Patent Publication Number: US-2022220516-A1

Title: Method for asymmetrically preparing l-phosphinothricin by oxidation-reduction reaction through biological multi-enzyme coupling

Description:
FIELD OF TECHNOLOGY 
     The present invention relates to the field of biotechnology, in particular to a method for asymmetrically preparing L-phosphinothricin by oxidation-reduction reaction through biological multi-enzyme coupling. 
     BACKGROUND TECHNOLOGY 
     Phosphinothricin (PPT for short), also known as glufosinate ammonium, with a chemical name of 2-amino-4-[hydroxy(methyl)phosphono]butyric acid, is the second largest herbicide tolerant for transgenic crops in the world and was developed and produced by Hoechst (which is now owned by Bayer after several mergers). Phosphinothricin, a phosphonate herbicide, is a glutamine synthetase inhibitor and a non-selective (biocidal) contact herbicide. 
     It is well known that the market for non-selective biocides is huge. At present, the three major herbicides in the world include paraquat, glyphosate, and phosphinothricin. In terms of market share, glyphosate ranks the first, but due to its long-term use, a large number of weeds have become resistant against it, and glyphosate tends to lose efficacy. Paraquat has been listed under the Rotterdam Convention due to its high toxicity and is banned or restricted in an increasing number of countries worldwide; and the Ministry of Agriculture of China issued a notice stating that paraquat production would be ceased as of 1 Jul. 2014 and banned as of 1 Jul. 2016. At present, the output of phosphinothricin is small yield, but phosphinothricin exhibits excellent herbicidal performance and less phytotoxicity and side effects, therefore, it has great market potential in the future. 
     Phosphinothricin consists of two optical isomers, L-phosphinothricin and D-phosphinothricin. However, only L-phosphinothricin has herbicidal activity, and it is easily decomposed in the soil, with less toxicity to humans and animals, and has a wide herbicidal spectrum, and small destructive effect on the environment. 
     Phosphinothricin currently available on the market is generally a racemic mixture. If the phosphinothricin product can be used as a pure optical isomer in the L-configuration, the consumption of phosphinothricin can be remarkably reduced, which is of great significance for improving atomic economy, reducing use cost and reducing environmental pressure. 
     There are three main methods for preparing chiral pure L-phosphinothricin: chiral resolution method, chemical synthesis method and bio-catalysis method. 
     The chiral resolution method is to separate the D-form and L-form isomers by chiral resolution of racemic D,L-phosphinothricin or its derivatives, thus obtaining the optically pure L-phosphinothricin. This process mainly has the following shortcomings: expensive chiral resolution reagents are needed, the theoretical yield can only reach 50%, the single resolution rate is low, and the process is relatively complex. 
     The chemical synthesis method is to synthesize optically pure L-phosphinothricin from chiral raw materials. The chemical asymmetric synthesis method has many process steps, low yield and high production cost due to expensive chiral raw materials, which are not conducive to large-scale preparation of L-phosphinothricin. 
     The bio-catalysis method for the production of phosphinothricin has the advantages of strict stereoselectivity, mild reaction conditions, high yield and the like, and is an advantageous method for producing L-phosphinothricin. It mainly includes the following three categories: 
     (1) With L-phosphinothricin derivatives as substrates, L-phosphinothricin is obtained through direct hydrolysis by an enzyme method. Its main advantages include high conversion rate and high ee value of the product; however, expensive and difficult-to-obtain chiral raw materials are needed as precursors. 
     (2) With racemic phosphinothricin precursor as the substrate, L-phosphinothricin is obtained by selective resolution with enzymes. Its main advantages are that raw materials are relatively easy to obtain, and the activity of the catalyst is high, but its theoretical yield can only reach 50%, thereby causing waste of raw materials. 
     (3) With D,L-phosphinothricin as the raw material, D-phosphinothricin is catalyzed by D-amino acid oxidase to obtain L-phosphinothricin precursor 2-carbonyl-4-[hydroxy(methyl)phosphono]butyric acid (PPO for short), which is then catalyzed with transaminase to obtain L-phosphinothricin. 
