cell wall. In this Highlight article we review recent advances in the validation of antibiotic targets, studies of the enzymes of the lysine biosynthetic pathway and development of inhibitors of these enzymes.

Dihydrodipicolinate synthase from Thermotoga maritima

DHDPS (dihydrodipicolinate synthase) catalyses the branch point in lysine biosynthesis in bacteria and plants and is feedback inhibited by lysine. DHDPS from the thermophilic bacterium Thermotoga maritima shows a high level of heat and chemical stability. When incubated at 90 degrees C or in 8 M urea, the enzyme showed little or no loss of activity, unlike the Escherichia coli enzyme. The active site is very similar to that of the E. coli enzyme, and at mesophilic temperatures the two enzymes have similar kinetic constants. Like other forms of the enzyme, T. maritima DHDPS is a tetramer in solution, with a sedimentation coefficient of 7.2 S and molar mass of 133 kDa. However, the residues involved in the interface between different subunits in the tetramer differ from those of E. coli and include two cysteine residues poised to form a disulfide bond. Thus the increased heat and chemical stability of the T. maritima DHDPS enzyme is, at least in part, explained by an increased number of inter-subunit contacts. Unlike the plant or E. coli enzyme, the thermophilic DHDPS enzyme is not inhibited by (S)-lysine, suggesting that feedback control of the lysine biosynthetic pathway evolved later in the bacterial lineage.

Crystallization, preliminary X-ray diffraction and structure solution of MosA, a dihydrodipicolinate synthase from Sinorhizobium meliloti L5-30

The structure of MosA, a dihydrodipicolinate synthase and reported methyltransferase from Sinorhizobium meliloti, has been solved using molecular replacement with Escherichia coli dihydrodipicolinate synthase as the model. A crystal grown in the presence of pyruvate diffracted X-rays to 2.3 A resolution using synchrotron radiation and belonged to the orthorhombic space group C222(1), with unit-cell parameters a = 69.14, b = 138.87, c = 124.13 A.

Dihydrodipicolinate synthase (DHDPS) from Escherichia coli displays partial mixed inhibition with respect to its first substrate, pyruvate

Dihydrodipicolinate synthase (DHDPS, E.C. 4.2.1.52) mediates the first unique reaction of (S)-lysine biosynthesis in plants and microbes-the condensation of (S)-aspartate-beta-semialdehyde ((S)-ASA) and pyruvate. It has been shown that DHDPS is partially feedback inhibited by (S)-lysine; it is suggested that this mechanism regulates flux through the DAP biosynthetic pathway. Others have characterised DHDPS from Escherichia coli with respect to (S)-lysine inhibition. They have concluded that, with respect to pyruvate, the first substrate of the reaction, DHDPS shows uncompetitive inhibition: as such, they further suggest that (S)-lysine inhibits DHDPS via interaction with the binding site for the second substrate, (S)-ASA. Yet, this finding is based on the assumption that (S)-lysine is a fully uncompetitive inhibitor. In light of crystallographic studies, which lead to the proposal that (S)-lysine affects the putative proton-relay of DHDPS, we re-evaluated the inhibition mechanism of DHDPS with respect to (S)-lysine by incorporating the observed hyperbolic inhibition. Our data showed that lysine is not an uncompetitive inhibitor, but a mixed inhibitor when pyruvate and (S)-lysine concentrations were varied. Thus, consistent with the crystallographic data, (S)-lysine must have an effect on the initial steps of the DHDPS reaction, including the binding of pyruvate and Schiff base formation.