Biochemical and structural characterization of alanine racemase from Bacillus anthracis (Ames)

Racemisation in Chemistry and Biology

The two enantiomers of a compound often have profoundly different biological properties and thus their liability to racemisation in aqueous solutions is an important piece of information. The authors reviewed the available data concerning the process of racemisation in vivo, in the presence of biological molecules (e.g., racemase enzymes, serum albumin, cofactors and derivatives) and under purely chemical but aqueous conditions (acid, base and other aqueous systems). Mechanistic studies are described critically in light of reported kinetic data. The types of experimental measurement that can be used to effectively determine rate constants of racemisation in various conditions are discussed and the data they provide is summarised. The proposed origins of enzymatic racemisation are presented and suggest ways to promote the process that are different from processes taking place in bulk water. Experimental and computational studies that provide understanding and quantitative predictions of racemisation risk are also presented.

Biofilm formation displays intrinsic offensive and defensive features of Bacillus cereus

Biofilm formation is a strategy of many bacterial species to adapt to a variety of stresses and has become a part of infections, contaminations, or beneficial interactions. In this study, we demonstrate that profound physiological changes permit Bacillus cereus to switch from a floating to a sessile lifestyle, to undergo further maturation of the biofilm and to differentiate into the offensive or defensive features. We report that floating and biofilm cells are populations that differentiate metabolically, with members of each subpopulation developing different branches of certain metabolic pathways. Secondly, biofilm populations rearrange nucleotides, sugars, amino acids, and energy metabolism. Thirdly, this metabolic rearrangement coexists with: the synthesis of the extracellular matrix, sporulation, reinforcement of the cell wall, activation of the ROS detoxification machinery and production of secondary metabolites. This strategy contributes to defend biofilm cells from competitors. However, floating cells maintain a fermentative metabolic status that ensures a higher aggressiveness against hosts, evidenced by the production of toxins. The maintenance of the two distinct subpopulations is an effective strategy to face different environmental conditions found in the life styles of B. cereus.

Purification, Characterization and Inhibition of Alanine Racemase from a Pathogenic Strain of Streptococcus iniae

Streptococcus iniae is a pathogenic and zoonotic bacteria that impacted high mortality to many fish species as well as capable of causing serious disease to humans. Alanine racemase (Alr, EC 5.1.1.1) is a pyridoxal-5’-phosphate (PLP)-containing homodimeric enzyme that catalyzes the racemization of L-alanine and D-alanine. In this study, we purified alanine racemase from S. iniae that was isolated from an infected Chinese sturgeon (Acipenser sinensis), as well as determined its biochemical characteristics and inhibitors. The alr gene has an open reading frame (ORF) of 1107 bp, encoding a protein of 369 amino acids, which has a molecular mass of 40 kDa. The enzyme has optimal activity at a temperature of 35°C and a pH of 9.5. It belongs to the PLP-dependent enzymes family and is highly specific to L-alanine. S. iniae Alr (SiAlr) could be inhibited by some metal ions, hydroxylamine and dithiothreitol (DTT). The kinetic parameters K_m and V_max of the enzyme were 33.11 mM, 2426 units/mg for L-alanine, and 14.36 mM, 963.6 units/mg for D-alanine. Finally, the 50% inhibitory concentrations (IC₅₀) values and antibiotic activity of two alanine racemase inhibitors (homogentisic acid and hydroquinone), were determined and found to be effective against both Gram-positive and Gram-negative bacteria employed in this study.