     The decomposition of L-phosphinothricin into 2-carbonyl-4-[hydroxy(methyl)phosphono]butyric acid has been found as early as in the study of the metabolic pathway of phosphinothricin in soil microorganisms. Therefore, it is a good method to produce L-phosphinothricin by reversible enzyme catalysis using 2-carbonyl-4-[hydroxy(methyl)phosphono]butyric acid as the substrate. 
     D-amino acid oxidase is a kind of enzyme that specifically and selectively catalyzes D-amino acid and its derivatives to generate α-keto acid, and the reaction is catalyzed and completed by its own coenzyme FAD. Due to its excellent catalytic efficiency and selectivity, D-amino acid oxidase is widely used in the production of L-amino acid and α-keto acid through biological resolution. For example, D-amino acid oxidase converts cephalosporin C to glutaryl-7-aminocephalosporanic acid. 
     Transaminases are pyridoxal phosphate (PLP)-dependent enzymes which are transferases that catalyze the exchange of amino and keto groups. They widely exist in nature and play an important role in amino transfer in the process of nitrogen metabolism in cells. Transaminases have many advantages, such as high enantioselectivity and regioselectivity, high reaction rate and stability. Transamination is a reversible reaction, and 2-carbonyl-4-[hydroxy(methyl)phosphono]butyric acid can generate L-phosphinothricin through a reverse reaction as catalyzed by transaminases. 
     SUMMARY OF THE INVENTION 
     In view of the defects in the prior art, the present invention provides a brand-new method for preparing L-phosphinothricin by resolving the racemic mixture. Meanwhile, the present invention provides a D-amino acid oxidase mutant which has high activity of catalyzing D-phosphinothricin to react to generate 2-carbonyl-4-[hydroxy(methyl)phosphono]butyric acid. 
     The present invention provides a method for asymmetrically preparing L-phosphinothricin by oxidation-reduction reaction through biological multi-enzyme coupling, where D,L-phosphinothricin as a raw material is catalyzed by an enzyme catalysis system to obtain L-phosphinothricin, wherein the enzyme catalysis system comprises a D-amino acid oxidase mutant for catalyzing D-phosphinothricin in D,L-phosphinothricin into 2-carbonyL-4-[hydroxy(methyl)phosphono]butyric acid and a transaminase for catalytic reduction of the 2-carbonyl-4-[hydroxy(methyl)phosphono]butyric acid into L-phosphinothricin, and 
     The D-amino acid oxidase mutant is obtained by mutation of D-amino acid oxidase in wild strain  Rhodotorula taiwanensis  at one of the following four sites: 
     (1) M213S; 
     (2) M213S-N54V-F58E; 
     (3) M213S-N54V-F58E-D207A; 
     (4) M213S -N54V-F58E-D207A-S60T. 
     The specific principles are as follows. Using a one-pot multi-enzyme catalytic system, with racemic D,L-phosphinothricin as a substrate, D-phosphinothricin is catalyzed into 2-carbonyL-4-[hydroxy(methyl)phosphono]butyric acid by D-amino acid oxidase. Meanwhile, the catalase added is used for removing by-product hydrogen peroxide, because accumulation of hydrogen peroxide can poison the catalyst. L-phosphinothricin does not participate in the reaction and is completely retained. Then, the 2-carbonyl-4-[hydroxy(methyl)phosphono]butyric acid is reduced into L-phosphinothricin as catalyzed by transaminases, thereby realizing in-situ racemization of D,L-phosphinothricin to obtain optically pure L-phosphinothricin. Pyridoxal phosphate is used as coenzyme and amino donor as co-substrate for transaminase catalysis. 
     Transaminases in the present application may employ sequences conventional in the art. Preferably, the amino acid sequence of the transaminase is as shown in SEQ ID No. 7. 
     For catalyzing D-phosphinothricin to react to generate 2-carbonyl-4-[hydroxy(methyl)phosphono]butyric acid, the expressed crude enzyme liquid or the expressed purified enzyme liquid can be used, and the catalytic reaction can also be directly carried out by using recombinant bacteria capable of expressing the D-amino acid oxidase mutant. Similarly, for the transaminase, the expressed crude enzyme liquid, the expressed purified pure enzyme liquid can be used, or recombinant bacteria capable of expressing the transaminase can be directly used. 
     More preferably, the D-amino acid oxidase mutant is obtained by adding a recombinant bacterium expressing the D-amino acid oxidase mutant in a reaction system; 
     The transaminase is obtained by adding a recombinant bacterium expressing the transaminase together with a coenzyme pyridoxal phosphate into the reaction system. 
