Stereospecific production of the herbicide phosphinothricin (glufosinate) by transamination: isolation and characterization of a phosphinothricin-specific transaminase from Escherichia coli

Enzymatic Synthesis of Biologically Active H-Phosphinic Analogue of α-Ketoglutarate

Amino acid analogues with a phosphorus-containing moiety replacing the carboxylic group are promising sources of biologically active compounds. The H-phosphinic group, with hydrogen-phosphorus-carbon (H-P-C) bonds and a flattened tetrahedral configuration, is a bioisostere of the carboxylic group. Consequently, amino-H-phosphinic acids undergo substrate-like enzymatic transformations, leading to new biologically active metabolites. Previous studies employing NMR-based metabolomic and proteomic analyses show that in Escherichia coli, α-KG-γ-PH (the distal H-phosphinic analogue of α-ketoglutarate) can be converted into L-Glu-γ-PH. Notably, α-KG-γ-PH and L-Glu-γ-PH are antibacterial compounds, but their intracellular targets only partially overlap. L-Glu-γ-PH is known to be a substrate of aspartate transaminase and glutamate decarboxylase, but its substrate properties with NAD+-dependent glutamate dehydrogenase (GDH) have never been investigated. Compounds containing P-H bonds are strong reducing agents; therefore, enzymatic NAD+-dependent oxidation is not self-evident. Herein, we demonstrate that L-Glu-γ-PH is a substrate of eukaryotic GDH and that the pH optimum of L-Glu-γ-PH NAD+-dependent oxidative deamination is shifted to a slightly alkaline pH range compared to L-glutamate. By 31P NMR, we observe that α-KG-γ-PH exists in a pH-dependent equilibrium of keto and germinal diol forms. Furthermore, the stereospecific enzymatic synthesis of α-KG-γ-PH from L-Glu-γ-PH using GDH is a possible route for its bio-based synthesis.

Biomanufacture of L-homoserine lactone building block: A strategy for preparing γ-substituted L-amino acids by modular reaction

A strain high-performance of esterase producing bacteria was screened from soil, which could selectively hydrolyze D-homoserine lactone from its racemate to achieve the resolution of L- homoserine lactone with more than 99% e.e. in 48% yield. L-homoserine lactone building block was then converted to L-α-amino-γ-bromobutyronic acid chiral blocks, which reacted with various nucleophilic reagent modules could to be applied to prepare L-γ- substituted α-amino acids such as L-selenomethionine, L-methionine, L-glufosinate and L-selenocystine. Its advantages included high selectivity of biocatalytic resolution reactions, high optical purity of products, racemic recycle of D-substrates and modular reaction, which simplified the production process of these products and highlighted the power of biological manufacturing.

A Desmethylphosphinothricin Dipeptide Derivative Effectively Inhibits Escherichia coli and Bacillus subtilis Growth

New antibiotics are unquestionably needed to fight the emergence and spread of multidrug-resistant bacteria. To date, antibiotics targeting bacterial central metabolism have been poorly investigated. By determining the minimal inhibitory concentration (MIC) of desmethylphosphinothricin (Glu-γ-PH), an analogue of glutamate with a phosphinic moiety replacing the γ-carboxyl group, we previously showed its promising antibacterial activity on Escherichia coli. Herein, we synthetized and determined the growth inhibition exerted on E. coli by an L-Leu dipeptide derivative of Glu-γ-PH (L-Leu-D,L-Glu-γ-PH). Furthermore, we compared the growth inhibition obtained with this dipeptide with that exerted by the free amino acid, i.e., Glu-γ-PH, and by their phosphonic and non-desmethylated analogues. All the tested compounds were more effective when assayed in a chemically-defined minimal medium. The dipeptide L-Leu-D,L-Glu-γ-PH had a significantly improved antibacterial activity (2 μg/mL), at a concentration between the non-desmethytaled (0.1 μg/mL) and the phosphonic (80 μg/mL) analogues. Also, in Bacillus subtilis, the dipeptide L-Leu-D,L-Glu-γ-PH displayed an activity comparable to that of the antibiotic amoxicillin. This work highlights the antibacterial relevance of the phosphinic pharmacophore and proposes new avenues for the development of novel antimicrobial drugs containing the phosphinic moiety.