Function of alanine racemase in the physiological activity and cariogenicity of Streptococcus mutans

The enzyme alanine racemase (Alr) has been a new target for the development of antibacterial drugs based on the involvement of D-Ala in bacterial cell wall biosynthesis. Our previous study noted that Alr is essential for the growth and interspecies competitiveness of S. mutans, the major causative organism of dental caries. However, physiological activity and cariogenicity of S. mutans affected by Alr remains unknown. The current study examined the biofilm biomass, biofilm structure, extracellular polysaccharide (EPS) synthesis, glucosyltransferase (gtf) gene expression, acid production and acid tolerance in the alr-mutant strain. We found that biofilm formation, biofilm structure, and EPS synthesis was in a D-Ala dose-dependent manner. Biofilm structure was loose in alr-mutant group and the ratio of EPS/bacteria was also elevated. Additionally, the expression levels of multiple gtfs were up-regulated, and acid tolerance was decreased. We also established in vivo models of dental caries and found that the incidence and severity of the caries were decreased in the alr-mutant group in comparison to the parental S. mutans group. Our in vivo and in vitro experiments demonstrate that Alr is essential for the cariogenicity of S. mutans and that Alr might be a potential target for the prevention and treatment of caries.

Identification and elucidation of in vivo function of two alanine racemases from Pseudomonas putida KT2440

The genome of Pseudomonas putida KT2440 contains two open reading frames (ORFs), PP_3722 and PP_5269, that encode proteins with a Pyridoxal phosphate binding motif and a high similarity to alanine racemases. Alanine racemases play a key role in the biosynthesis of D-alanine, a crucial amino acid in the peptidoglycan layer. For these ORFs, we generated single and double mutants and found that inactivation of PP_5269 resulted in D-alanine auxotrophy, while inactivation of PP_3722 did not. Furthermore, as expected, the PP_3722/PP_5269 double mutant was a strict auxotroph for D-alanine. These results indicate that PP_5269 is an alr allele and that it is the essential alanine racemase in P. putida. We observed that the PP_5269 mutant grew very slowly, while the double PP_5269/PP_3722 mutant did not grow at all. This suggests that PP_3722 may replace PP_5269 in vivo. In fact, when the ORF encoding PP_3772 was cloned into a wide host range expression vector, ORF PP_3722 successfully complemented P. putida PP_5269 mutants. We purified both proteins to homogeneity and while they exhibit similar K_M values, the V_max of PP_5269 is fourfold higher than that of PP_3722. Here, we propose that PP_5269 and PP_3722 encode functional alanine racemases and that these genes be named alr-1 and alr-2 respectively.

Characterization of Lactobacillus salivarius alanine racemase: short-chain carboxylate-activation and the role of A131

Many strains of lactic acid bacteria produce high concentrations of d-amino acids. Among them, Lactobacillus salivarius UCC 118 produces d-alanine at a relative concentration much greater than 50 % of the total d, l-alanine (100d/d, l-alanine). We characterized the L. salivarius alanine racemase (ALR) likely responsible for this d-alanine production and found that the enzyme was activated by carboxylates, which is an unique characteristic among ALRs. In addition, alignment of the amino acid sequences of several ALRs revealed that A131 of L. salivarius ALR is likely involved in the activation. To confirm that finding, an L. salivarius ALR variant with an A131K (ALR(A131K)) substitution was prepared, and its properties were compared with those of ALR. The activity of ALR(A131K) was about three times greater than that of ALR. In addition, whereas L. salivarius ALR was strongly activated by low concentrations (e.g., 1 mM) of short chain carboxylates, and was inhibited at higher concentrations (e.g., 10 mM), ALR(A131K) was clearly inhibited at all carboxylate concentrations tested (1-40 mM). Acetate also increased the stability of ALR such that maximum activity was observed at 35 °C and pH 8.0 without acetate, but at 50 °C in the presence of 1 mM acetate. On the other hand, maximum ALR(A131K) activity was observed at 45 °C and around pH 9.0 with or without acetate. It thus appears that A131 mediates the activation and stabilization of L. salivarius ALR by short chain carboxylates.

