# Patent

## Title
Recombinant D-amino acid oxidases

## Abstract

The present invention discloses two novel D-amino acid oxidases, whose activities of converting cephalosporin C to glutaryl-7-aminocephalosporanic acid are at least 25% higher than that of wild-type Trigonopsis variabilis D-amino acid oxidase and the encoding DNA sequences thereof.

## Background
FIELD OF THE INVENTION
The present invention belongs to a field of biotechnology and, more specifically, relates to the preparation and use of novel D-amino acid oxidases that have higher catalytic activity and are suitable for the conversion of cephalosporin C to glutaryl-7-aminocephalosporanic acid.
BACKGROUND OF THE INVENTION
7-Aminocephalosporanic acid (7-ACA), the core of many semi-synthetic cephalosporins, is conventionally manufactured chemically from cephalosporin C. The chemical process uses chemical reagents that are highly toxic and heavily pollute the environment and the chemical process is low in conversion rate, as the process requires multiple steps of reactions. Bio-process offers an attractive alternative for the production of 7-ACA. The bioconversion of cephalosporin C to 7-ACA is conducted in two steps: (1) cephalosporin C is first oxidized by D-amino acid oxidase to glutaryl-7-ACA; (2) the glutaryl-7-ACA is then cleaved at the bond between the glutaryl moiety and the 7-ACA moiety by glutaryl-7ACA acylase to 7-ACA. Rhodotorula gracilis and Trigonopsis variabilis are the two major sources of D-amino acid oxidase for industrial application. However, these enzymes are low in their catalytic activities for oxidizing cephalosporin C (Simonetta, et al., Biochim. Biophys. Acta, 914: 136-142 (1987); U.S. Pat. Nos. 5,453,374; and 5,208,155). It is, therefore, desirable for the industry to have a D-amino acid oxidase of higher catalytic activity to reduce the cost of the bioconversion of cephalosporin C to 7-ACA.
SUMMARY OF THE INVENTION
The object of the invention is to provide two recombinant D-amino acid oxidases with higher catalytic activities to oxidize cephalosporin C. Another object of the invention is to apply the recombinant D-amino acid oxidases for the efficient conversion of cephalosporin C to glutaryl-7-aminocephalosporanic acid.
This invention cloned a gene for encoding T. variabilis FA10 D-amino acid oxidase (the wild-type; Li, W. et al., Acta Microbiologica Sinica, 31:251-253, 1991) into a suitable expression vector and performed the site-directed mutagenesis of the gene to generate a novel D-amino acid oxidase with higher catalytic activity. More specifically, the site-directed mutagenesis was targeted at the 53rd amino acid residue of the peptide in order to generate recombinant D-amino acid oxidases with higher catalytic activities.
In one aspect, the present invention provides a DNA sequence for encoding a recombinant D-amino acid oxidase, which is characterized by a nucleotide variation in comparison to the SEQ ID NO.: 1; resulting in that the amino acid residue at position 53 (i.e. threonine) of the peptide encoded therefrom is replaced by other natural amino acids, and that catalytic activity for oxidizing cephalosporin C, which is used as a substrate, is at least 25% higher; preferably, at least 35% higher; more preferably, at least 50% higher; and most preferably, at least 100% higher than that of its wild-type D-amino acid oxidase.
Preferably, the DNA sequence of the present invention includes the nucleic acid sequence for encoding the peptide sequence as shown in SEQ ID NO.:4 or SEQ ID NO.:6, More preferably, the DNA sequence includes the nucleic acid sequence as shown in SEQ ID NO.: 3 or SEQ ID NO.:5.
In another aspect, the present invention provides a polypeptide characterized in that, with SEQ ID NO.: 2 as a reference sequence, the amino acid residue corresponding to the threonine at position 53 of the reference sequence is changed to other natural amino acids, and that its catalytic activity for oxidizing cephalosporin C is at least 25% higher; preferably, at least 35% higher; more preferably, at least 50% higher; and most preferably, at least 100% higher than that of the wild-type D-amino acid oxidase.