A Single-Transaminase-Catalyzed Biocatalytic Cascade for Efficient Asymmetric Synthesis of l-Phosphinothricin

A single-transaminase-catalyzed biocatalytic cascade was developed by employing the desired biocatalyst, ATA-117-Rd11, that showed high activity toward 2-oxo-4-[(hydroxy)(methyl)phosphinoyl] butyric acid (PPO) and α-ketoglutarate, and low activity against pyruvate. The cascade successfully promotes a highly asymmetric amination reaction for the synthesis of l-phosphinothricin (l-PPT) with high conversion (>95 %) and>99 % ee. In a scale-up experiment, using 10 kg pre-frozen E. coli cells harboring ATA-117-Rd11 as catalyst, 80 kg PPO was converted to ≈70 kg l-PPT after 24 hours with a high ee value (>99 %).

Enzymatic kinetic resolution of desmethylphosphinothricin indicates that phosphinic group is a bioisostere of carboxyl group

Escherichia coli glutamate decarboxylase (EcGadB), a pyridoxal 5'-phosphate (PLP)-dependent enzyme, is highly specific for L-glutamate and was demonstrated to be effectively immobilised for the production of γ-aminobutyric acid (GABA), its decarboxylation product. Herein we show that EcGadB quantitatively decarboxylates the L-isomer of D,L-2-amino-4-(hydroxyphosphinyl)butyric acid (D,L-Glu-γ-P_H), a phosphinic analogue of glutamate containing C-P-H bonds. This yields 3-aminopropylphosphinic acid (GABA-P_H), a known GABA_B receptor agonist and provides previously unknown D-Glu-γ-P_H, allowing us to demonstrate that L-Glu-γ-P_H, but not D-Glu-γ-P_H, is responsible for D,L-Glu-γ-P_H antibacterial activity. Furthermore, using GABase, a preparation of GABA-transaminase and succinic semialdehyde dehydrogenase, we show that GABA-P_H is converted to 3-(hydroxyphosphinyl)propionic acid (Succinate-P_H). Hence, PLP-dependent and NADP⁺-dependent enzymes are herein shown to recognise and metabolise phosphinic compounds, leaving unaffected the P-H bond. We therefore suggest that the phosphinic group is a bioisostere of the carboxyl group and the metabolic transformations of phosphinic compounds may offer a ground for prodrug design.

Asymmetric synthesis of l-phosphinothricin using thermostable alpha-transaminase mined from Citrobacter koseri

α-Transaminase (α-TA) responsible for catalyzing the reversible transfer of amino groups between amine donors and amine acceptors, is applicable to enzymatic route for asymmetric synthesis of herbicide l-phosphinothricin (l-PPT). In the search for α-TAs with better catalysis performance, three α-TAs were discovered by genome mining approach using a known sequence encoding Escherichia coli tyrosine TA (TyrB) as probe. Through detailed comparison of their expression amount, activities and characteristics, Citrobacter koseri TA (CkTA) exhibited better activity and thermostability, which retain 65.9% of initial activity after incubation at 57 °C for 4 h. The K_m and k_cat/K_m values of CkTA were 36.75 mM and 34.29 mM^-1 min^-1, respectively. In addition, recombinant CkTA cells were immobilized onto Celite 545 using tris(hydroxymethyl)phosphine as crosslinker. During five repetitive asymmetric synthesis of l-PPT from 20 g/L prostereogenic ketone using l-Glu as amine donor, all the yields of l-PPT reached up to 91.2% (＞99% ee). These characteristics made CkTA a valuable addition to the currently scarce α-TA library for stereospecific synthesis of l-PPT.

Enzymatic asymmetric synthesis of chiral amino acids

This review summarizes the progress achieved in the enzymatic asymmetric synthesis of chiral amino acids from prochiral substrates.

