Meso-diaminopimelate Dehydrogenase Research Articles

Enhancing the amination activity of meso-diaminopimelate dehydrogenase from Symbiobacterium thermophilum by modifying the crucial residue His154 for deamination

During the deamination and amination processes of meso-diaminopimelate dehydrogenase (meso-DAPDH) from Symbiobacterium thermophilum (StDAPDH), residue R71 was observed to display distinct functions. H154 has been proposed as a basic residue that facilitates water molecules to attack the D-chiral carbon of meso-DAP during deamination. Inspired by the phenomenon of R71, the effects of H154 during deamination and amination were investigated in this study with the goal of enhancing the amination activities of StDAPDH. Single site saturation mutagenesis indicated that almost all of the H154 mutants completely lost their deamination activity towards meso-DAP. However, some H154 variants showed enhanced kcat/Km values towards pyruvic acid and other bulky 2-keto acids, such as 2-oxovaleric acid, 4-methyl-2-oxopentanoic acid, 2-ketobutyric acid, and 3-methyl-2-oxobutanoic acid. When combined with the previously reported W121L/H227I mutant, triple mutants with significantly improved kcat/Km values (2.4-, 2.5-, 2.5-, and 4.0-fold) towards these 2-keto acids were obtained. Despite previous attempts, mutations at the H154 site did not yield the desired results. Moreover, this study not only recognizes the distinctive impact of H154 on both the deamination and amination reactions, but also provides guidance for further high-throughput screening in protein engineering and understanding the catalytic mechanism of StDAPDH.

Mechanism-Guided Computational Design Drives meso-Diaminopimelate Dehydrogenase to Efficient Synthesis of Aromatic d-amino Acids.

Aromatic d-amino acids (d-AAs) play a pivotal role as important chiral building blocks and key intermediates in fine chemical and drug synthesis. Meso-diaminopimelate dehydrogenase (DAPDH) serves as an excellent biocatalyst in the synthesis of d-AAs and their derivatives. However, its strict substrate specificity and the lack of efficient engineering methods have hindered its widespread application. Therefore, this study aims to elucidate the catalytic mechanism underlying DAPDH from Proteus vulgaris (PvDAPDH) through the examination of its crystallographic structure, computational simulations of potential energies and molecular dynamics simulations, and site-directed mutagenesis. Mechanism-guided computational design showed that the optimal mutant PvDAPDH-M3 increased specific activity and catalytic efficiency (kcat/Km) for aromatic keto acids up to 124-fold and 92.4-fold, respectively, compared to that of the wild type. Additionally, it expanded the substrate scope to 10 aromatic keto acid substrates. Finally, six high-value-added aromatic d-AAs and their derivatives were synthesized using a one-pot three-enzyme cascade reaction, exhibiting a good conversion rate ranging from 32 to 84% and excellent stereoselectivity (enantiomeric excess >99%). These findings provide a potential synthetic pathway for the green industrial production of aromatic d-AAs.

Studies on Insertion/Deletion Residues for Functional Analysis and Improved Amination Activity in Meso-DAPDH from Corynebacterium glutamicum

Meso-diaminopimelate dehydrogenase (meso-DAPDH) from Corynebacterium glutamicum ATCC13032 (CgDAPDH) is a type I meso-DAPDH that shows obvious preference toward meso-diaminopimelate (meso-DAP) and exhibits almost no amination activity toward 2-keto acids. There are seven distinct conserved insertions and deletions (indels) between type I and type II meso-DAPDH. The current functional analysis of indels is not comprehensive in meso-DAPDH. Continuing from our previous work on these indels, we first examined the functions of the other indels shown as insertion residues in type I CgDAPDH. Alanine mutations in M216, T240, K289, and Q290 lost at least 40% of their activity, highlighting the importance of these four sites in CgDAPDH. Molecular dynamic analysis indicated that the four non-active sites altered the dynamic network of interactions within the protein. Subsequently, these four sites together with the previously identified indel-related residues R180, L176, and H193 were targeted by site-saturation mutagenesis to improve the amination ability of CgDAPDH toward pyruvic acid. The most significant improvement was observed with the mutant CgL176R, which showed a six-fold increase toward pyruvic acid in kcat/Km compared to wild-type CgDAPDH. Overall, our study provides new hotspots and ideas for the subsequent protein engineering of CgDAPDH, which may also be applied to other meso-DAPDHs.