     More preferably,  E. coli  BL21(DE3) is used as a host cell for both of the recombinant bacteria. 
     More preferably, in the reaction system, the D,L-phosphinothricin has a final concentration of 100-400 mM, the recombinant bacterium expressing the D-amino acid oxidase mutant is added at an amount of 20-40 g/L, the catalase has a concentration of 0.1 g/L, the recombinant bacterium expressing the transaminase is added at an amount of 30-50 g/L, and the coenzyme pyridoxal phosphate has a concentration of 1 mM. The reaction is carried out at a temperature of 30° C. and a pH of 8 for 10 hours. 
     After the reaction is completed, the final product L-phosphinothricin is separated and extracted by pretreatment-ion exchange-crystallization. 
     The present invention further provides a D-amino acid oxidase mutant obtained by mutation of D-amino acid oxidase in wild-type  Rhodotorula taiwanensis  at one of the following four sites: 
     (1) M213S; 
     (2) M213S-N54V-F58E; 
     (3) M2135-N54V-F58E-D207A; 
     (4) M213S-N54V-F58E-D207A-S60T. 
     The present invention also provides a gene encoding the D-amino acid oxidase mutant, wherein the gene has a nucleotide sequence as shown in one of SEQ ID No. 3 to 6. 
     The present invention also provides a recombinant bacterium comprising the gene. 
     Compared with the prior art, the present invention has the following beneficial effects: 
     (1) The D-amino acid oxidase mutant of the present invention has better catalytic efficiency, and when racemic D,L-phosphinothricin is used as a substrate for catalytic reaction, the conversion rate is much higher than that of the wild type enzyme, and the yield of PPO is also greatly improved. 
     (2) In the present invention, with racemic D,L-phosphinothricin as a substrate, D-phosphinothricin is oxidized into 2-carbonyl-4-[hydroxy(methyl)phosphono]butyric acid by using D-amino acid oxidase, and L-phosphinothricin is completely reserved because it does not participate in the reaction; the product 2-carbonyl-4-[hydroxy(methyl)phosphono]butyric acid can be further reduced into L-phosphinothricin as catalyzed by transaminases, thereby realizing in-situ racemization of D,L-phosphinothricin. On the contrary, the traditional oxidation method needs to convert both D-phosphinothricin and L-phosphinothricin into 2-carbonyl-4-[hydroxy(methyl)phosphono]butyric acid, causing the waste of raw materials. 
     (3) The present invention can directly take D,L-phosphinothricin as a substrate for resolution, does not require expensive resolution reagents, does not need to synthesize phosphinothricin derivatives, and does not need to carry out the steps of separation, re-racemization, re-resolution and the like on the D-phosphinothricin. 
     (4) The method of the present invention overcomes the defect in synthesis of the L-phosphinothricin precursor 2-carbonyl-4-[hydroxy(methyl)phosphono]butyric acid by the chemical method, is a green, environment-friendly and low-carbon process route, and is suitable for large-scale industrial production and application. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a reaction formula for producing L-phosphinothricin by resolution with the multi-enzyme system adopted in the method of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Reagents for upstream genetic engineering: the genome extraction kit, plasmid extraction kit, and DNA purification and recovery kit used in the examples of the present invention were purchased from Corning Life Sciences (Wujiang) Co., Ltd.;  E. coli  DH5α,  E. coli  BL21 (DE3), plasmid pET-24a(+) were purchased from Shanghai Xuguan Biotechnology Development Co., Ltd.; DNA marker, low-molecular-weight standard protein, and protein gel were purchased from Beijing GenStar Co., Ltd.; and the primer synthesis and sequencing were completed by Hangzhou TSINGKE Biological Technology Co., Ltd. For the use of the above reagents, refer to the corresponding product instructions. 
     The reagent used in the downstream catalytic process, 2-carbonyl-4-[hydroxy(methyl)phosphono]butyric acid (PPO for short), was synthesized in the laboratory; D,L-phosphinothricin was purchased from Sigma-Aldrich; other commonly used reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. 
     The structural formula of D-phosphinothricin (D-PPT for short) is as shown in Formula (1); the structural formula of L-phosphinothricin (L-PPT for short) is as shown in Formula (2); the structural formula of 2-carbonyl-4-[hydroxy(methyl)phosphono]butyric acid (PPO for short) is as shown in Formula (3). 
     