Preferably, the present invention provides a polypeptide characterized in that, with SEQ ID NO.: 2 as the reference sequence, the amino acid residue corresponding to the threonine at position 53 of the reference sequence is serine or proline. In the Examples of the present invention, two novel recombinant D-amino acid oxidases, recombinant D-amino acid oxidase GHA and recombinant D-amino acid oxidase GHB, are provided. Recombinant D-amino acid oxidase GHA has an amino acid sequence as shown in SEQ ID NO.: 4 and its catalytic activity is 105% higher than that of the wild-type D-amino acid oxidase. The recombinant D-amino acid oxidase GHB has an amino acid sequence as shown in SEQ ID NO.: 6 and its catalytic activity is 35% higher than that of the wild-type D-amino acid oxidase. In the present invention, the wild-type D-amino acid oxidase gene refers to the one derived from the D-amino acid oxidase gene of Trigonopsis variabilis FA10 (Li, W. et al., Acta Microbiologica Sinica, 31:251-253, 1991). The nucleic acid of the wild-type D-amino acid oxidase is shown in SEQ ID NO.: 1 and its amino acid sequence is shown in SEQ ID NO.: 2.
Also covered by the invention are the derivatives of GHA or GHB, including those obtained by conservative substitutions or the addition or deletion of one or more amino acids.
For the preparation of the recombinant D-amino acid oxidases GHA or GHB of the present invention, suitable vectors include, but are not limited to, prokaryotic expression vectors, such as pRSET-A and pET; eukaryotic expression vectors, such as pYD1 and pYES2; and cloning vectors, such as pGEM®-T Easy, pUC18; pUC19 and pBluescript®-SK(+/−).
For the preparation of the novel D-amino acid oxidases of the present invention, either prokaryotic or eukaryotic cells can be employed as host cells. The suitable prokaryotic cells include, but are not limited to, E. coli, Bacillus subtilis, Bacillus brevis and Streptomyces; and the suitable eukaryotic cells include, but are not limited to, Saccharomyces cerevisiae, Rhodotorula gracilis, Trigonopsis variabilis, Aspergillus niger, Kluyveromyces lactis and Pichia pastoris. 
For the preparation of the novel D-amino acid oxidases GHA or GHB of the present invention, the recombinant D-amino acid oxidases can be readily introduced into appropriate microbial hosts, e.g. E. coli, S. cerevisiae; and expressed as intra-cellular or extra-cellular proteins in prokaryotic or eukaryotic cells. Suitable techniques and means for expressing the polypeptide are well known in the art.
In the case in which the recombinant D-amino acid oxidase is expressed as an extra-cellular protein, the enzyme can be partially-purified by conventional biochemical procedures, such as ammonium sulfate fractionation and acetone precipitation, and can be further purified with conventional purification procedures, such as ion exchange and affinity columns. The recombinant D-amino acid oxidase can be used in the form of un-purified/crude extracts, as a partially-purified enzyme, or as a purified enzyme.
In the case in which the recombinant D-amino acid oxidase is expressed as an intra-cellular protein, the enzyme can be purified after the disruption of host cells, followed by centrifugation (to remove cell debris) and fractionation.
For various industrial applications, the recombinant enzymes can be prepared as immobilized cells. Suitable methods or processes for immobilizing enzymes or cells expressing the enzymes are well known to those skilled in the field and may be used. Immobilized cells can be prepared by immobilizing transformed cells containing the recombinant D-amino acid oxidase of the present invention into or onto a solid support or carrier according to the known immobilization methodologies in the art. In the present invention, crude enzyme preparation, partially-purified enzyme preparation, or completely purified preparation of the recombinant D-amino acid oxidase can also be immobilized onto a solid support or carrier using conventional methodologies. Immobilized enzymes can be prepared by adsorption of the recombinant D-amino acid oxidase of the present invention onto an ion-exchange resin.
In addition, the recombinant enzymes can also be prepared as crystal enzymes, as described by Miyake, Y., Aki, K., Hashimoto, S., Yamano, T., Biochim. Biophys. Acta, 105:86-99, 1965, The use of the immobilized enzymes and/or crystal enzymes is also within the scope of the present invention.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is the restriction map of plasmid pRSET-kan.
FIG. 2 shows the DNA sequence of plasmid pRSET-kan.
FIG. 3 is a photograph of the SDS-PAGE eletrophoresis of the recombinant D-amino acid oxidases GHA and GHB. Lane I shows BenchMarkTM Pre-Stained Protein Ladder (Invitrogen) with protein sizes in kDa. Lane 2 shows the electrophoresis result of T. variabilis FA10 D-amino acid oxidase. Lane 3 shows the electrophoresis result of D-amino acid oxidase GHA. Finally, lane 4 shows the electrophoresis result of D-amino acid oxidase GHB.