Creating NADP+ -Specific Formate Dehydrogenases from Komagataella phaffii by Enzymatic Engineering.

Most natural formate dehydrogenases (FDHs) exhibit NAD+ specificity, making it imperative to explore the engineering of FDH cofactor specificity for NADPH regeneration systems. The endogenous FDH of Komagataella phaffii (K. phaffii), termed KphFDH, is a typical NAD+ -specific FDH. However, investigations into engineering the cofactor specificity of KphFDH have yet to be conducted. To develop an NADP+ -specific variant of KphFDH, we selected D195, Y196, and Q197 as mutation sites and generated twenty site-directed variants. Through kinetic characterization, KphFDH/V19 (D195Q/Y196R/Q197H) was identified as the variant with the highest specificity towards NADP+ , with a ratio of catalytic efficiency (kcat /KM )NADP+ /(kcat /KM )NAD+ of 129.226. Studies of enzymatic properties revealed that the optimal temperature and pH for the reduction reaction of NADP+ catalyzed by KphFDH/V19 were 45 °C and 7.5, respectively. The molecular dynamics (MD) simulation was performed to elucidate the mechanism of high catalytic activity of KphFDH/V19 towards NADP+ . Finally, KphFDH/V19 was applied to an in vitro NADPH regeneration system with Meso-diaminopimelate dehydrogenase from Symbiobacterium thermophilum (StDAPDH/H227V). This study successfully created a KphFDH variant with high NADP+ specificity and demonstrated its practical applicability in an in vitro NADPH regeneration system.

Rational Design of Meso-Diaminopimelate Dehydrogenase with Enhanced Reductive Amination Activity for Efficient Production of d-p-Hydroxyphenylglycine.

d-p-hydroxyphenylglycine (d-HPG) is an important intermediate in the pharmaceutical industry. In this study, a tri-enzyme cascade for the production of d-HPG from l-HPG was designed. However, the amination activity of Prevotella timonensis meso-diaminopimelate dehydrogenase (PtDAPDH) toward 4-hydroxyphenylglyoxylate (HPGA) was identified as the rate-limiting step. To overcome this issue, the crystal structure of PtDAPDH was solved, and a "binding pocket and conformation remodeling" strategy was developed to improve the catalytic activity toward HPGA. The best variant obtained, PtDAPDHM4, exhibited a catalytic efficiency (kcat/Km) that was 26.75-fold higher than that of the wild type. This improvement was due to the enlarged substrate-binding pocket and enhanced hydrogen bond networks around the active center; meanwhile, the increased number of interdomain residue interactions drove the conformation distribution toward the closed state. Under optimal transformation conditions, PtDAPDHM4 produced 19.8 g/L d-HPG from 40 g/L racemate DL-HPG in a 3 L fermenter within 10 h, with 49.5% conversion and >99% enantiomeric excess. Our study provides an efficient three-enzyme cascade pathway for the industrial production of d-HPG from racemate DL-HPG. IMPORTANCE d-p-hydroxyphenylglycine (d-HPG) is an important intermediate in the synthesis of antimicrobial compounds. d-HPG is mainly produced via chemical and enzymatic approaches, and enzymatic asymmetric amination employing diaminopimelate dehydrogenase (DAPDH) is considered an attractive method. However, the low catalytic activity of DAPDH toward bulky 2-keto acids limits its applications. In this study, we identified a DAPDH from Prevotella timonensis and created a mutant, PtDAPDHM4, which exhibited a catalytic efficiency (kcat/Km) toward 4-hydroxyphenylglyoxylate that was 26.75-fold higher than that of the wild type. The novel strategy developed in this study has practical value for the production of d-HPG from inexpensive racemate DL-HPG.