       
         
         
             
             
         
       
     
     The reaction formula for D-phosphinothricin to produce 2-carbonyl-4-[hydroxy(methyl)phosphono]butyric acid, ammonia and hydrogen peroxide as catalyzed by D-amino acid oxidase is as shown in  FIG. 1 . 
     The present invention detected the progress of the reaction and analyzed the product by high performance liquid chromatography (HPLC). HPLC analysis: chromatographic column/AQ-C18; column temperature/30° C.; flow rate/1 mL/min; detection wavelength/205 nm; mobile phase: 50 mM (NH 4 ) 2 HPO 4 , to which 1% of 10% tetrabutyl ammonium bromide in water was added, pH adjusted to 3.8 with phosphoric acid, and 12% of acetonitrile was added. 
     The content of phosphinothricin of either configuration was examined by chiral HPLC analysis which comprised: chromatographic column//Pntulips QS-C18; mobile phase/50 mM ammonium acetate solution:methanol=9:1; detection wavelength/338 nm; flow rate/1 mL/min; column temperature/30° C. Derivatization reagent: 0.1 g of o-phthalaldehyde and 0.12 g GN-acetyl-L-cysteine were separately weighed and 10 ml of ethanol was added to facilitate dissolving, followed by addition of 40 mL of 0.1 M boric acid buffer (pH 9.8). The mixture was shaken to fully dissolve, and stored in a refrigerator at 4° C. for later use (not more than 3 days). Derivatization reaction and determination: 200 μL of the sample was mixed with 400 μL of the derivatization reagent at 30° C. for 5 minutes, and then mixed with 400 μL of ultra-pure water. Then, 10 μL of the sample was injected for analysis. 
     Example 1 
     Construction and Screening of D-Amino Acid Oxidase Mutant Library 
     1. Construction of Recombinant Bacteria 
     The gene sequence of D-amino acid oxidase (Gen Bank No.: POY70719.1) derived from  Rhodotorula taiwanensis  was sent to Sangon Biotech (Shanghai) Co., Ltd. for whole-gene synthesis after codon optimization and cloned into the recombinant expression plasmid pET-24a(+). The recombinant plasmid was transferred to the expression host  E. coli  BL21 (DE3) after sequencing verification, for subsequent use in expression of recombinant D-amino acid oxidase. The gene sequence of the D-amino acid oxidase after codon optimization is as shown in SEQ ID No. 2. 
     2. Construction of D-Amino Acid Oxidase Mutant Library 
     In the first round, with the D-amino acid oxidase gene with optimized codons obtained by the above whole gene synthesis as a template, and using the primers used for mutations of M213R and M213S in Table 1 respectively, site-specific mutation PCR was carried out, followed by transformation and plating. The dominant strain obtained by screening was a mutant with M213S mutation, and the plasmid of the dominant mutant was named as D-amino acid oxidase mutant pRtDAAO-M213S. 
     In the second round, with the mutant pRtDAAO-M213S as a template, and using the primers for mutations of N54G, N54L, N54V and N54A in Table 1 respectively, site-directed mutation PCR was carried out, followed by transformation and plating. The dominant strain obtained through screening was a mutant with double mutations of M213S and N54V, and the plasmid of the dominant mutant was named as D-amino acid oxidase mutant pRtDAAO-M213S-N54V. 
     In the third round, with the mutant pRtDAAO-M213R-N54V as a template, and using the primers used for mutations of F58E, F58A, F58R and F58S in Table 1 respectively, site-directed mutation PCR was carried out, followed by transformation and plating. The dominant strain obtained by screening was a mutant with three mutations of M213S, N54V and F58E, and the plasmid of the dominant mutant was named as D-amino acid oxidase mutant pRtDAAO-M213S-N54V-F58E. 
     In the fourth round, with the mutant pRtDAAO-M213S-N54V-F58E as a template, using the primers used for mutations of D207A, D207T and D207E in Table 1 respectively, site-directed mutation PCR was carried out, followed by transformation and plating. The dominant strain obtained by screening carried four mutations of M213S, N54V, F58E and D207A, and the plasmid of the dominant mutant was named as D-amino acid oxidase mutant pRtDAAO-M213R-N54V-F58E-D207A. 
     In the fifth round, with the mutant pRtDAAO-M213S-N54V-F58E-D207A as a template, and using the primers used for mutations of S60A, S60E and S60T in Table 1 respectively, site-directed mutation PCR was carried out, followed by transformation and plating. The dominant strains obtained by screening carried five mutations of M213S, N54V, F58E, D207A and S60T, and the plasmid of the dominant mutant was named as D-amino acid oxidase mutant pRtDAAO-M213S-N54V-F58E-D207A-S60T. The dominant single mutants in the later experiments were all constructed by the same method. 
     Specifically, the PCR reaction system is as follow: 
     2×Phanta Max buffer: 25 μL; 
     dNTPs: 1 μL; 
     Upstream primer: 1 μL; 
     Downstream primer: 1 μL; 
     Template: 1 μL; 
     Phanta Super-Fidelity DNA polymerase: 0.5 μL; 
     ddH 2 O: 20.5 μL. 
     PCR reaction conditions: pre-denaturation at 95° C. for 5 minutes; denaturation at 95° C. for 15 seconds, annealing at 56° C. for 30 seconds, and elongation at 72° C. for 6 minutes, with a total of 30 cycles; post-elongation at 72° C. for 10 minutes; storage at 4° C. 
     DNA agarose gel electrophoresis was performed for positive verification of PCR results, and the results showed that the amplified products were single bands, with the size of about 1300 bp, respectively. The PCR product was treated with Dpn I enzyme to digest the template, and the amplification product was purified and recovered by a DNA recovery and purification kit. For the specific steps, refer to the purification kit instructions. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Primer 
                   
               
               
                 Mutation 
                 name 
                 Primer sequence (5′-3′) 
               
               
                   
               
             
            
               
                 M213S 
                 M213S-Pf 
                 CGTTGCACC TCT GACAGCAGCGATCCGAAC 
               
               
                   
                 M213S-Pr 
                 CGCTGCTGTC AGA GGTGCAACGTTTGCA 
               
               
                   
               
               
                 M213R 
                 M213R-Pf 
                 CGTTGCACC CGT GACAGCAGCGATCCGAAC 
               
               
                   
                 M213R-Pr 
                 CGCTGCTGTC ACG GGTGCAACGTTTGCA 
               
               
                   
               
               
                 N54V 
                 N54V-Pf 
                 CGGGTGCG GTT TGGACCCCGGAAATGAGCA 
               
               
                   
                   
                 AGGAA 
               
               
                   
                 N54V-Pr 
                 ATTTCCGGGGTCCA AAC CGCACCCGCCCAC 
               
               
                   
                   
                 GGGCT 
               
               
                   
               
               
                 N54G 
                 N54G-Pf 
                 CGGGTGCG GGT TGGACCCCGGAAATGAGCA 
               
               
                   
                   
                 AGGAA 
               
               
                   
                 N54G-Pr 
                 ATTTCCGGGGTCCA ACC CGCACCCGCCCAC 
               
               
                   
                   
                 GGGCT 
               
               
                   
               
               
                 N54L 
                 N54L-Pf 
                 CGGGTGCG CTT TGGACCCCGGAAATGAGCA 
               
               
                   
                   
                 AGGAA 
               
               
                   
                 N54L-Pr 
                 ATTTCCGGGGTCCA AAG CGCACCCGCCCAC 
               
               
                   
                   
                 GGGCT 
               
               
                   
