Catalytic mechanism and cofactor preference of dihydrodipicolinate reductase from methicillin-resistant Staphylococcus aureus

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.

Taking Advantage of Promiscuity of Cold-Active Enzymes

Cold-active enzymes increase their catalytic efficiency at low-temperature, introducing structural flexibility at or near the active sites. Inevitably, this feat seems to be accompanied by lower thermal stability. These characteristics have made cold-active enzymes into attractive targets for the industrial applications, since they could reduce the energy cost in the reaction, attenuate side-reactions, and simply be inactivated. In addition, the increased structural flexibility could result in broad substrate specificity for various non-native substrates, which is called substrate promiscuity. In this perspective, we deal with a less addressed aspect of cold-active enzymes, substrate promiscuity, which has enormous potential for semi-synthesis or enzymatic modification of fine chemicals and drugs. Further structural and directed-evolutional studies on substrate promiscuity of cold-active enzymes will provide a new workhorse in white biotechnology.

Comparative structural and mechanistic studies of 4-hydroxy-tetrahydrodipicolinate reductases from Mycobacterium tuberculosis and Vibrio vulnificus

BACKGROUND

The products of the lysine biosynthesis pathway, meso-diaminopimelate and lysine, are essential for bacterial survival. This paper focuses on the structural and mechanistic characterization of 4-hydroxy-tetrahydrodipicolinate reductase (DapB), which is one of the enzymes from the lysine biosynthesis pathway. DapB catalyzes the conversion of (2S, 4S)-4-hydroxy-2,3,4,5-tetrahydrodipicolinate (HTPA) to 2,3,4,5-tetrahydrodipicolinate in an NADH/NADPH dependent reaction. Genes coding for DapBs were identified as essential for many pathogenic bacteria, and therefore DapB is an interesting new target for the development of antibiotics.

METHODS

We have combined experimental and computational approaches to provide novel insights into mechanism of the DapB catalyzed reaction.

RESULTS

Structures of DapBs originating from Mycobacterium tuberculosis and Vibrio vulnificus in complexes with NAD⁺, NADP⁺, as well as with inhibitors, were determined and described. The structures determined by us, as well as currently available structures of DapBs from other bacterial species, were compared and used to elucidate a mechanism of reaction catalyzed by this group of enzymes. Several different computational methods were used to provide a detailed description of a plausible reaction mechanism.

CONCLUSIONS

This is the first report presenting the detailed mechanism of reaction catalyzed by DapB.

GENERAL SIGNIFICANCE

Structural data in combination with information on the reaction mechanism provide a background for development of DapB inhibitors, including transition-state analogues.

Studying NAD(P)H cofactor-binding to alcohol dehydrogenases through global analysis of circular dichroism spectra

The initial step in reactions catalyzed by NAD(P)H-dependent alcohol dehydrogenases (ADHs) is the binding of the cofactor to the active site. To study this process, we measured NAD(P)H concentration-dependent circular dichroism (CD) in the presence of purified enzymes (ADH from horse liver, HLADH; ADH-A from Rhodococcus ruber; YGL157w from Saccharomyces cerevisiae) or enzyme-containing whole cell extract (ADH from Lactobacillus brevis, LbADH). We determined the proportions of binding and non-binding NAD(P)H and the associated dissociation constants (Kd) from matrix least-squares global fitting of law of mass action-derived model. Furthermore, the fitting allowed the back calculation of CD spectra corresponding to the cofactor in its bound conformation. With increasing pH and/or increasing ionic strength, we detected an increase in Kd for the NADH·HLADH complex with the shape of the bound cofactor conformation spectrum remaining unaffected. While the bound cofactor spectrum for the ADH-A·NADH complex was similar to that for HLADH, the corresponding spectra obtained for the NADPH-dependent enzymes YGL157w and LbADH exhibited opposite signs of the most prominent band. In comparison to CD spectra calculated on cofactor geometries from the crystal structures at the sTD-DFT level, we found that the sign of the bound cofactor spectrum correlates with the orientation of the nicotinamide ring of the cofactor in the active site. These results demonstrate the usefulness of the global analysis of cofactor titration CD spectra to study the role of cofactor binding and its geometry in ADH catalysis.