A multi-enzyme cascade for efficient production of d-p-hydroxyphenylglycine from l-tyrosine

In this study, a four-enzyme cascade pathway was developed and reconstructed in vivo for the production of d-p-hydroxyphenylglycine (D-HPG), a valuable intermediate used to produce β-lactam antibiotics and in fine-chemical synthesis, from l-tyrosine. In this pathway, catalytic conversion of the intermediate 4-hydroxyphenylglyoxalate by meso-diaminopimelate dehydrogenase from Corynebacterium glutamicum (CgDAPDH) was identified as the rate-limiting step, followed by application of a mechanism-guided “conformation rotation” strategy to decrease the hydride-transfer distance d(C6HDAP−C4NNADP) and increase CgDAPDH activity. Introduction of the best variant generated by protein engineering (CgDAPDHBC621/D120S/W144S/I169P with 5.32 ± 0.85 U·mg−1 specific activity) into the designed pathway resulted in a D-HPG titer of 42.69 g/L from 50-g/L l-tyrosine in 24 h, with 92.5% conversion, 71.5% isolated yield, and > 99% enantiomeric excess in a 3-L fermenter. This four-enzyme cascade provides an efficient enzymatic approach for the industrial production of D-HPG from cheap amino acids.

Characterization of an NAD(P)+-dependent meso-diaminopimelate dehydrogenase from Thermosyntropha lipolytica

meso-Diaminopimelate dehydrogenase (meso-DAPDH) catalyzes the reversible NADP+-dependent oxidative deamination of meso-2,6-diaminopimelate (meso-DAP) to produce l-2-amino-6-oxopimelate. meso-DAPDH is divided into two major clusters, types I and II, based on substrate specificity and structural characteristic. Here, we describe a novel type II meso-DAPDH from Thermosyntropha lipolytica (TlDAPDH). The gene encoding a putative TlDAPDH was expressed in Escherichia coli cells, and then the enzyme was purified 7.3-fold to homogeneity from the crude cell extract. The molecule of TlDAPDH seemed to form a hexamer, which is the typical structural characteristic of type II meso-DAPDHs. The purified enzyme exhibited oxidative deamination activity toward meso-DAP with both NADP+ and NAD+ as coenzymes. TlDAPDH exhibited reductive amination activity of corresponding 2-oxo acid to produce d-amino acid. In particular, the productivities for d-aspartate and d-glutamate have not been reported in the type II enzymes. The optimum pH and temperature for oxidative deamination of meso-DAP were 10.5 and 55°C, respectively. TlDAPDH retained more than 80% of its activity after incubation for 30 min at temperatures between 50°C and 65°C and in the pH range of 4.5–9.5. Moreover, the coenzyme and substrate recognition mechanisms of TlDAPDH were elucidated based on a multiple sequence alignment and the homology model. The results of these analyses suggested that the molecular mechanisms for coenzyme and substrate recognition of TlDAPDH were similar to those of meso-DAPDH from S. thermophilum, which is the representative type II enzyme. Based on the kinetic characteristics and structural comparison, TlDAPDH was considered to be a novel type II meso-DAPDH.

Altered Cofactor Preference of Thermostable StDAPDH by a Single Mutation at K159.

D-amino acid production from 2-keto acid by reductive amination is an attractive pathway because of its high yield and environmental safety. StDAPDH, a meso-diaminopimelate dehydrogenase (meso-DAPDH) from Symbiobacterium thermophilum, was the first meso-DAPDH to show amination of 2-keto acids. Furthermore, StDAPDH shows excellent thermostability compared to other meso-DAPDHs. However, the cofactor of StDAPDH is NADP(H), which is less common than NAD(H) in industrial applications. Therefore, cofactor engineering for StDAPDH is needed. In this study, the highly conserved cofactor binding sites around the adenosine moiety of NADPH were targeted to determine cofactor specificity. Lysine residues within a loop were found to be critical for the cofactor specificity of StDAPDH. Replacement of lysine with arginine resulted in the activity of pyruvic acid with NADH as the cofactor. The affinity of K159R to pyruvic acid was equal with NADH or NADPH as the cofactor, regardless of the mutation. Molecular dynamics simulations revealed that the large steric hindrance of arginine and the interaction of the salt bridge between NADH and arginine may have restricted the free movement of NADH, which prompted the formation of a stable active conformation of mutant K159R. These results provide further understanding of the catalytic mechanism of StDAPDH and guidance for the cofactor engineering of StDAPDH.