               
               
                 N54A 
                 N54A-Pf 
                 CGGGTGCG GCT TGGACCCCGGAAATGAGCA 
               
               
                   
                   
                 AGGAA 
               
               
                   
                 N54A-Pr 
                 ATTTCCGGGGTCCA AGC CGCACCCGCCCAC 
               
               
                   
                   
                 GGGCT 
               
               
                   
               
               
                 F58E 
                 F58E-Pf 
                 GACCCCG GAA ATGAGCAAGGAAGACGG 
               
               
                   
                 F58E-Pr 
                 CTTGCTCAT TTC CGGGGTCCAAACCGC 
               
               
                   
               
               
                 F58A 
                 F58A-Pf 
                 GACCCCG GCT ATGAGCAAGGAAGACGG 
               
               
                   
                 F58A-Pr 
                 CTTGCTCAT AGC CGGGGTCCAAACCGC 
               
               
                   
               
               
                 F58R 
                 F58R-Pf 
                 GACCCCG CGT ATGAGCAAGGAAGACGG 
               
               
                   
                 F58R-Pr 
                 CTTGCTCAT ACG CGGGGTCCAAACCGC 
               
               
                   
               
               
                 F58S 
                 F58S-Pf 
                 GACCCCG TCG ATGAGCAAGGAAGACGG 
               
               
                   
                 F58S-Pr 
                 CTTGCTCAT CGA CGGGGTCCAAACCGC 
               
               
                   
               
               
                 D207A 
                 D207A-Pf 
                 GTGAAGAGC GCT TGCAAACGTTGCACCTCT 
               
               
                   
                 D207A-Pr 
                 CAACGTTTGCA AGC GCTCTTCACCAGAAC 
               
               
                   
               
               
                 D207T 
                 D207T-Pf 
                 GTGAAGAGC ACG TGCAAACGTTGCACCTCT 
               
               
                   
                 D207T-Pr 
                 CAACGTTTGCA CGT GCTCTTCACCAGAAC 
               
               
                   
               
               
                 D207E 
                 D207E-Pf 
                 GTGAAGAGC GAG TGCAAACGTTGCACCTCT 
               
               
                   
                 D207E-Pr 
                 CAACGTTTGCA CTC GCTCTTCACCAGAAC 
               
               
                   
               
               
                 S60T 
                 S60T-Pf 
                 CCGGAAATG ACT AAGGAAGACGGTCCGCGT 
               
               
                   
                 S60T-Pr 
                 GTCTTCCTT AGT CATTTCCGGGGTCCAAAC 
               
               
                   
               
               
                 S60A 
                 S60A-Pf 
                 CCGGAAATG GCT AAGGAAGACGGTCCGCGT 
               
               
                   
                 S60A-Pr 
                 GTCTTCCTT AGC CATTTCCGGGGTCCAAAC 
               
               
                   
               
               
                 S60E 
                 S60E-Pf 
                 CCGGAAATG GAG AAGGAAGACGGTCCGCGT 
               
               
                   
                 S60E-Pr 
                 GTCTTCCTT CTC CATTTCCGGGGTCCAAAC 
               
               
                   
               
            
           
         
       
     
     Example 2 
     Construction of Recombinant Bacterium Expressing Transaminase 
     1. Amplification of Target Gene Transaminase 
     The transaminase gene was cloned from the genome of  Pseudomonas  sp. and the corresponding PCR upstream and downstream primers were designed based on the corresponding genomic DNA sequence (GenBank Accession No.: WP_076423369.1). 
     
       
         
           
               
               
            
               
                   
                 Upstream primer: ATGAACACCAACAACGCTC 
               
               
                   
                   
               
               
                   
                 Downstream Primer: TTAAGCCTGTTTAGCTTC 
               
            
           
         
       