PREFERRED EMBODIMENT
Enzymes such as restriction enzymes, T4 DNA ligase, DNA polymerase, and other materials, were obtained from commercial suppliers and used in accordance with manufacturers' instructions. DNA cloning, transformation of host cells, and other procedures are well known in the art or can be adapted from literature.
The following examples are given for the purpose of illustrating this invention but are not limited thereto. Unless indicated in the following examples, conventionally known conditions were adopted or the procedures are performed as recommended by the manufacturer, especially when some specific conditions are not given.
EXAMPLE 1
Construction of a Recombinant Vector pRSET-A-DAO for Expressing T. variabilis D-Amino Acid Oxidase
Sequence-specific primers were designed in accordance with the known sequences of the 5′- and 3′-ends of the T. variabilis D-amino acid oxidase gene (Gonzalez, F. J., Montes, J., Martin, F., Lopez, M. C., Ferminan, E., Catalan, J., Galan, M. A. and Dominguez, A., Molecular cloning of TvDAO1; a gene encoding a D-amino acid oxidase from Trigonopsis variabilis and its expression in Saccharomyces cerevisiae and Kluyveromyces lactis. Yeast 13:1399-1408, 1997). The primer pair is shown as follows:

    5′-NdeI (incorporated NdeI restriction site)
    5′-TAGGGCTGACATATGGCTAAAATCGTTGTTATTGGTGC-3′ (SEQ ID NO.: 7)
    3′-BglII (incorporated BglII restriction site)
    5′-TAGGGCTGAAGATCTCTAAAGGTTTGGACGAGTAAGAGC-3′ (SEQ ID NO.: 8)


T. variabilis D-amino acid oxidase gene was synthesized by PCR, in which plasmid pJL (Yang, Y. L. et al., Patent Application Publication number: CN1371999A, 2002) was a template. The above-mentioned primer pair and Pfu DNA polymerase (Promega) were used to amplify the gene. Plasmid pJL contains T. variabilis FA10 D-amino acid oxidase gene (Li, W. et al., Acta Microbiologica Sinica, 31:251-253, 1991). The PCR mixture contained 40 ng pJL, 0.4 μM 5′-NdeI, 0.4 μM 3′-BglII, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 20 mM Tris-HCl (pH8.8), 10 mM KCl, 10 mM (NH4)2SO4, 2 mM MgSO4, 0.1% Triton X-100; and 2.5 U Pfu DNA polymerase; and the volume of the mixture was made up to 50 μL with sterile deionized water.
PCR profile was as follows:




As a result, a PCR fragment of 1,098 bp was obtained, with NdeI and BglII at the 5′- and 3′-ends respectively. The PCR product was resolved in a 1% agarose gel, purified and cut with NdeI and BglII, then ligated to a 2.9 kb restricted fragment obtained by digesting plasmid pRSET-A (Invitrogen) with NdeI and BglII, so as to generate plasmid pRSET-A-DAO. The plasmid was transformed into a competent E. coli BL21(DE3)pLysS (Novagen). The transformants were spread onto LB/amp agar plate and incubated at 37° C. overnight. Plasmid was extracted in accordance with the procedures described in Molecular Cloning—A Laboratory Manual, ed. by J. Sambrook, et al., 1989; CSHL Press. The plasmid was sequenced and confirmed to be T. variabilis D-amino acid oxidase gene (see SEQ ID NO.: 1). Its deduced amino acid sequence is shown in as SEQ ID NO.:2.
EXAMPLE 2
Construction of Plasmid pRSET-kan
To remove the ampicillin resistance gene from pRSET-A, the following primers were synthesized based on the sequence of pRSET-A:

    VET-F
    5′-CTGTCAGACCAAGTTTACTCATATATACTTTAG-3′ (SEQ ID NO.:9)
    VET-R
    5′-ACTCTTCCTTTTTCAATATTATTGAAGC-3′ (Sequence 10)


To amplify the kanamycin resistance gene from plasmid pET-28b (Novagen), the following primers were synthesized based on the sequence of pET-28b:

    KAN-F
    5′-ATGAGTCATATTCAACGGGAAAC-3′ (SEQ ID NO.:11)
    KAN-R
    5′-TTAGAAAAACTCATCGAGCATCAAATG-3′ (SEQ ID NO.:12)