Insight into the Highly Conserved and Differentiated Cofactor-Binding Sites of meso-Diaminopimelate Dehydrogenase StDAPDH.

meso-Diaminopimelate dehydrogenase ( meso-DAPDH) is a good candidate for one-step synthesis of d-amino acid from 2-keto acids. Our previous research revealed the classification of meso-DAPDH family and showed that type II meso-DAPDH, such as the meso-DAPDH from Symbiobacterium thermophilum (StDAPDH), could catalyze reductive amination. In this article, seven residues of StDAPDH, which are highly conserved in each subfamily but are different between two subfamilies, were targeted to explore the relationships between structure and function. Determination of kinetic parameters showed that the amino acid residues, including P69, K159, V68, S90, V14, and V156, played very important roles in the catalytic function of StDAPDH. Molecular dynamics simulation revealed that these point mutations reduced the productive conformations by the newly formed or eliminated interactions between the residues and ligands. These results strengthen our understanding of the catalytic mechanism and evolution of meso-DAPDH and can aid future endeavors in enzyme engineering.

Overexpression of thermostable meso-diaminopimelate dehydrogenase to redirect diaminopimelate pathway for increasing L-lysine production in Escherichia coli

Dehydrogenase pathway, one of diaminopimelate pathway, is important to the biosynthesis of L-lysine and peptidoglycan via one single reaction catalyzed by meso-diaminopimelate dehydrogenase (DapDH). In this study, the thermostable DapDH was introduced into diaminopimelate pathway that increased the final titer (from 71.8 to 119.5 g/L), carbon yield (from 35.3% to 49.1%) and productivity (from 1.80 to 2.99 g/(L∙h)) of L-lysine by LATR12-2∆rpiB::ddhSt in fed-batch fermentation. To do this, the kinetic properties and the effects of different DapDHs on L-lysine production were investigated, and the results indicated that overexpression of StDapDH in LATR12-2 was beneficial to construct an L-lysine producer with good productive performance because it exhibited the best of kinetic characteristics and optimal temperature as well as thermostability in reductive amination. Furthermore, ammonium availability was optimized, and found that 20 g/L of (NH4)2SO4 was the optimal ammonium concentration for improving the efficiency of L-lysine production by LATR12-2∆rpiB::ddhSt. Metabolomics analysis showed that introducing the StDapDH significantly enhanced carbon flux into pentose phosphate pathway and L-lysine biosynthetic pathway, thus increasing the levels of NADPH and precursors for L-lysine biosynthesis. This is the first report of a rational modification of diaminopimelate pathway that improves the efficiency of L-lysine production through overexpression of thermostable DapDH in E. coli.

Highly selective synthesis of d-amino acids from readily available l-amino acids by a one-pot biocatalytic stereoinversion cascade.

d-Amino acids are key intermediates required for the synthesis of important pharmaceuticals. However, establishing a universal enzymatic method for the general synthesis of d-amino acids from cheap and readily available precursors with few by-products is challenging. In this study, we constructed and optimized a cascade enzymatic route involving l-amino acid deaminase and d-amino acid dehydrogenase for the biocatalytic stereoinversions of l-amino acids into d-amino acids. Using l-phenylalanine (l-Phe) as a model substrate, this artificial biocatalytic cascade stereoinversion route first deaminates l-Phe to phenylpyruvic acid (PPA) through catalysis involving recombinant Escherichia coli cells that express l-amino acid deaminase from Proteus mirabilis (PmLAAD), followed by stereoselective reductive amination with recombinant meso-diaminopimelate dehydrogenase from Symbiobacterium thermophilum (StDAPDH) to produce d-phenylalanine (d-Phe). By incorporating a formate dehydrogenase-based NADPH-recycling system, d-Phe was obtained in quantitative yield with an enantiomeric excess greater than 99%. In addition, the cascade reaction system was also used to stereoinvert a variety of aromatic and aliphatic l-amino acids to the corresponding d-amino acids by combining the PmLAAD whole-cell biocatalyst with the StDAPDH variant. Hence, this method represents a concise and efficient route for the asymmetric synthesis of d-amino acids from the corresponding l-amino acids.