Structural and Functional Adaptation of Vancomycin Resistance VanT Serine Racemases

Vancomycin resistance in Gram-positive bacteria results from the replacement of the d-alanyl–d-alanine target of peptidoglycan precursors with d-alanyl–d-lactate or d-alanyl–d-serine (d-Ala-d-Ser), to which vancomycin has low binding affinity. VanT is one of the proteins required for the production of d-Ala-d-Ser-terminating precursors by converting l-Ser to d-Ser. VanT is composed of two domains, an N-terminal membrane-bound domain, likely involved in l-Ser uptake, and a C-terminal cytoplasmic catalytic domain which is related to bacterial alanine racemases. To gain insight into the molecular function of VanT, the crystal structure of the catalytic domain of VanT_G from VanG-type resistant Enterococcus faecalis BM4518 was determined. The structure showed significant similarity to type III pyridoxal 5′-phosphate (PLP)-dependent alanine racemases, which are essential for peptidoglycan synthesis. Comparative structural analysis between VanT_G and alanine racemases as well as site-directed mutagenesis identified three specific active site positions centered around Asn₆₉₆ which are responsible for the l-amino acid specificity. This analysis also suggested that VanT racemases evolved from regular alanine racemases by acquiring additional selectivity toward serine while preserving that for alanine. The 4-fold-lower relative catalytic efficiency of VanT_G against l-Ser versus l-Ala implied that this enzyme relies on its membrane-bound domain for l-Ser transport to increase the overall rate of d-Ser production. These findings illustrate how vancomycin pressure selected for molecular adaptation of a housekeeping enzyme to a bifunctional enzyme to allow for peptidoglycan remodeling, a strategy increasingly observed in antibiotic-resistant bacteria.

Vancomycin is one of the drugs of last resort against Gram-positive antibiotic-resistant pathogens. However, bacteria have evolved a sophisticated mechanism which remodels the drug target, the d-alanine ending precursors in cell wall synthesis, into precursors terminating with d-lactate or d-serine, to which vancomycin has less affinity. d-Ser is synthesized by VanT serine racemase, which has two unusual characteristics: (i) it is one of the few serine racemases identified in bacteria and (ii) it contains a membrane-bound domain involved in l-Ser uptake. The structure of the catalytic domain of VanT_G showed high similarity to alanine racemases, and we identified three specific active site substitutions responsible for l-Ser specificity. The data provide the molecular basis for VanT evolution to a bifunctional enzyme coordinating both transport and racemization. Our findings also illustrate the evolution of the essential alanine racemase into a vancomycin resistance enzyme in response to antibiotic pressure.

Crystal Structure of a Thermostable Alanine Racemase from Thermoanaerobacter tengcongensis MB4 Reveals the Role of Gln360 in Substrate Selection

Pyridoxal 5’-phosphate (PLP) dependent alanine racemase catalyzes racemization of L-Ala to D-Ala, a key component of the peptidoglycan network in bacterial cell wall. It has been extensively studied as an important antimicrobial drug target due to its restriction in eukaryotes. However, many marketed alanine racemase inhibitors also act on eukaryotic PLP-dependent enzymes and cause side effects. A thermostable alanine racemase (Alr_Tt) from Thermoanaerobacter tengcongensis MB4 contains an evolutionarily non-conserved residue Gln360 in inner layer of the substrate entryway, which is supposed to be a key determinant in substrate specificity. Here we determined the crystal structure of Alr_Tt in complex with L-Ala at 2.7 Å resolution, and investigated the role of Gln360 by saturation mutagenesis and kinetic analysis. Compared to typical bacterial alanine racemase, presence of Gln360 and conformational changes of active site residues disrupted the hydrogen bonding interactions necessary for proper PLP immobilization, and decreased both the substrate affinity and turnover number of Alr_Tt. However, it could be complemented by introduction of hydrophobic amino acids at Gln360, through steric blocking and interactions with a hydrophobic patch near active site pocket. These observations explained the low racemase activity of Alr_Tt, revealed the essential role of Gln360 in substrate selection, and its preference for hydrophobic amino acids especially Tyr in bacterial alanine racemization. Our work will contribute new insights into the alanine racemization mechanism for antimicrobial drug development.