Catabolism of Amino Acids and Related Compounds

This review considers the pathways for the degradation of amino acids and a few related compounds (agmatine, putrescine, ornithine, and aminobutyrate), along with their functions and regulation. Nitrogen limitation and an acidic environment are two physiological cues that regulate expression of several amino acid catabolic genes. The review considers Escherichia coli, Salmonella enterica serovar Typhimurium, and Klebsiella species. The latter is included because the pathways in Klebsiella species have often been thoroughly characterized and also because of interesting differences in pathway regulation. These organisms can essentially degrade all the protein amino acids, except for the three branched-chain amino acids. E. coli, Salmonella enterica serovar Typhimurium, and Klebsiella aerogenes can assimilate nitrogen from D- and L-alanine, arginine, asparagine, aspartate, glutamate, glutamine, glycine, proline, and D- and L-serine. There are species differences in the utilization of agmatine, citrulline, cysteine, histidine, the aromatic amino acids, and polyamines (putrescine and spermidine). Regardless of the pathway of glutamate synthesis, nitrogen source catabolism must generate ammonia for glutamine synthesis. Loss of glutamate synthase (glutamineoxoglutarate amidotransferase, or GOGAT) prevents utilization of many organic nitrogen sources. Mutations that create or increase a requirement for ammonia also prevent utilization of most organic nitrogen sources.

Bioinformatic analysis of a PLP-dependent enzyme superfamily suitable for biocatalytic applications

In this review we analyse structure/sequence-function relationships for the superfamily of PLP-dependent enzymes with special emphasis on class III transaminases. Amine transaminases are highly important for applications in biocatalysis in the synthesis of chiral amines. In addition, other enzyme activities such as racemases or decarboxylases are also discussed. The substrate scope and the ability to accept chemically different types of substrates are shown to be reflected in conserved patterns of amino acids around the active site. These findings are condensed in a sequence-function matrix, which facilitates annotation and identification of biocatalytically relevant enzymes and protein engineering thereof.

Ferrous and ferric ions-based high-throughput screening strategy for nitrile hydratase and amidase

Rapid and direct screening of nitrile-converting enzymes is of great importance in the development of industrial biocatalytic process for pharmaceuticals and fine chemicals. In this paper, a combination of ferrous and ferric ions was used to establish a novel colorimetric screening method for nitrile hydratase and amidase with α-amino nitriles and α-amino amides as substrates, respectively. Ferrous and ferric ions reacted sequentially with the cyanide dissociated spontaneously from α-amino nitrile solution, forming a characteristic deep blue precipitate. They were also sensitive to weak basicity due to the presence of amino amide, resulting in a yellow precipitate. When amino amide was further hydrolyzed to amino acid, it gave a light yellow solution. Mechanisms of color changes were further proposed. Using this method, two isolates with nitrile hydratase activity towards 2-amino-2,3-dimethyl butyronitrile, one strain capable of hydrating 2-amino-4-(hydroxymethyl phosphiny) butyronitrile and another microbe exhibiting amidase activity against 2-amino-4-methylsulfanyl butyrlamide were obtained from soil samples and culture collections of our laboratory. Versatility of this method enabled it the first direct and inexpensive high-throughput screening system for both nitrile hydratase and amidase.

Resistance to Phosphinothricin (Glufosinate) and Its Utilization as a Nitrogen Source by Chlamydomonas reinhardtii