Artificial Thermostable D-Amino Acid Dehydrogenase: Creation and Application.

Many kinds of NAD(P)+-dependent L-amino acid dehydrogenases have been so far found and effectively used for synthesis of L-amino acids and their analogs, and for their sensing. By contrast, similar biotechnological use of D-amino acid dehydrogenase (D-AADH) has not been achieved because useful D-AADH has not been found from natural resources. Recently, using protein engineering methods, an NADP+-dependent D-AADH was created from meso-diaminopimelate dehydrogenase (meso-DAPDH). The artificially created D-AADH catalyzed the reversible NADP+-dependent oxidative deamination of D-amino acids to 2-oxo acids. The enzyme, especially thermostable one from thermophiles, was efficiently applicable to synthesis of D-branched-chain amino acids (D-BCAAs), with high yields and optical purity, and was useful for the practical synthesis of 13C- and/or 15N-labeled D-BCAAs. The enzyme also made it possible to assay D-isoleucine selectively in a mixture of isoleucine isomers. Analyses of the three-dimensional structures of meso-DAPDH and D-AADH, and designed mutations based on the information obtained made it possible to markedly enhance enzyme activity and to create D-AADH homologs with desired reactivity profiles. The methods described here may be an effective approach to artificial creation of biotechnologically useful enzymes.

Exploring the binding mechanisms of diaminopimelic acid analogs to meso-diaminopimelate dehydrogenase by molecular modeling

Meso-Diaminopimelic acid (meso-2,6-diamino-heptanedioic acid, DAP) is an important component of the cell wall of many bacteria. Meso-diaminopimelate dehydrogenase (m-Ddh) is a critical enzyme in the process of converting tetrahydrodipicolinate to DAP. Here, we are proposing that DAP analogs targeting m-Ddh may be considered as potential antibiotics. Four DAP analogs without significant structural change from DAP have been obtained and their inhibitory potencies against m-Ddh from the P. gingivalis strain W83 show significant differences from that of DAP. However, their inhibitory mechanisms as for how simple structural change influences the inhibitory potency remain unknown. Therefore, we employed molecular modeling methods to obtain insight into the inhibitory mechanisms of DAP and analogs with m-Ddh. The predicted binding mode of DAP was highly consistent with the experimental structural data and disclosed the important roles played by the binding pocket residues. According to our predictions, the isoxazoline ring of compounds 1 and 2 and the double bonds in compounds 3 and 4 had distinct influences on these compounds' binding to m-Ddh. This enriched understanding of the inhibitory mechanisms of DAP and these four analogs to m-Ddh has provided new and relevant information for future rational development of potent inhibitors targeting m-Ddh.

Essential role of amino acid position 71 in substrate preference by meso-diaminopimelate dehydrogenase from Symbiobacterium thermophilum IAM14863.

meso--Diaminopimelate dehydrogenase (meso-DAPDH) catalyzes the reversible oxidative deamination of the d-configuration of meso-2,6-diaminopimelate (meso-DAP) and is thought to have substrate specificity toward meso-DAP. The discovery of the meso-DAPDH from Symbiobacterium thermophilum IAM14863 (StDAPDH) revealed meso-DAPDH members with broad substrate specificity. In order to elucidate the substrate-preference mechanism of StDAPDH, it is necessary to identify the key residues related to this mechanism. Our previous work suggested that the non-active-site R71 of StDAPDH was related to substrate preference. Here, we report the key roles of the non-active site on the catalysis of StDAPDH. In order to explore the mechanism through which non-active-site R71 only affected the amination activity of StDAPDH, we performed molecular dynamic simulations and investigated the functional role of R71 in the type II meso-DAPDH StDAPDH. Site-directed mutagenesis with the allelic site A69 of CgDAPDH as a target proved that when replaced by Arg at position 71 of StDAPDH, the CgA69R mutant showed higher catalytic efficiencies toward a series of 2-keto acids, ranging from 1.2- to 1.5-fold. These findings provide some guidelines for improving our understanding of the broad substrate specificity of StDAPDH.