The structure of alanine racemase from Acinetobacter baumannii

Acinetobacter baumannii is an opportunistic Gram-negative bacterium which is a common cause of hospital-acquired infections. Numerous antibiotic-resistant strains exist, emphasizing the need for the development of new antimicrobials. Alanine racemase (Alr) is a pyridoxal 5'-phosphate dependent enzyme that is responsible for racemization between enantiomers of alanine. As D-alanine is an essential component of the bacterial cell wall, its inhibition is lethal to prokaryotes, making it an excellent antibiotic drug target. The crystal structure of A. baumannii alanine racemase (AlrAba) from the highly antibiotic-resistant NCTC13302 strain has been solved to 1.9 Å resolution. Comparison of AlrAba with alanine racemases from closely related bacteria demonstrates a conserved overall fold. The substrate entryway and active site of the enzymes were shown to be highly conserved. The structure of AlrAba will provide the template required for future structure-based drug-design studies.

Structural and biochemical analyses of alanine racemase from the multidrug-resistant Clostridium difficile strain 630

Clostridium difficile, a Gram-positive, spore-forming anaerobic bacterium, is the leading cause of infectious diarrhea among hospitalized patients. C. difficile is frequently associated with antibiotic treatment, and causes diseases ranging from antibiotic-associated diarrhea to life-threatening pseudomembranous colitis. The severity of C. difficile infections is exacerbated by the emergence of hypervirulent and multidrug-resistant strains, which are difficult to treat and are often associated with increased mortality rates. Alanine racemase (Alr) is a pyridoxal-5'-phosphate (PLP)-dependent enzyme that catalyzes the reversible racemization of L- and D-alanine. Since D-alanine is an essential component of the bacterial cell-wall peptidoglycan, and there are no known Alr homologs in humans, this enzyme is being tested as an antibiotic target. Cycloserine is an antibiotic that inhibits Alr. In this study, the catalytic properties and crystal structures of recombinant Alr from the virulent and multidrug-resistant C. difficile strain 630 are presented. Three crystal structures of C. difficile Alr (CdAlr), corresponding to the complex with PLP, the complex with cycloserine and a K271T mutant form of the enzyme with bound PLP, are presented. The structures are prototypical Alr homodimers with two active sites in which the cofactor PLP and cycloserine are localized. Kinetic analyses reveal that the K271T mutant CdAlr has the highest catalytic constants reported to date for any Alr. Additional studies are needed to identify the basis for the high catalytic activity. The structural and activity data presented are first steps towards using CdAlr for the development of structure-based therapeutics for C. difficile infections.

Structural basis for the broad specificity of a new family of amino-acid racemases

Broad-spectrum amino-acid racemases (Bsrs) enable bacteria to generate noncanonical D-amino acids, the roles of which in microbial physiology, including the modulation of cell-wall structure and the dissolution of biofilms, are just beginning to be appreciated. Here, extensive crystallographic, mutational, biochemical and bioinformatic studies were used to define the molecular features of the racemase BsrV that enable this enzyme to accommodate more diverse substrates than the related PLP-dependent alanine racemases. Conserved residues were identified that distinguish BsrV and a newly defined family of broad-spectrum racemases from alanine racemases, and these residues were found to be key mediators of the multispecificity of BrsV. Finally, the structural analysis of an additional Bsr that was identified in the bioinformatic analysis confirmed that the distinguishing features of BrsV are conserved among Bsr family members.

Involvement of alanine racemase in germination of Bacillus cereus spores lacking an intact exosporium

The L-alanine mediated germination of food isolated Bacillus cereus DSA 1 spores, which lacked an intact exosporium, increased in the presence of D-cycloserine (DCS), which is an alanine racemase (Alr) inhibitor, reflecting the activity of the Alr enzyme, capable of converting L-alanine to the germination inhibitor D-alanine. Proteomic analysis of the alkaline extracts of the spore proteins, which include exosporium and coat proteins, confirmed that Alr was present in the B. cereus DSA 1 spores and matched to that encoded by B. cereus ATCC 14579, whose spore germination was strongly affected by the block of conversion of L- to D-alanine. Unlike ATCC 14579 spores, L-alanine germination of B. cereus DSA 1 spores was not affected by the preincubation with DCS, suggesting a lack of restriction in the reactant accessibility.