4-Hydroxy-tetrahydrodipicolinate reductase from Neisseria gonorrhoeae - structure and interactions with coenzymes and substrate analog

Neisseria gonorrhoeae, an obligate human pathogen, is a leading cause of communicable diseases globally. Due to rapid development of drug resistance, the rate of successfully curing gonococcal infections is rapidly decreasing. Hence, research is being directed toward finding alternative drugs or drug targets to help eradicate these infections. 4-Hydroxy-tetrahydrodipicolinate reductase (DapB), an important enzyme in the meso-diaminopimelate pathway, is a promising target for the development of new antibiotics. This manuscript describes the first structure of DapB from N. gonorrhoeae determined at 1.85 Å. This enzyme uses NAD(P)H as cofactor. Details of the interactions of the enzyme with its cofactors and a substrate analog/inhibitor are discussed. A large scale bioinformatics analysis of DapBs' sequences is also described.

Characterization of recombinant dihydrodipicolinate synthase from the bread wheat Triticum aestivum

Recombinant wheat DHDPS was produced for the first time in milligram quantities and shown to be an enzymatically active tetramer in solution using analytical ultracentrifugation and small angle X-ray scattering. Wheat is an important cereal crop with an extensive role in global food supply. Given our rapidly growing population, strategies to increase the nutritional value and production of bread wheat are of major significance in agricultural science to satisfy our dietary requirements. Lysine is one of the most limiting essential amino acids in wheat, thus, a thorough understanding of lysine biosynthesis is of upmost importance to improve its nutritional value. Dihydrodipicolinate synthase (DHDPS; EC 4.3.3.7) catalyzes the first committed step in the lysine biosynthesis pathway of plants. Here, we report for the first time the expression and purification of recombinant DHDPS from the bread wheat Triticum aestivum (Ta-DHDPS). The optimized protocol yielded 36 mg of > 98% pure recombinant Ta-DHDPS per liter of culture. Enzyme kinetic studies demonstrate that the recombinant Ta-DHDPS has a K_M (pyruvate) of 0.45 mM, K_M (l-aspartate-4-semialdehyde) of 0.07 mM, k_cat of 56 s^-1, and is inhibited by lysine (IC ₅₀ ^LYS of 0.033 mM), which agree well with previous studies using labor-intensive purification from wheat suspension cultures. We subsequently employed circular dichroism spectroscopy, analytical ultracentrifugation and small angle X-ray scattering to show that the recombinant enzyme is folded with 60% α/β structure and exists as a 7.5 S tetrameric species with a R_g of 33 Å and D_max of 118 Å. This study is the first to report the biophysical properties of the recombinant Ta-DHDPS in aqueous solution and offers an excellent platform for future studies aimed at improving nutritional value and primary production of bread wheat.

Crystal structure of dihydrodipicolinate reductase (PaDHDPR) from Paenisporosarcina sp. TG-14: structural basis for NADPH preference as a cofactor

Dihydrodipicolinate reductase (DHDPR) is a key enzyme in the diaminopimelate- and lysine-synthesis pathways that reduces DHDP to tetrahydrodipicolinate. Although DHDPR uses both NADPH and NADH as a cofactor, the structural basis for cofactor specificity and preference remains unclear. Here, we report that Paenisporosarcina sp. TG-14 PaDHDPR has a strong preference for NADPH over NADH, as determined by isothermal titration calorimetry and enzymatic activity assays. We determined the crystal structures of PaDHDPR alone, with its competitive inhibitor (dipicolinate), and the ternary complex of the enzyme with dipicolinate and NADPH, with results showing that only the ternary complex had a fully closed conformation and suggesting that binding of both substrate and nucleotide cofactor is required for enzymatic activity. Moreover, NADPH binding induced local conformational changes in the N-terminal long loop (residues 34–59) of PaDHDPR, as the His35 and Lys36 residues in this loop interacted with the 2′-phosphate group of NADPH, possibly accounting for the strong preference of PaDHDPR for NADPH. Mutation of these residues revealed reduced NADPH binding and enzymatic activity, confirming their importance in NADPH binding. These findings provide insight into the mechanism of action and cofactor selectivity of this important bacterial enzyme.