Enzymatic synthesis of d-alanine from a renewable starting material by co-immobilized dehydrogenases

d-alanine is an important pharmaceutical intermediate. In this study, an effective method was developed to prepare optically pure d-alanine from sodium pyruvate with co-immobilized meso-diaminopimelate dehydrogenase from Symbiobacterium thermophilum and formate dehydrogenase from Candida Boidinii onto a resin. After optimizing the conditions of immobilization and biotransformation, the co-immobilized enzymes exhibited excellent stereoselectivity, yield and reusability. The co-immobilized enzymes kept 80% of its activity after 12 reuses, and d-alanine was isolated in 75.7% average yield and 99.5% ee for the first 20 cycles in the batch reactions. This suggests the co-immobilized enzyme system as a promising biocatalyst for the green production of d-alanine from pyruvic acid, a bio-based starting material.

Diaminopimelic acid (DAP) analogs bearing isoxazoline moiety as selective inhibitors against meso-diaminopimelate dehydrogenase (m-Ddh) from Porphyromonas gingivalis

Two diastereomeric analogs (1 and 2) of diaminopimelic acid (DAP) bearing an isoxazoline moiety were synthesized and evaluated for their inhibitory activities against meso-diaminopimelate dehydrogenase (m-Ddh) from the periodontal pathogen, Porphyromonas gingivalis. Compound 2 showed promising inhibitory activity against m-Ddh with an IC50 value of 14.9µM at pH 7.8. The two compounds were further tested for their antibacterial activities against a panel of periodontal pathogens, and compound 2 was shown to be selectively potent to P. gingivalis strains W83 and ATCC 33277 with minimum inhibitory concentration (MIC) values of 773µM and 1.875mM, respectively. Molecular modeling studies revealed that the inversion of chirality at the C-5 position of these compounds was the primary reason for their different biological profiles. Based on these preliminary results, we believe that compound 2 has properties consistent with it being a lead compound for developing novel pathogen selective antibiotics to treat periodontal diseases.

A Newly Determined Member of the meso-Diaminopimelate Dehydrogenase Family with a Broad Substrate Spectrum.

meso-Diaminopimelate dehydrogenase (meso-DAPDH) from Symbiobacterium thermophilum (StDAPDH) is the first member of the meso-DAPDH family known to catalyze the asymmetric reductive amination of 2-keto acids to produce d-amino acids. It is important to understand the catalytic mechanisms of StDAPDH and other enzymes in this family. In this study, based on an evolutionary analysis and examination of catalytic activity, the meso-DAPDH enzymes can be divided into two types. Type I showed highly preferable activity toward meso-diaminopimelate (meso-DAP), and type II exhibited obviously reversible amination activity with a broad substrate spectrum. StDAPDH belongs to type II. A quaternary structure analysis revealed that insertions/deletions (indels) and a loss of quaternary structure resulted in divergence among members of the meso-DAPDH family. A structure alignment of StDAPDH with a representative of type I, the meso-DAPDH from Corynebacterium glutamicum (CgDAPDH), indicated that they had the same folding. Based on sequence and conservation analyses, two amino acid residues of StDAPDH, R35 and R71, were found to be highly conserved within type II while also distinct from each other between the subtypes. Site mutagenesis studies identified R71 as a substrate preference-related residue of StDAPDH, which may serve as an indicator of the amination preference of type II. These results deepen the present understanding of the meso-DAPDH family and provide a solid foundation for the discovery and engineering of meso-DAPDH for d-amino acid biosynthesis.IMPORTANCE The l-form of amino acids is typically more abundant than the d-form. However, the d-form has many important pharmaceutical applications. meso-Diaminopimelate dehydrogenase (meso-DAPDH) from Symbiobacterium thermophilum (StDAPDH) was the first member of meso-DAPDH known to catalyze the amination of 2-keto acids to produce d-amino acids. Accordingly, we analyzed the evolution of meso-DAPDH proteins and found that they form two groups, i.e., type I proteins, which show high preference toward meso-diaminopimelate (meso-DAP), and type II proteins, which show a broad substrate spectrum. We examined the differences in sequence, ternary structure, and quaternary structure to determine the mechanisms underlying the functional differences between the type I and type II lineages. These results will facilitate the identification of additional meso-DAPDHs and may provide guidance to protein engineering studies for d-amino acid biosynthesis.