Crystallization and preliminary X-ray study of a thermostable alanine racemase from Thermoanaerobacter tengcongensis MB4

Alanine racemase (Alr(MB4)), a dimeric PLP-dependent thermostable enzyme from the anaerobic eubacterium Thermoanaerobacter tengcongensis MB4, was expressed and purified with a His(6) tag in a form suitable for X-ray crystallographic analysis. Crystals were grown by the hanging-drop vapour-diffusion method at 289 K using a solution consisting of 0.1 M bis-tris pH 7.0, 22%(w/v) polyethylene glycol 4000. X-ray diffraction data were collected to 2.6 Å resolution. The crystal belonged to the orthorhombic space group P2(1)2(1)2(1), with two protein molecules in an asymmetric unit.

Characterization and preliminary mutation analysis of a thermostable alanine racemase from Thermoanaerobacter tengcongensis MB4

A thermostable alanine racemase from Thermoanaerobacter tengcongensis MB4 was successfully expressed in Escherichia coli and characterized. The full-length gene MBalr2 (1164 bp) encodes 388 amino acid residues including 6 out of 8 highly conserved amino acid residues at the entryway to the active site of alanine racemase. Recombinant MBAlr2 and three mutants (S171A, H359Y and double mutation S171A/H359Y) of MBAlr2 were purified by His6-tag affinity column and gel filtration chromatography. The purified protein MBAlr2 was a dimeric PLP-dependent enzyme with broad substrate specificity. The optimal racemization temperature and pH were 70-75 °C and 11.0, respectively. The kinetic parameters K m and V max of MBAlr2 at 70 °C, determined by HPLC, were 20.16 mM and 1414 μmol min(-1) for L-alanine, and 9.95 mM and 702.6 μmol min(-1) for D-alanine, respectively. Enzymatic assays showed that the activity of both mutants (S171A and H359Y) was lost, but the activity of mutant S171A/H359Y was recovered to 69.8 % of wild type, which suggested that residues Ser171 and His359 might be the important residues for catalytic mechanisms of MBAlr2.

Structure of alanine racemase from Oenococcus oeni with bound pyridoxal 5'-phosphate

The crystal structure of alanine racemase from Oenococcus oeni has been determined at 1.7 Å resolution using the single-wavelength anomalous dispersion (SAD) method and selenium-labelled protein. The protein exists as a symmetric dimer in the crystal, with both protomers contributing to the two active sites. Pyridoxal 5'-phosphate, a cofactor, is bound to each monomer and forms a Schiff base with Lys39. Structural comparison of alanine racemase from O. oeni (Alr) with homologous family members revealed similar domain organization and cofactor binding.

Structural features and kinetic characterization of alanine racemase from Staphylococcus aureus (Mu50)

Staphylococcus aureus is an opportunistic Gram-positive bacterium which causes a wide variety of diseases ranging from minor skin infections to potentially fatal conditions such as pneumonia, meningitis and septicaemia. The pathogen is a leading cause of nosocomial acquired infections, a problem that is exacerbated by the existence of methicillin- and glycopeptide antibiotic-resistant strains which can be challenging to treat. Alanine racemase (Alr) is a pyridoxal-5'-phosphate-dependent enzyme which catalyzes reversible racemization between enantiomers of alanine. As D-alanine is an essential component of the bacterial cell-wall peptidoglycan, inhibition of Alr is lethal to prokaryotes. Additionally, while ubiquitous amongst bacteria, this enzyme is absent in humans and most eukaryotes, making it an excellent antibiotic drug target. The crystal structure of S. aureus alanine racemase (Alr(Sas)), the sequence of which corresponds to that from the highly antibiotic-resistant Mu50 strain, has been solved to 2.15 Å resolution. Comparison of the Alr(Sas) structure with those of various alanine racemases demonstrates a conserved overall fold, with the enzyme sharing most similarity to those from other Gram-positive bacteria. Structural examination indicates that the active-site binding pocket, dimer interface and active-site entryway of the enzyme are potential targets for structure-aided inhibitor design. Kinetic constants were calculated in this study and are reported here. The potential for a disulfide bond in this structure is noted. This structural and biochemical information provides a template for future structure-based drug-development efforts targeting Alr(Sas).

