Identification of the catalytic residues of alpha-amino acid ester hydrolase from Acetobacter turbidans by labeling and site-directed mutagenesis

l-tryptophan-histidine synthesis by Pseudomonas serine peptidase, an amino acid ester hydrolase of the peptidase family S9

Pseudomonas sp. KM1 produces an amino acid ester hydrolase (KM1AEH) that catalyzes peptide bond formation by acting on carboxylic ester bonds. The KM1AEH gene was cloned from genomic DNA and expressed in Escherichia coli. The recombinant enzyme (rKM1AEH) was purified, and gel filtration showed that it is a 68 kDa monomeric protein. rKM1AEH can synthesize the vasoactive dipeptide tryptophan-histidine from tryptophan methyl ester and histidine as acyl donor and acceptor, respectively. The enzyme showed maximum activity at pH 9.5 and 45 °C and was specifically inhibited by silver (Ag⁺). Mutation of the catalytic Ser459 residue in the active site of rKM1AEH with Ala, Cys, or Thr eliminated all catalytic activity. The enzyme is a novel ester hydrolase that belongs to the peptidase family S9 based on the phylogenetic analysis.

Characterization and Thermal Denaturation Kinetic Analysis of Recombinant l-Amino Acid Ester Hydrolase from Stenotrophomonas maltophilia

Stenotrophomonas maltophilia HS1 exhibits l-amino acid ester hydrolase (SmAEH) activity, which can synthesize dipeptides such as Ile-Trp, Val-Gly, and Trp-His from the corresponding amino acid methyl esters and amino acids. The gene encoding SmAEH was cloned and expressed in Escherichia coli and was purified and characterized. SmAEH shared 77% sequence identity with a known amino acid ester hydrolase (AEH) from Xanthomonas citri, which belongs to a class of β-lactam antibiotic acylases. The thermal stability of SmAEH was evaluated using various mathematical models to assess its industrial potential. First-order kinetics provided the best description for the inactivation of the enzyme over a temperature range of 35-50 °C. Decimal reduction time ranged from 212.76 to 3.44 min, with a z value of 8.06 °C, and the deactivation energy was 204.1 kJ mol^-1.

Enzymatic strategies and biocatalysts for amide bond formation: tricks of the trade outside of the ribosome

Amide bond-containing (ABC) biomolecules are some of the most intriguing and functionally significant natural products with unmatched utility in medicine, agriculture and biotechnology. The enzymatic formation of an amide bond is therefore a particularly interesting platform for engineering the synthesis of structurally diverse natural and unnatural ABC molecules for applications in drug discovery and molecular design. As such, efforts to unravel the mechanisms involved in carboxylate activation and substrate selection has led to the characterization of a number of structurally and functionally distinct protein families involved in amide bond synthesis. Unlike ribosomal synthesis and thio-templated synthesis using nonribosomal peptide synthetases, which couple the hydrolysis of phosphoanhydride bond(s) of ATP and proceed via an acyl-adenylate intermediate, here we discuss two mechanistically alternative strategies: ATP-dependent enzymes that generate acylphosphate intermediates and ATP-independent transacylation strategies. Several examples highlighting the function and synthetic utility of these amide bond-forming strategies are provided.

Amino ester hydrolase from Xanthomonas campestris pv. campestris, ATCC 33913 for enzymatic synthesis of ampicillin

α-Amino ester hydrolases (AEH) are a small class of proteins, which are highly specific for hydrolysis or synthesis of α-amino containing amides and esters including β-lactam antibiotics such as ampicillin, amoxicillin, and cephalexin. A BLAST search revealed the sequence of a putative glutaryl 7-aminocephalosporanic acid (GL-7-ACA) acylase 93% identical to a known AEH from Xanthomonas citri. The gene, termed gaa, was cloned from the genomic DNA of Xanthomonas campestris pv. campestris sp. strain ATCC 33913 and the corresponding protein was expressed into Escherichia coli. The purified protein was able to perform both hydrolysis and synthesis of a variety of α-amino β-lactam antibiotics including (R)-ampicillin and cephalexin, with optimal ampicillin hydrolytic activity at 25 °C and pH 6.8, with kinetic parameters of k(cat) of 72.5 s(-1) and K(M) of 1.1 mM. The synthesis parameters α, β(o), and γ for ampicillin, determined here first for this class of proteins, are α = 0.25, β(o) = 42.8 M(-1), and γ = 0.23, and demonstrate the excellent synthetic potential of these enzymes. An extensive study of site-directed mutations around the binding pocket of X. campestris pv. campestris AEH strongly suggests that mutation of almost any first-shell amino acid residues around the active site leads to inactive enzyme, including Y82, Y175, D207, D208, W209, Y222, and E309, in addition to those residues forming the catalytic triad, S174, H340, and D307.