Molecular evolution of an oligomeric biocatalyst functioning in lysine biosynthesis

Dihydrodipicolinate synthase (DHDPS) is critical to the production of lysine through the diaminopimelate (DAP) pathway. Elucidation of the function, regulation and structure of this key class I aldolase has been the focus of considerable study in recent years, given that the dapA gene encoding DHDPS has been found to be essential to bacteria and plants. Allosteric inhibition by lysine is observed for DHDPS from plants and some bacterial species, the latter requiring a histidine or glutamate at position 56 (Escherichia coli numbering) over a basic amino acid. Structurally, two DHDPS monomers form the active site, which binds pyruvate and (S)-aspartate β-semialdehyde, with most dimers further dimerising to form a tetrameric arrangement around a solvent-filled centre cavity. The architecture and behaviour of these dimer-of-dimers is explored in detail, including biophysical studies utilising analytical ultracentrifugation, small-angle X-ray scattering and macromolecular crystallography that show bacterial DHDPS tetramers adopt a head-to-head quaternary structure, compared to the back-to-back arrangement observed for plant DHDPS enzymes. Finally, the potential role of pyruvate in providing substrate-mediated stabilisation of DHDPS is considered.

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.

Plant DHDPR forms a dimer with unique secondary structure features that preclude higher-order assembly

Dihydrodipicolinate reductase (DHDPR) catalyses the second reaction in the diaminopimelate pathway of lysine biosynthesis in bacteria and plants. In contrast with the tetrameric bacterial DHDPR enzymes, we show that DHDPR from Vitis vinifera (grape) and Selaginella moellendorffii are dimeric in solution. In the present study, we have also determined the crystal structures of DHDPR enzymes from the plants Arabidopsis thaliana and S. moellendorffii, which are the first dimeric DHDPR structures. The analysis of these models demonstrates that the dimer forms through the intra-strand interface, and that unique secondary features in the plant enzymes block tetramer assembly. In addition, we have also solved the structure of tetrameric DHDPR from the pathogenic bacteria Neisseria meningitidis Measuring the activity of plant DHDPR enzymes showed that they are much more prone to substrate inhibition than the bacterial enzymes, which appears to be a consequence of increased flexibility of the substrate-binding loop and higher affinity for the nucleotide substrate. This higher propensity to substrate inhibition may have consequences for ongoing efforts to increase lysine biosynthesis in plants.

Structure and Function of Cyanobacterial DHDPS and DHDPR

Lysine biosynthesis in bacteria and plants commences with a condensation reaction catalysed by dihydrodipicolinate synthase (DHDPS) followed by a reduction reaction catalysed by dihydrodipicolinate reductase (DHDPR). Interestingly, both DHDPS and DHDPR exist as different oligomeric forms in bacteria and plants. DHDPS is primarily a homotetramer in all species, but the architecture of the tetramer differs across kingdoms. DHDPR also exists as a tetramer in bacteria, but has recently been reported to be dimeric in plants. This study aimed to characterise for the first time the structure and function of DHDPS and DHDPR from cyanobacteria, which is an evolutionary important phylum that evolved at the divergence point between bacteria and plants. We cloned, expressed and purified DHDPS and DHDPR from the cyanobacterium Anabaena variabilis. The recombinant enzymes were shown to be folded by circular dichroism spectroscopy, enzymatically active employing the quantitative DHDPS-DHDPR coupled assay, and form tetramers in solution using analytical ultracentrifugation. Crystal structures of DHDPS and DHDPR from A. variabilis were determined at 1.92 Å and 2.83 Å, respectively, and show that both enzymes adopt the canonical bacterial tetrameric architecture. These studies indicate that the quaternary structure of bacterial and plant DHDPS and DHDPR diverged after cyanobacteria evolved.

Cloning, Expression, and Purification of Histidine-Tagged Escherichia coli Dihydrodipicolinate Reductase

The enzyme dihydrodipicolinate reductase (DHDPR) is a component of the lysine biosynthetic pathway in bacteria and higher plants. DHDPR catalyzes the NAD(P)H dependent reduction of 2,3-dihydrodipicolinate to the cyclic imine L-2,3,4,5,-tetrahydropicolinic acid. The dapB gene that encodes dihydrodipicolinate reductase has previously been cloned, but the expression of the enzyme is low and the purification is time consuming. Therefore the E. coli dapB gene was cloned into the pET16b vector to improve the protein expression and simplify the purification. The dapB gene sequence was utilized to design forward and reverse oligonucleotide primers that were used to PCR the gene from Escherichia coli genomic DNA. The primers were designed with NdeI or BamHI restriction sites on the 5’and 3’ terminus respectively. The PCR product was sequenced to confirm the identity of dapB. The gene was cloned into the expression vector pET16b through NdeI and BamHI restriction endonuclease sites. The resulting plasmid containing dapB was transformed into the bacterial strain BL21 (DE3). The transformed cells were utilized to grow and express the histidine-tagged reductase and the protein was purified using Ni-NTA affinity chromatography. SDS/PAGE gel analysis has shown that the protein was 95% pure and has approximate subunit molecular weight of 28 kDa. The protein purification is completed in one day and 3 liters of culture produced approximately 40–50 mgs of protein, an improvement on the previous protein expression and multistep purification.