Crystal structure of a zinc-dependent D-serine dehydratase from chicken kidney

D-serine is a physiological co-agonist of the N-methyl-D-aspartate receptor. It regulates excitatory neurotransmission, which is important for higher brain functions in vertebrates. In mammalian brains, D-amino acid oxidase degrades D-serine. However, we have found recently that in chicken brains the oxidase is not expressed and instead a D-serine dehydratase degrades D-serine. The primary structure of the enzyme shows significant similarities to those of metal-activated D-threonine aldolases, which are fold-type III pyridoxal 5'-phosphate (PLP)-dependent enzymes, suggesting that it is a novel class of D-serine dehydratase. In the present study, we characterized the chicken enzyme biochemically and also by x-ray crystallography. The enzyme activity on D-serine decreased 20-fold by EDTA treatment and recovered nearly completely by the addition of Zn(2+). None of the reaction products that would be expected from side reactions of the PLP-D-serine Schiff base were detected during the >6000 catalytic cycles of dehydration, indicating high reaction specificity. We have determined the first crystal structure of the D-serine dehydratase at 1.9 Å resolution. In the active site pocket, a zinc ion that coordinates His(347) and Cys(349) is located near the PLP-Lys(45) Schiff base. A theoretical model of the enzyme-D-serine complex suggested that the hydroxyl group of D-serine directly coordinates the zinc ion, and that the ε-NH(2) group of Lys(45) is a short distance from the substrate Cα atom. The α-proton abstraction from D-serine by Lys(45) and the elimination of the hydroxyl group seem to occur with the assistance of the zinc ion, resulting in the strict reaction specificity.

The crystal structure of alanine racemase from Streptococcus pneumoniae, a target for structure-based drug design

Background

Streptococcus pneumoniae is a globally important pathogen. The Gram-positive diplococcus is a leading cause of pneumonia, otitis media, bacteremia, and meningitis, and antibiotic resistant strains have become increasingly common over recent years. Alanine racemase is a ubiquitous enzyme among bacteria and provides the essential cell wall precursor, D-alanine. Since it is absent in humans, this enzyme is an attractive target for the development of drugs against S. pneumoniae and other bacterial pathogens.

Results

Here we report the crystal structure of alanine racemase from S. pneumoniae (Alr_SP). Crystals diffracted to a resolution of 2.0 Å and belong to the space group P3₁21 with the unit cell parameters a = b = 119.97 Å, c = 118.10 Å, α = β = 90° and γ = 120°. Structural comparisons show that Alr_SP shares both an overall fold and key active site residues with other bacterial alanine racemases. The active site cavity is similar to other Gram positive alanine racemases, featuring a restricted but conserved entryway.

Conclusions

We have solved the structure of Alr_SP, an essential step towards the development of an accurate pharmacophore model of the enzyme, and an important contribution towards our on-going alanine racemase structure-based drug design project. We have identified three regions on the enzyme that could be targeted for inhibitor design, the active site, the dimer interface, and the active site entryway.

Correlation between catalytic activity and monomer-dimer equilibrium of bacterial alanine racemases

From the reaction mechanism and crystal structure analysis, a bacterial alanine racemase is believed to work as a homodimer with a substrate, l-alanine or d-alanine. We analysed oligomerization states of seven alanine racemases, biosynthetic and catabolic, from Escherichia coli, Salmonella typhimurium, Pseudomonas aeruginosa, P. putida and P. fluorescens, with three different methods, gel filtration chromatography, native PAGE and analytical ultracentrifugation. All alanine racemases were proved to be in a dynamic equilibrium between monomeric and dimeric form with every methods used in this study. In both biosynthetic and catabolic alanine racemases, association constants for dimerization were high for the enzymes with high V(max) values. The enzymes with low V(max) values gave the low association constants. We proposed that alanine racemases are classified into two types; the enzymes with low and high-equilibrium association constants for dimerization.