Identification of Small-Molecule Inhibitors against Meso-2, 6-Diaminopimelate Dehydrogenase from Porphyromonas gingivalis.

Species-specific antimicrobial therapy has the potential to combat the increasing threat of antibiotic resistance and alteration of the human microbiome. We therefore set out to demonstrate the beginning of a pathogen-selective drug discovery method using the periodontal pathogen Porphyromonas gingivalis as a model. Through our knowledge of metabolic networks and essential genes we identified a “druggable” essential target, meso-diaminopimelate dehydrogenase, which is found in a limited number of species. We adopted a high-throughput virtual screen method on the ZINC chemical library to select a group of potential small-molecule inhibitors. Meso-diaminopimelate dehydrogenase from P. gingivalis was first expressed and purified in Escherichia coli then characterized for enzymatic inhibitor screening studies. Several inhibitors with similar structural scaffolds containing a sulfonamide core and aromatic substituents showed dose-dependent inhibition. These compounds were further assayed showing reasonable whole-cell activity and the inhibition mechanism was determined. We conclude that the establishment of this target and screening strategy provides a model for the future development of new antimicrobials.

Structural Analysis Reveals the Substrate‐Binding Mechanism for the Expanded Substrate Specificity of Mutant meso‐Diaminopimelate Dehydrogenase

A meso-diaminopimelate dehydrogenase (DAPDH) from Clostridium tetani E88 (CtDAPDH) was found to have low activity toward the D-amino acids other than its native substrate. Site-directed mutagenesis similar to that carried out on the residues mutated by Vedha-Peters et al. resulted in a mutant enzyme with highly improved catalytic ability for the synthesis of D-amino acids. The crystal structures of the CtDAPDH mutant in apo form and in complex with meso-diaminopimelate (meso-DAP), D-leucine (D-leu), and 4-methyl-2-oxopentanoic acid (MOPA) were solved. meso-DAP was found in an area outside the catalytic cavity; this suggested a possible two-step substrate-binding mechanism for meso-DAP. D-leu and MOPA each bound both to Leu154 and to Gly155 in the open form of CtDAPDH, and structural analysis revealed the molecular basis for the expanded substrate specificity of the mutant meso-diaminopimelate dehydrogenases.

Structural and Mutational Studies on the Unusual Substrate Specificity of meso‐Diaminopimelate Dehydrogenase from Symbiobacterium thermophilum

Wild-type meso-diaminopimelate dehydrogenase (DAPDH) is usually specific to the native substrate, meso-2,6-diaminopimelate. Recently, a DAPDH from Symbiobacterium thermophilum (StDAPDH) was found to exhibit expanded substrate specificity. As such, its crystal structures in apo form and in complex with NADP(+) and both NADPH and meso-DAP were investigated to reveal the structural basis of its unique catalytic properties. Structural analysis results show that StDAPDH should prefer an ordered kinetic catalytic mechanism. A second substrate entrance tunnel with Met152 at its bottleneck was found, through which pyruvate/D-alanine might bind and enter the catalytic cavity, providing some structural insights into its high activity toward pyruvate. The side chain of Met152 might interact with Asp92 and Asn253, thus affecting the domain motion and catalysis. These results offer useful information for understanding the unique catalytic properties of StDAPDH and guiding further engineering of this enzyme.