Wild-type strain 21gr of the green alga Chlamydomonas reinhardtii was resistant to the ammonium salt of l-phosphinothricin (PPT, also called glufosinate), an irreversible inhibitor of glutamine synthetase activity and the main active component of the herbicide BASTA (AgrEvo, Frankfurt am Main, Germany). Under the same conditions, however, this strain was highly sensitive to l-methionine-S-sulfoximine, a structural analog of PPT which has been reported to be 5 to 10 times less effective than PPT as an inhibitor in plants. Moreover, this alga was able to grow with PPT as the sole nitrogen source when this compound was provided at low concentrations. This utilization of PPT was dependent upon the addition of acetate and light and did not take place in the presence of ammonium. Resistance was due neither to the presence of N-acetyltransferase or transaminase activity nor to the presence of glutamine synthetase isoforms resistant to PPT. By using l-[methyl-(sup14)C]PPT, we demonstrated that resistance is due to lack of PPT transport into the cells. This strongly suggests that PPT and l-methionine-S-sulfoximine enter the cells through different systems. Growth with PPT is supported by its deamination by an l-amino acid oxidase activity which has been previously described to be located at the periplasm.

Compensations for diminished terminal oxidase activity in Escherichia coli: cytochrome bd-II-mediated respiration and glutamate metabolism

Escherichia coli possesses cytochrome bo' (CyoABCDE), cytochrome bd-I (CydAB), and cytochrome bd-II (AppBC) quinol oxidases, all of which can catalyze the terminal step in the aerobic respiratory chain, the reduction of oxygen by ubiquinol. Although CydAB has a role in the generation of DeltapH, AppBC has been proposed to alleviate the accumulation of electrons in the quinone pool during respiratory stress via electroneutral ubiquinol oxidation. A cydB mutant strain exhibited lower respiration rates while maintaining a wild type growth rate. Transcriptomic analysis revealed a dramatic up-regulation of AppBC in the cydB strain, accompanied by the induction of genes involved in glutamate/gamma-aminobutyric acid (GABA) antiport, the GABA shunt, the glyoxylate shunt, respiration (including appBC), motility, and osmotic stress. Transcription factor modeling suggests that the underpinning regulation is largely controlled by H-NS, GadX, FlhDC, and AppY. The transcriptional adaptations imply that cydB cells contribute to the proton motive force via consumption of intracellular protons and glutamate/GABA antiport. Indeed, supplementation of culture medium with l-glutamate stimulates growth in a cydB strain. Phenotype analyses of the cydB strain confirm decreased motility and elevated acid resistance and also an elevated cytochrome d spectroscopic signal in cells grown at low pH. We propose a mechanism via which E. coli can compensate for the loss of cytochrome bd-I activity; cytochrome bd-II-mediated quinol oxidation prevents the accumulation of NADH, whereas GABA synthesis/antiport maintains the proton motive force for ATP production.

Novel biosynthetic approaches to the production of unnatural amino acids using transaminases

Transaminase enzymes are being increasingly applied to the large-scale synthesis of unnatural and nonproteinogenic amino acids. Typically displaying relaxed substrate specificity, rapid reaction rates and lacking the need for cofactor regeneration, they possess many characteristics that make them desirable as effective biocatalysts. By judiciously combining the transaminase reaction with additional enzymatic steps, this approach can be used very efficiently to prepare a broad range of D- and L-amino acids.

Stereospecific production of the herbicide phosphinothricin (glufosinate): purification of aspartate transaminase from Bacillus stearothermophilus, cloning of the corresponding gene, aspC, and application in a coupled transaminase process

We have isolated and characterized an aspartate transaminase (glutamate:oxalacetate transaminase, EC 2.6.1.1) from the thermophilic microorganism Bacillus stearothermophilus. The purified enzyme has a molecular mass of 40.5 kDa by sodium dodecyl sulfate gel analysis, a temperature optimum of 95 degrees C, and a pH optimum of 8.0. The corresponding gene, aspC, was cloned and overexpressed in Escherichia coli. The recombinant glutamate:oxalacetate transaminase protein was used in immobilized form together with 4-aminobutyrate:2-ketoglutarate transaminase (EC 2.6.1.19) from E. coli for the production of L-phosphinothricin [L-homoalanin-4-yl-(methyl)phosphinic acid], the active ingredient of the herbicide Basta (AgrEvo GmbH), from its nonchiral 2-keto acid precursor 2-oxo-4-[(hydroxy)(methyl)phosphinoyl]butyric acid (PPO). In this new coupled process conversion rates of ca. 85% were obtained with substrate solutions containing 10% PPO by using only slight excesses of the amino donors glutamate and aspartate. The contamination of the reaction broth with amino acid by-products was < 3%.