Ampicillin Synthesis Using a Two-Enzyme Cascade with Both α-Amino Ester Hydrolase and Penicillin G Acylase

The current enzymatic production of semisynthetic β-lactam antibiotics requires isolation and purification of the intermediate 6-aminopenicillanic acid which adds cost and complexity to the manufacturing process. In this work, we took advantage of the unique substrate specificity of a-amino ester hydrolases to perform a purely aqueous one-pot production of ampicillin from penicillin G and D-phenylglycine methyl ester, catalyzed by α-amino ester hydrolase and penicillin G acylase. The synthesis was performed in both a one-pot, one-step synthesis resulting in a maximum conversion of 39%, and a one-pot, two-step process resulting in a maximum conversion of 47%. The two-enzyme cascade reported in this paper is a promising alternative to the current enzymatic two-step, two-pot manufacturing process for semisynthetic β-lactam antibiotics which requires intermittent isolation of 6-aminopenicillanic acid.

Bacterial degradation of N,N-diethyl-m-toluamide (DEET): cloning and heterologous expression of DEET hydrolase

Pseudomonas putida DTB grew aerobically with N,N-diethyl-m-toluamide (DEET) as a sole carbon source, initially breaking it down into 3-methylbenzoate and diethylamine. The former was further metabolized via 3-methylcatechol and meta ring cleavage. A gene from DTB, dthA, was heterologously expressed and shown to encode the ability to hydrolyze DEET into 3-methylbenzoate and diethylamine.

J1 acylase, a glutaryl-7-aminocephalosporanic acid acylase from Bacillus laterosporus J1, is a member of the alpha/beta-hydrolase fold superfamily

J1 acylase, a glutaryl-7-aminocephalosporanic acid acylase (GCA) isolated from Bacillus laterosporus J1, has been conventionally grouped as the only member of class V GCA, although its amino acid sequence shares less than 10% identity with members of other classes of GCA. Instead, it shows higher sequence similarities with Rhodococcus sp. strain MB1 cocaine esterase (RhCocE) and Acetobacter turbidans alpha-amino acid ester hydrolase (AtAEH), members of the alpha/beta-hydrolase fold superfamily. Homology modeling and secondary structure prediction indicate that the N-terminal region of J1 acylase has an alpha/beta-hydrolase folding pattern. The catalytic triads in RhCocE and AtAEH were identified in J1 acylase as S125, D264 and H309. Mutations to alanine at these positions were found to completely inactivate the enzyme. These results suggest that J1 acylase is a member of the alpha/beta-hydrolase fold superfamily with a serine-histidine-aspartate catalytic triad.

Kinetics of β-lactam antibiotics synthesis by penicillin G acylase (PGA) from the viewpoint of the industrial enzymatic reactor optimization

Competition with well-established, fine-tuned chemical processes is a major challenge for the industrial implementation of the enzymatic synthesis of beta-lactam antibiotics. Enzyme-based routes are acknowledged as an environmental-friendly approach, avoiding organochloride solvents and working at room temperatures. Among different alternatives, the kinetically controlled synthesis, using immobilized penicillin G acylase (PGA) in aqueous environment, with the simultaneous crystallization of the product, is the most promising one. However, PGA may act either as a transferase or as a hydrolase, catalyzing two undesired side reactions: the hydrolysis of the acyl side-chain precursor (an ester or amide, a parallel reaction) and the hydrolysis of the antibiotic itself (a consecutive reaction). This review focuses specially on aspects of the reactions' kinetics that may affect the performance of the enzymatic reactor.

Acetobacter turbidans alpha-amino acid ester hydrolase: how a single mutation improves an antibiotic-producing enzyme

The alpha-amino acid ester hydrolase (AEH) from Acetobacter turbidans is a bacterial enzyme catalyzing the hydrolysis and synthesis of beta-lactam antibiotics. The crystal structures of the native enzyme, both unliganded and in complex with the hydrolysis product D-phenylglycine are reported, as well as the structures of an inactive mutant (S205A) complexed with the substrate ampicillin, and an active site mutant (Y206A) with an increased tendency to catalyze antibiotic production rather than hydrolysis. The structure of the native enzyme shows an acyl binding pocket, in which D-phenylglycine binds, and an additional space that is large enough to accommodate the beta-lactam moiety of an antibiotic. In the S205A mutant, ampicillin binds in this pocket in a non-productive manner, making extensive contacts with the side chain of Tyr(112), which also participates in oxyanion hole formation. In the Y206A mutant, the Tyr(112) side chain has moved with its hydroxyl group toward the catalytic serine. Because this changes the properties of the beta-lactam binding site, this could explain the increased beta-lactam transferase activity of this mutant.

Improved β-lactam acylases and their use as industrial biocatalysts

Whereas the beta-lactam acylases are traditionally used for the hydrolytic processing of penicillin G and cephalosporin C, new and mutated acylases can be used for the hydrolysis of alternative fermentation products as well as for the synthesis of semisynthetic beta-lactam antibiotics. Three-dimensional structural analyses and site-directed mutagenesis studies have increased the understanding of the catalytic mechanism of these enzymes. The yield of hydrolysis and synthesis has been greatly improved by process design, including immobilization of the enzyme and the use of alternative reaction media. Significant advances have also been made in the resolution of racemic mixtures by means of stereoselective acylation/hydrolysis using beta-lactam acylases.