     
     PCR amplification system: 
     2×Phanta Max buffer: 25 μL; 
     dNTPs: 1 μL; 
     Upstream primer: 1 μL; 
     Downstream primer: 1 μL; 
     Template: 1 μL; 
     Phanta Super-Fidelity DNA polymerase: 0.5 μL; 
     ddH 2 O: 20.5 μL. 
     PCR reaction conditions: pre-denaturation at 95° C. for 5 minutes; denaturation at 95° C. for 30 seconds, annealing at 60° C. for 30 seconds, and elongation at 72° C. for 6 minutes, with a total of 30 cycles; post-elongation at 72° C. for 10 minutes; storage at 4° C. 
     DNA agarose gel electrophoresis was performed for positive verification of PCR results, and the results showed that the amplified products were single bands, with the size of about 1300 bp, respectively. The PCR product was treated with Dpn I enzyme to digest the template, and the amplification product was purified and recovered by a DNA recovery and purification kit. For the specific steps, references were made to the purification kit instructions. 
     2. Construction of Expression Vector and Engineered Bacteria 
     The expression vector pET-28a(+) and PCR amplification products were double-cleaved with the corresponding restriction enzymes, respectively, and the cleaved products were purified and recovered by a DNA purification kit after enzyme cleavage to remove the restriction enzymes and the digested nucleotide small fragments. The PCR amplification product after double enzyme digestion was connected to expression vector pET-28a(+) with a corresponding incision by T4 DNA ligase to construct expression vector pET-28a(+)-gabT. The constructed expression vector was transformed into  E. coli  BL21 (DE3), coated on an LB plate containing 50 mg/ml kanamycin resistance, and cultured at 37° C. for 8-12 hours. The cloned and extracted plasmids were randomly selected and sequenced for identification, and the recombinant  E. coli  BL21 (DE)/pET-28a(+)-gabT expressing the recombinant plasmid pET-28a (+)-gabT was screened. 
     Example 3 
     Culture of Microorganisms 
     1. Culture of Bacterial Cells 
     Engineered bacteria containing D-amino acid oxidase gene and transaminase gene were activated by plate streaking, and single colonies were selected and inoculated into 10 mL of LB liquid medium containing 50 μg/mL kanamycin, and cultured under shaking at 37° C. for 10 hours. The samples were transferred at an inoculation amount of 2% to 50 mL of LB liquid medium also containing 50 μg/mL kanamycin, and cultured under shaking at 37° C. until OD 600  reached about 0.8. Thereafter, IPTG with a final concentration of 0.5 mM was added and the mixture was cultured under shaking at 28° C. for 12 hours. After culture, the culture solution was centrifuged at 8,000 rpm for 10 minutes, the supernatant was discarded, and the cells were collected and stored in an ultra-low temperature refrigerator at −80° C. for later use. 
     2. Preparation of Crude Enzyme Liquid 
     The collected bacterial cells after culture were washed twice with phosphate buffer (50 mM) at pH 8, and then the cells were re-suspended in PBS (50 mM) with pH=8 and ultrasonicated for 30 times under the conditions of a powder of 400 W for 2 seconds with an internal of 5 seconds. The fragmented cell suspension was centrifuged a 4° C. and 8,000 rpm for 10 minutes, the precipitate was removed, and the resultant supernatant was the crude enzyme liquid. 
     3. Purification of Enzyme 
     The crude enzyme liquid was combined with Ni affinity chromatography resin balanced with a loading buffer (50 mM phosphate buffer with pH=8, containing 500 mM NaCl and 20 mM imidazole), then rinsed with a rinsing buffer (50 mM phosphate buffer with pH=8, containing 50 mM imidazole and 500 mM NaCl) until essentially free of foreign proteins, and subsequently eluted with an elution buffer (50 mM phosphate buffer with pH=8, containing 200 mM imidazole and 500 mM NaCl), and the target proteins were collected. After purity identification by electrophoresis, the target proteins were combined and dialyzed against a dialysis buffer (50 mM phosphate buffer with pH=8) for 24 hours. The trapped solution had a protein content of 2.7 mg/mL as determined by the Coomassie brilliant blue method, and the enzyme liquid was diluted to a final concentration of 0.5 mg/mL, sub-packaged and cryopreserved at −80° C., thereby obtaining the recombinant pure enzyme. 
     Each of the D-amino acid oxidase mutants was also prepared as described above. 
     Example 4 
     Determination of D-Amino Acid Oxidase Activity 
     Definition of enzyme activity: According to the regulation of the 1961 International Enzymology Conference, one enzyme activity unit refers to the amount of enzyme that transforms one micromolar of substrate or one micromolar of related groups in the transformed substrate within one minute under specific conditions (30° C.). 
     Determination of enzyme activity of D-amino acid oxidase: 400 μl of a substrate solution (50 mM D, L-phosphinothricin) dissolved in 50 mM phosphate buffer was placed in a metal bath oscillator and kept at 30° C. for 10 minutes. Then 50 μl of pure enzyme and 0.25 μl of catalase (Sigma-Aldrich, Art. No. 60634) were added, and timing was started. The reaction was carried out at 30° C. for 10 minutes, 5 μL of 6 M hydrochloric acid was added thereto, the mixture was taken out, shaken and mixed evenly, and then the reaction was terminated. The mixture was centrifuged at 12,000 rpm for 3 min and the supernatant was 2-fold diluted with deionized water for HPLC detection. Enzyme activity was calculated based on the concentration of 2-carbonyl-4-[hydroxy(methyl)phosphono]butyric acid determined by HPLC. The results are shown in Table 2. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Enzyme activity determination results 
               
            
           
           
               
               
               
            
               
                 Number 
                 Mutation type 
                 Enzyme activity (U/L) 
               
               
                   
               
               
                 Control 1 
                 Non-mutation 
                 0.23 
               
               
                 E1 
                 M213S 
                 0.78 
               
               
                 E2 
                 M213S-N54V-F58E 
                 3.21 
               
               
                 E3 
                 M213S-N54V-F58E-D207A 
                 3.89 
               
               
                 E4 
                 M213S-N54V-F58E-D207A-S60T 
                 4.63 
               
               
                   
               
            
           
         
       
     