Glufosinate (phosphinothricin), a natural amino acid with unexpected herbicidal properties

Glufosinate ammonium (phosphinothricin ammonium) (GLA) is the active ingredient of Basta and several other herbicides used worldwide. It is produced as part of the tripeptide L-phosphinothricyl-L-alanyl-L-alanin, which was first isolated from Streptomyces viridichromogenes or Streptomyces hygroscopicus. Its structure is confirmed by degradation and synthesis. Several processes for the preparation of D,L- and L-phosphinothricin are described. Glufosinate is a structural analog of glutamate and inhibits the glutamine synthetase. The result is a rapid build-up of a high ammonia level and a concomitant depletion of glutamine and several other amino acids in the plant. These effects are accompanied by a rapid decline of photosynthetic CO2-fixation and are followed by chlorosis and desiccation. The results of numerous toxicological studies show that glufosinate ammonium and its commercial formulations are safe for users and consumers under the conditions of recommended use. The fast and complete degradation in soil and surface water prevents movement of residues into groundwater. The toxicological threshold levels for all the nontarget organisms tested are well above the potential exposure levels and therefore do not reflect any hazard for nontarget organisms in the ecosystem. Basta is a nonselective foliar applied herbicide for the control of undesirable mono- and dicotyledonous plants in orchards, vineyards, and plantations for minimum tillage, and as a harvest aid. A synthetic phosphinothricin acetyltransferase (PAT) gene has been introduced via Agrobacterium tumefaciens into dicot crops, such as like tobacco, tomato, spring and winter rapeseed, alfalfa, and several horticultural crops. The PAT gene was also successfully introduced into maize protoplasts that could be regenerated into fertile plants. All transgenic crop plants tolerated a two- to threefold field dosage of Basta.

Molecular organization of the Escherichia coli gab cluster: nucleotide sequence of the structural genes gabD and gabP and expression of the GABA permease gene

We have determined the nucleotide sequences of two structural genes of the Escherichia coli gab cluster, which encodes the enzymes of the 4-aminobutyrate degradation pathway: gabD, coding for succinic semialdehyde dehydrogenase (SSDH, EC 1.2.1.16) and gabP, coding for the 4-aminobutyrate (GABA) transport carrier (GABA permease). We have previously reported the nucleotide sequence of the third structural gene of the cluster, gabT, coding for glutamate: succinic semialdehyde transaminase (EC 2.6.1.19). All three gab genes are transcribed unidirectionally and their orientation within the cluster is 5'-gabD-gabT-gabP-3'. gabT and gabP are separated by an intergenic region of 234-bp, which contains three repetitive extragenic palindromic (REP) sequences. The gabD gene consists of 1,449 nucleotides specifying a protein of 482 amino acids with a molecular mass of 51.7 kDa. The protein shows significant homologies to the NAD(+)-dependent aldehyde dehydrogenase (EC 1.2.1.3) from Aspergillus nidulans and several mammals, and to the tumor associated NADP(+)-dependent aldehyde dehydrogenase (EC 1.2.1.4) from rat. The permease gene gabP comprises 1,401 nucleotides coding a highly hydrophobic protein of 466 amino acids with a molecular mass of 51.1 kDa. The GABA permease shows features typical for an integral membrane protein and is highly homologous to the aromatic acid carrier from E. coli, the proline, arginine and histidine permeases from Saccharomyces cerevisiae and the proline transport protein from A. nidulans. Uptake of GABA was increased ca. 5-fold in transformants of E. coli containing gabP plasmids.(ABSTRACT TRUNCATED AT 250 WORDS)

Transgenic plants containing the phosphinothricin-N-acetyltransferase gene metabolize the herbicide L-phosphinothricin (glufosinate) differently from untransformed plants