BdcA, a protein important for Escherichia coli biofilm dispersal, is a short-chain dehydrogenase/reductase that binds specifically to NADPH

The Escherichia coli protein BdcA (previously referred to as YjgI) plays a key role in the dispersal of cells from bacterial biofilms, and its constitutive activation provides an attractive therapeutic target for dismantling these communities. In order to investigate the function of BdcA at a molecular level, we integrated structural and functional studies. Our 2.05 Å structure of BdcA shows that it is a member of the NAD(P)(H)-dependent short-chain dehydrogenase/reductase (SDR) superfamily. Structural comparisons with other members of the SDR family suggested that BdcA binds NADP(H). This was demonstrated experimentally using thermal denaturation studies, which showed that BcdA binds specifically to NADPH. Subsequent ITC experiments further confirmed this result and reported a Kd of 25.9 µM. Thus, BdcA represents the newest member of the limited number of oxidoreductases shown to be involved in quorum sensing and biofilm dispersal.

NADP+ binding to the regulatory subunit of methionine adenosyltransferase II increases intersubunit binding affinity in the hetero-trimer

Mammalian methionine adenosyltransferase II (MAT II) is the only hetero-oligomer in this family of enzymes that synthesize S-adenosylmethionine using methionine and ATP as substrates. Binding of regulatory β subunits and catalytic α2 dimers is known to increase the affinity for methionine, although scarce additional information about this interaction is available. This work reports the use of recombinant α2 and β subunits to produce oligomers showing kinetic parameters comparable to MAT II purified from several tissues. According to isothermal titration calorimetry data and densitometric scanning of the stained hetero-oligomer bands on denatured gels, the composition of these oligomers is that of a hetero-trimer with α2 dimers associated to single β subunits. Additionally, the regulatory subunit is able to bind NADP(+) with a 1:1 stoichiometry, the cofactor enhancing β to α2-dimer binding affinity. Mutants lacking residues involved in NADP(+) binding and N-terminal truncations of the β subunit were able to oligomerize with α2-dimers, although the kinetic properties appeared altered. These data together suggest a role for both parts of the sequence in the regulatory role exerted by the β subunit on catalysis. Moreover, preparation of a structural model for the hetero-oligomer, using the available crystal data, allowed prediction of the regions involved in β to α2-dimer interaction. Finally, the implications that the presence of different N-terminals in the β subunit could have on MAT II behavior are discussed in light of the recent identification of several splicing forms of this subunit in hepatoma cells.

Characterisation of the first enzymes committed to lysine biosynthesis in Arabidopsis thaliana

In plants, the lysine biosynthetic pathway is an attractive target for both the development of herbicides and increasing the nutritional value of crops given that lysine is a limiting amino acid in cereals. Dihydrodipicolinate synthase (DHDPS) and dihydrodipicolinate reductase (DHDPR) catalyse the first two committed steps of lysine biosynthesis. Here, we carry out for the first time a comprehensive characterisation of the structure and activity of both DHDPS and DHDPR from Arabidopsis thaliana. The A. thaliana DHDPS enzyme (At-DHDPS2) has similar activity to the bacterial form of the enzyme, but is more strongly allosterically inhibited by (S)-lysine. Structural studies of At-DHDPS2 show (S)-lysine bound at a cleft between two monomers, highlighting the allosteric site; however, unlike previous studies, binding is not accompanied by conformational changes, suggesting that binding may cause changes in protein dynamics rather than large conformation changes. DHDPR from A. thaliana (At-DHDPR2) has similar specificity for both NADH and NADPH during catalysis, and has tighter binding of substrate than has previously been reported. While all known bacterial DHDPR enzymes have a tetrameric structure, analytical ultracentrifugation, and scattering data unequivocally show that At-DHDPR2 exists as a dimer in solution. The exact arrangement of the dimeric protein is as yet unknown, but ab initio modelling of x-ray scattering data is consistent with an elongated structure in solution, which does not correspond to any of the possible dimeric pairings observed in the X-ray crystal structure of DHDPR from other organisms. This increased knowledge of the structure and function of plant lysine biosynthetic enzymes will aid future work aimed at improving primary production.

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.