     Determination of transaminase activity: 400 μl of substrate solution (50 mM 2-carbonyl-4-[hydroxy(methyl)phosphono]butyric acid) dissolved in 50 mM phosphate buffer was placed in a metal bath oscillator, kept at 30° C. for 10 minutes, 50 μl of pure enzyme, 1 mM PLP and 80 mM L-alanine were added, and timing was started. The reaction was carried out at 30° C. for 10 minutes, 5 μL of 6 M hydrochloric acid was added, the mixture was taken out, shaken and mixed evenly, and then the reaction was terminated. The mixture was centrifuged at 12,000 rpm for 3 minutes, the supernatant was 2-fold diluted with deionized water, and detected by HPLC. The enzyme activity was calculated based on the L-phosphinothricin concentration measured by HPLC. 
     Example 4 
     Large-Scale Preparation of Bacterial Cells 
     Since a large amount of biocatalyst is required in the process of producing L-phosphinothricin, large-scale preparation of bacterial cells is required. The medium used was LB medium. 
     A glycerol tube containing the recombinant D-amino acid oxidase engineered bacterium was activated by plate streaking, and then single colonies were selected and inoculated into a 50 mL LB liquid medium containing 50 μg/mL kanamycin, and cultured under shaking at 37° C. for 12 hours. The sample was transferred at an inoculation amount of 2% to 1 L of fresh 
     LB liquid medium also containing 50 μg/mL kanamycin, and cultured under shaking at 37° C. until OD 600  reached about 0.8. Thereafter, IPTG with a final concentration of 0.5 mM was added and the mixture was cultured under shaking at 28° C. for 16 hours. After the culture, the culture solution was centrifuged at 8,000 rpm for 10 minutes, the supernatant was discarded, and the cells were collected and stored in an ultra-low temperature refrigerator at −80° C. for later use. 
     Example 5 
     Preparation of L-Phosphinothricin Using D-Amino Acid Oxidase Mutant (E1) and Transaminase 
     Recombinant bacteria capable of expressing D-amino acid oxidase (E1) and transaminase were cultured in the same manner as in Example 4, and the bacterial cells were collected by centrifugation. 
     D,L-phosphinothricin was quantitatively weighed into a 50 mM phosphate buffer with pH=8 and placed into a reactor, so that the final concentration of D,L-phosphinothricin was 100 mM, the D-amino acid oxidase (E1) cell concentration was 20 g/L, the catalase concentration was 0.1 g/L, and the transaminase cell concentration was 30 g/L. 400 mM L-alanine and 1 mM PLP were added. The reaction temperature was controlled at 30° C. by a water bath. PPO production was detected by achiral liquid chromatography with timing sampling. Also, the decrease of D-PPT and increase and ee value of L-PPT were detected by pre-column derivatization high performance liquid chromatography. 
     At the end of 10 hours of reaction, 41.36 mM D-PPT remained, with conversion of 7.33% (the maximum theoretical conversion is 50%), and 58.77 mM L-PPT was generated (ee 99%). 
     Example 6 
     Preparation of L-Phosphinothricin Using D-Amino Acid Oxidase Mutant (E2) and Transaminase. 
     Recombinant bacteria capable of expressing D-amino acid oxidase (E2) and transaminase were cultured in the same manner as in Example 4, and the bacterial cells were collected by centrifugation. 
     D,L-phosphinothricin was quantitatively weighed into a 50 mM phosphate buffer with pH=8 and placed into a reactor, so that the final concentration of D,L-phosphinothricin was 100 mM, the D-amino acid oxidase (E2) cell concentration was 20 g/L, the catalase concentration was 0.1 g/L, and the transaminase cell concentration was 30 g/L. 400 mM L-alanine and 1 mM PLP were added. The reaction temperature was controlled at 30° C. by a water bath. PPO production was detected by achiral liquid chromatography with timing sampling. Also, the decrease of D-PPT and increase and ee value of L-PPT were detected by pre-column derivatization high performance liquid chromatography. 
     At the end of 10 hours of reaction, 30.26 mM D-PPT remained, with conversion of 29% (the maximum theoretical conversion is 50%), 78.53 mM L-PPT was generated (99% ee), with yield of 28.5%, and less than 0.05 mM PPO formed. 
     Example 7 
     Preparation of L-Phosphinothricin Using D-Amino Acid Oxidase Mutant (E3) and Transaminase 
     Recombinant bacteria capable of expressing D-amino acid oxidase (E3) and transaminase were cultured in the same manner as in Example 4, and the bacterial cells were collected by centrifugation. 
     D,L-phosphinothricin was quantitatively weighed into a 50 mM phosphate buffer with pH=8 and placed into a reactor, so that the final concentration of D,L-phosphinothricin was 100 mM, the D-amino acid oxidase (E3) cell concentration was 20 g/L, the catalase concentration was 0.1 g/L, and the transaminase cell concentration was 30 g/L. 400 mM L-alanine and 1 mM PLP were added. The reaction temperature was controlled at 30° C. by a water bath. PPO production was detected by achiral liquid chromatography with timing sampling. Also, the decrease of D-PPT and increase and ee value of L-PPT were detected by pre-column derivatization high performance liquid chromatography. 