L-Phosphinothricin (L-Pt)-resistant plants were constructed by introducing a modified phosphinothricin-N-acetyl-transferase gene (pat) via Agrobacterium-mediated gene transfer into tobacco (Nicotiana tabacum L), and via direct gene transfer into carrot (Daucus carota L). The metabolism of L-Pt was studied in these transgenic, Pt-resistant plants, as well as in the untransformed species. The degradation of L-Pt, (14)C-labeled specifically at different C-atoms, was analysed by measuring the release of (14)CO2 and by separating the labeled degradation products on thin-layer-chromatography plates. In untransformed tobacco and carrot plants, L-Pt was deaminated to form its corresponding oxo acid 4-methylphosphinico-2-oxo-butanoic acid (PPO), which subsequently was decarboxylated to form 3-methylphosphinico-propanoic acid (MPP). This compound was stable in plants. A third metabolite remained unidentified. The L-Pt was rapidly N-acetylated in herbicide-resistant tobacco and carrot plants, indicating that the degradation pathway of L-Pt into PPO and MPP was blocked. The N-acetylated product, L-N-acetyl-Pt remained stable with regard to degradation, but was found to exist in a second modified form. In addition, there was a pH-dependent, reversible change in the mobility of L-N-acetyl-Pt thin-layer during chromatography.

Molecular analysis of two genes of the Escherichia coli gab cluster: nucleotide sequence of the glutamate:succinic semialdehyde transaminase gene (gabT) and characterization of the succinic semialdehyde dehydrogenase gene (gabD)

We have characterized two genes of the Escherichia coli K-12 gab cluster, which encodes the enzymes of the 4-aminobutyrate degradation pathway. The nucleotide sequence of gabT, coding for glutamate:succinic semialdehyde transaminase (EC 2.6.1.19), alternatively known as 4-aminobutyrate transaminase, was determined. The structural gene consists of 1,281 nucleotides specifying a protein of 426 amino acids with a molecular mass of 45.76 kDa. The protein shows significant homologies to the ornithine transaminases from Saccharomyces cerevisiae and from rat and human mitochondria. Three functionally and structurally important amino acid residues of the transaminase were identified by sequence comparison studies, and evolutionary relationships of the aminotransferases are discussed. The gabD gene, encoding succinic semialdehyde dehydrogenase (EC 1.2.1.16), was cloned and shown to be located adjacent to the 5' end of gabT. Expression studies with subfragments of the initially cloned DNA region revealed a maximal size of 1.7 kb for gabD. Both genes are cotranscribed from a promoter located upstream of gabD.

Stereospecific production of the herbicide phosphinothricin (glufosinate) by transamination: cloning, characterization, and overexpression of the gene encoding a phosphinothricin-specific transaminase from Escherichia coli

We have cloned the gene encoding a 43-kilodalton transaminase from Escherichia coli K-12 with a specificity for L-phosphinothricin [L-homoalanine-4-yl-(methyl)phosphinic acid], the active ingredient of the herbicide Basta (Hoechst AG). The structural gene was isolated, together with its own promoter, and shown to be localized on a 1.6-kilobase DraI-BamHI fragment. The gene is subject to catabolite repression by glucose; however, repression could be relieved completely when 4-aminobutyrate (GABA) served as the sole nitrogen source. The regulation pattern obtained and a comparison of the restriction map of the initially cloned 15-kilobase SalI fragment with the physical map of the E. coli K-12 genome suggest that the cloned gene is identical with gabT, a locus on the gab gene cluster of E. coli K-12 which codes for the GABA:2-ketoglutartate transaminase (EC 2.6.1.19). A number of expression plasmids carrying the isolated transaminase gene were constructed. With these constructs, the transaminase expression in transformants of E. coli could be increased up to 80-fold compared with that in a wild-type control, and the transaminase constituted up to 20% of the total soluble protein of the bacteria. Thus, the protein crude extracts of the transformants could be used, after a simple heat precipitation step, for the biotechnological production of L-phosphinothricin in an enzyme reactor.