     At the end of 10 hours of reaction, 21.86 mM D-PPT remained, with conversion of 37.33% (the maximum theoretical conversion is 50%), 85.47 mM L-PPT was generated (99% ee), with yield of 73%, and the residual PPO concentration was less than 0.05 mM. 
     Example 8 
     Preparation of L-Phosphinothricin Using D-Amino Acid Oxidase Mutant (E4) and Transaminase. 
     Recombinant bacteria capable of expressing D-amino acid oxidase (E4) and transaminase were cultured in the same manner as in Example 4, and the bacterial cells were collected by centrifugation. 
     D,L-phosphinothricin was quantitatively weighed into a 50 mM phosphate buffer with pH=8 and placed into a reactor, so that the final concentration of D,L-phosphinothricin was 100 mM, the D-amino acid oxidase (E4) cell concentration was 20 g/L, the catalase concentration was 0.1 g/L, and the transaminase cell concentration was 30 g/L. 400 mM L-alanine and 1 mM PLP were added. The reaction temperature was controlled at 30° C. by a water bath. PPO production was detected by achiral liquid chromatography with timing sampling. Also, the decrease of D-PPT and increase and ee value of L-PPT were detected by pre-column derivatization high performance liquid chromatography. 
     At the end of 10 hours of reaction, 0 mM D-PPT remained, that is, complete conversion of D-PPT was achieved, and the resulting concentration of L-PPT was 97.79 mM, i.e., yield of 94%, with ee being as high as 99%. The residual PPO concentration was less than 0.05 mM. 
     Example 9 
     Preparation of L-Phosphinothricin With High Concentration Using D-Amino Acid Oxidase Mutant (E4) and Transaminase 
     Recombinant bacteria capable of expressing D-amino acid oxidase (E4) and transaminase were cultured in the same manner as in Example 4, and the bacterial cells were collected by centrifugation. 
     D,L-phosphinothricin was quantitatively weighed into 100 mM phosphate buffer with pH=8 and placed into a reactor, so that the final concentration of D,L-phosphinothricin was 400 mM, the D-amino acid oxidase (E4) cell concentration was 40 g/L, the catalase concentration was 0.1 g/L, and the transaminase cell concentration was 50 g/L. 600 mM L-alanine and 1 mM PLP were added. The reaction temperature was controlled at 30° C. by a water bath. PPO production was detected by achiral liquid chromatography with timing sampling. Also, the decrease of D-PPT and increase and ee value of L-PPT were detected by pre-column derivatization high performance liquid chromatography. 
     At the end of 10 hours of reaction, 0 mM D-PPT remained, that is, complete conversion of D-PPT was realized, and the resulting concentration of L-PPT was 386.79 mM with a yield of 95%, with ee of the product being as high as 99%. The residual PPO concentration was less than 0.05 mM (0.9‰). 
     Example 9 
     The L-phosphinothricin reaction mixture prepared from the D-amino acid oxidase mutant (E4) and transaminase in Example 8 was adjusted to have a pH of 5-6, and loaded onto 001×7 sodium-type or ammonium-type strong acid cation exchange resin at a flow rate of 0.5 BV/h. Alanine would be adsorbed onto the resin. After loading, the system was rinsed with ultrapure water, and L-phosphinothricin and other impurities would flow out with ultrapure water. The effluent of L-phosphinothricin was collected, and distilled under reduced pressure at 50-65° C. Then, the reaction mixture was adjusted to a pH of 2-3.5, and slowly stirred at 0-45° C. to crystallize for 1-24 hours. The treated L-phosphinothricin reaction mixture was adjusted to a pH of 1.2-2.5, and then the filtrate was loaded onto a strong acid cation exchange resin to remove a small amount of organic matters, D-type substrate and a large amount of inorganic ions, and then washed with water and eluted. The L-phosphinothricin effluent was collected, and concentrated under reduced pressure at 50-65° C. to reach a constant weight. Anhydrous methanol was added to dissolve L-phosphinothricin at 50-65° C., and then the solution was slowly stirred in an ice bath to crystallize. After stirred for 12-24 hours, the mixture was freeze-dried in a freezing vacuum dryer to obtain L-phosphinothricin crystals. The purity of the final product L-phosphinothricin reached more than 98%, and the yield reached more than 98%. 
     Comparative Example 1 
     The recombinant bacteria capable of expressing the non-mutated D-amino acid oxidase and transaminase were cultured according to the method of Example 4, and the bacterial cells were collected by centrifugation. 
     D,L-phosphinothricin was quantitatively weighed into a 50 mM phosphate buffer with pH=8 and placed into a reactor, so that the final concentration of D,L-phosphinothricin was 100 mM, the non-mutated D-amino acid oxidase cell concentration was 20 g/L, the catalase concentration was 0.1 g/L, and the transaminase cell concentration was 30 g/L. 400 mM L-alanine and 1 mM PLP were added. The reaction temperature was controlled at 30° C. by a water bath. PPO production was detected by achiral liquid chromatography with timed sampling. Also, the decrease of D-PPT and ee value were determined by pre-column derivatization high performance liquid chromatography. 
     At the end of 10 hours of reaction, 46.77 mM D-PPT remained, with conversion of 2.16% (the maximum theoretical conversion is 50%), and the total L-PTT concentration was only 53.58 mM (ee 99%), with yield of only 51.67%.