The PCR mixture for amplifying pRSET-A fragment devoid of ampicillin resistance gene contained: 50 ng pRSET-A, 0.4 μM VET-F, 0.4 μM VET-R, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 20 mM Tris-HCl (pH8.8), 10 mM KCl, 10 mM (NH4)2SO4, 2 mM MgSO4, 0.1% Triton X-100; and 2.5 U Pfu DNA polymerase; and the volume of the mixture was made up to 50 μL with sterile deionized water. The PCR mixture for amplifying the kanamycin resistance gene from plasmid pET-28b contained: 50 ng pET-28b, 0.4 μM KAN-F, 0.4 μM KAN-R, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 20 mM Tris-HCl (pH8.8), 10 mM KCl, 10 mM (NH4)2SO4, 2 mM MgSO4, 0.1% Triton X-100; and 2.5 U Pfu DNA polymerase; and the volume of the mixture was made up to 50 μL with sterile deionized water.
PCR profile was as follows:




The two PCR products (pRSET-A fragment devoid of ampicillin resistance gene, 2,036 bp in size; kanamycin resistance gene, 816 bp in size) were resolved in 1% agarose and purified and ligated to each other. As a result, plasmid pRSET-kan (shown in FIG. 1) was obtained. The plasmid was transformed into a competent E. coli BL21(DE3)pLysS (Novagen) and the transformants were spread onto LB/kan agar plate and incubated at 37° C. overnight. The plasmid was extracted and sequenced. The sequence was shown as FIG. 2 and SEQ ID NO.:13.
EXAMPLE 3
Construction of Recombinant D-Amino Acid Oxidase GHA
Recombinant D-amino acid oxidase GHA was constructed by site-directed mutagenesis, which was based on the procedures in PCR Protocols (Ed. John M. S. Bartlett and David Stirling, Totowa, N.J.: Humana Press, 2003).
Primers were synthesized on the basis of the sequence of the cloned T. variabilis D-amino acid oxidase SEQ ID NO.:1):

    Primer A:
    5′-TAGGGCTGACATATGGCTAAAATCGTTGTTATTG-3′ (SEQ ID NO.:14)
    Primer B:
    5′-TAGGGCTGAAGATCTCTAAAGGTTTGGACGAG-3′ (SEQ ID NO.:15)
    Primer C1:
    5′-GCAGGTGCCAACTGGCTCCCGTTTTACGATGGAGGCAAG-3′ (SEQ ID NO.:16)
    Primer D:
    5′-GAGCCAGTTGGCACCTGCCCAAGG-3′ (SEQ ID NO.:17)


Primers A and B are a pair of outer primers. Primer A contains NdeI restriction site, with a portion of nucleotides overlapping with the 5′-end of the D-amino acid oxidase gene. Primer B contains BglII restriction site, with a portion of nucleotides overlapping with the 3′-end of the D-amino acid oxidase gene. Primers C1 and D are a pair of inner primers. Primer C1 converts the 53rd amino acid residue of wild-type D-amino acid oxidase from threonine (Thr) to proline (Pro). Primer D contains a portion of nucleotides overlapping with primer C1.
With pRSET-A-DAO as a template, primers A and D were used to perform PCR to synthesize fragment 1; and primers B and C1 were used to synthesize fragment 2, The PCR mixture contained: 20 ng pRSET-A-DAO, 20 mM Tris-HCl (pH8.8), 10 mM KCl, 10 mM (NH4)2SO4, 2 mM MgSO4, 0.1% Triton X-100, 0.4 μM primer A and 0.4 μM primer D (for synthesizing fragment 1) or 0.4 μM primer B and 0.4 μM primer C1 (for synthesizing fragment 2), 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 1.5 U Pfu DNA polymerase, the volume of the mixture was made up to 50 μL with sterile deionized water.
The PCR profile was as follows:




The amplified fragment 1 and fragment 2 were resolved in and purified from 1% agarose and were used to generate the full-length D-amino acid oxidase gene. The PCR mixture for synthesizing the full-length gene contained: 20 ng fragment 1, 20 ng fragment 2, 20 mM Tris-HCl (pH8.8), 10 mM KCl, 10 mM (NH4)2SO4, 2 mM MgSO4, 0.1% Triton X-100, 0.4 μM primer A and 0.4 μM primer B, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 1.5 U Pfu DNA polymerase, the volume of the mixture was made up to 50 μL with sterile deionized water.
The PCR profile was as follows:




As a result, the full-length recombinant D-amino acid oxidase GHA gene was obtained. The gene was cut with NdeI and BglII and then inserted into pRSET-kan, generating pRSET-kan-DAOGHA. The plasmid was transformed into a competent E. coli BL21(DE3)pLysS (Novagen). The resulted transformants were spread onto LB/kan agar plate and incubated at 37° C. overnight. Plasmid was extracted, and the insert was sequenced and confirmed as recombinant D-amino acid oxidase GHA. The DNA sequence for encoding GHA was shown in S SEQ ID NO.:3 and the deduced amino acid sequence was shown in S SEQ ID NO.:4.
EXAMPLE 4
Construction of Recombinant D-Amino Acid Oxidase GHB
Recombinant D-amino acid oxidase was constructed by site-directed mutagenesis, which is based on the procedures in PCR Protocols (Ed. John M. S. Bartlett and David Stirling, Totowa, N.J.: Humana Press, 2003).
Primers were synthesized on the basis of the sequence of the cloned T. variabilis D-amino acid oxidase (SEQ ID NO.:1):

    Primer A:
    5′-TAGGGCTGACATATGGCTAAAATCGTTGTTATTG-3′ (Sequence 14)
    Primer B:
    5′-TAGGGCTGAAGATCTCTAAAGGTTTGGACGAG-3′ (Sequence 15)
    Primer C2:
    5′-GCAGGTGCCAACTGGCTCAGCTTTTACGATGGAGGCAAG-3′ (Sequence 18)
    Primer D:
    5′-GAGCCAGTTGGCACCTGCCCAAGG-3′ (SEQ ID NO.: 17)


The above primers A, B and D were identical as in Example 3, Primer C2 is an inner primer, which converts the 53rd amino acid residue of wild-type D-amino acid oxidase from threonine (Thr) to serine (Ser). Primer D contains a portion of nucleotides overlapping with primer C2, Amplification of fragment 1 was identical as in Example 3. Fragment 3 was amplified by PCR, using pRSET-A-DAO, as a template, and primers B and C2, The PCR mixture contained: 20 ng pRSET-A-DAO, 20 mM Tris-HCl (pH8.8), 10 mM KCl, 10 mM (NH4)2SO4, 2 mM MgSO4, 0.1% Triton X-100, 0.4 μM primer B and 0.4 μM primer C2, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 1.5 U Pfu DNA polymerase, the volume of the mixture was made up to 50 μL with sterile deionized water.
The PCR profile was as follows:




The amplified fragment 1 and fragment 3 were resolved in and purified from 1% agarose and were used to generate the full-length D-amino acid oxidase gene GHB. The PCR mixture for synthesizing the full-length gene GHB contained: 20 ng fragment 1, 20 ng fragment 3, 20 mM Tris-HCl (pH8.8), 10 mM KCl, 10 mM (NH4)2SO4, 2 mM MgSO4, 0.1% Triton X-100, 0.4 μM primer A and 0.4 μM primer B, 50 μM dATP, 50 μM dTTP, 50 μM dCTP, 50 μM dGTP, 1.5 U Pfu DNA polymerase, the volume of the mixture was made up to 50 μL with sterile deionized water.
The PCR profile was as follows:




As a result, the full-length recombinant D-amino acid oxidase GHB gene was obtained. The gene was cut with NdeI and BglII and then inserted into pRSET-kan. The resultant plasmid was transformed into a competent E. coli BL21(DE3)pLysS (Novagen). The transformants were spread onto LB/kan agar plate and incubated overnight at 37° C. The plasmid was extracted, and the insert was sequenced and confirmed as recombinant D-amino acid oxidase GHB. The DNA sequence for encoding GHB was shown in SEQ ID NO.:5 and the deduced amino acid sequence was shown in SEQ ID NO.:6.
EXAMPLE 5
Purification of D-Amino Acid Oxidase
The purification of D-amino acid oxidase was essentially performed in accordance with Alonso, J., Barredo, J. L., Diez, B., Mellado, E., Salto, F., Garcia, J. L., Cortes, E. (D-amino acid oxidase gene from Rhodotorula gracilis [Rhodosporidium toruloides] ATCC 26217, Microbiology 144:1095-1101, 1998). A single colony of pRSET-kan-DAOGHA containing E. coli BL21(DE3)pLysS (Example 3) was inoculated and cultivated in 200 mL LB medium supplemented with ampicillin at 250 rpm for 12 hours. Following this, 1 mM IPTG was added and it was then induced for another 6 hours. The bacterial cells were pelleted and resuspended in 20 mL Buffer A [20 mM sodium phosphate buffer (pH8.0) containing 20% glycerol, 5 mM 2-mercaptoethanol, 1 mM PMSF and 2 mM EDTA], disrupted with ultrasonication, and centrifuged at 13,000 g for 20 min at 4° C. to collect the supernatant as crude D-amino acid oxidase. The crude D-amino acid oxidase was applied to DEAE-cellulose (Sigma, 6×2.5 cm), eluted by Buffer A. The eluate was considered as partially-purified recombinant D-amino acid oxidase GHA. The partially-purified recombinant D-amino acid oxidase GHA can be further purified by the following method: the partially-purified recombinant D-amino acid oxidase GHA is applied to Buffer A-equilibrated Cibacron Blue 3GA-sepharose (Pharmacia LKB Biotechnology, 4×1 cm), washed by 30 mL 1M sodium phosphate (pH8.0), followed by elution with 10 mL Buffer A containing 50 μM FAD. The purity of the protein was analyzed by SDS-PAGE (FIG. 3). T. variabilis D-amino acid oxidase and recombinant D-amino acid oxidase GHB were also extracted and purified in accordance with the above-described procedures (FIG. 3).
EXAMPLE 6
Determination of Activity of D-Amino Acid Oxidase
The procedures were performed in accordance with Isogai, T., Ono, H., Ishitani, Y., Kojo, H., Ueda, Y. and Kohsaka, M. (Structure and expression of cDNA for D-amino acid oxidase active against cephalosporin C from Fusarium solani. J. Biochem, [Tokyo] 108:1063-1069, 1990), with modifications made to a particular step. Three mL sodium phosphate (50 mM, pH7.5) with 75 mM cephalosporin C sodium was combined with 3 mL partially-purified T. variabilis D-amino acid oxidase (Example 5) to prepare a reaction mixture. The mixture was oxygenated and vortexed at 22° C. for 60 min. Aliquots (100 μL) were withdrawn at different times (1, 5, 10, 30, 60 min), mixed with 10 μL 3% hydrogen peroxide, and 50 μL 10% trichloroacetic acid was added to stop the reaction. The mixture was centrifuged (10,000 g, 3 min) and 10 μL of supernatant was mixed with 990 μL HPLC mobile phase and then it was analyzed by HPLC. HPLC column: DiamonsilTM C18, 250×4.6 mm (Beijing); mobile phase: 50 mM K2HPO4/KH2PO4 (pH7.0), 5% acetonitrile; column temperature: 30° C.; flow rate: 1 mL/min; scanning: 260 nm UV. One unit of enzyme activity was defined as the amount of enzyme that converted 1 μmole of cephalosporin C to glutary-7-aminocephalosporanic acid per minute under the above reaction condition. The activities of recombinant D-amino acid oxidase GHA and recombinant D-amino acid oxidase GHB were also determined by the above-mentioned method. The activity of the recombinant D-amino acid oxidase GHA was determined as 205% of that of the wild-type T. variabilis D-amino acid oxidase; the activity of the recombinant D-amino acid oxidase GHB was determined 135% of that of the wild-type T. variabilis D-amino acid oxidase.
EXAMPLE 7
Preparation of Immobilized D-Amino Acid Oxidase
Extraction and purification of the D-amino acid oxidase was performed in accordance with Example 5, Preparation of the immobilized D-amino acid oxidase was performed in accordance with the description from Rohm (Darmstadt, Germany). A 50 mL partially-purified recombinant D-amino acid oxidase GHA solution (containing 200 mg total protein) was added to K2HPO4 and KH2PO4 in order to adjust it to contain 0.5M phosphate with a pH of 7.5, Five g dry EupergitTM C (Röhm GmBH, Darmstadt, Germany), was added into the enzyme solution, stirred (75 rpm) at room temperature (17° C.-23° C.) for 72 hours and then filtered to remove the supernatant. The resultant immobilized enzyme was washed several times by deionized distilled water and collected by filtration. The total immobilized enzyme generated was 19.4 g. The activity of the immobilized recombinant D-amino acid oxidase GHA was determined in the same way as described in Example 6; with the exception of using 19.4 g of immobilized recombinant D-amino acid oxidase GHA in a reaction volume of 1,000 mL. The activity of the immobilized recombinant D-amino acid oxidase GHA was 65 units/g wet carrier.
This invention is not limited by the detailed description provided in the Examples above. Various modifications can be made by those skilled in the field and these modifications should be also regarded as being within the scope of the invention, which is defined by the claims.

## Claims

1. A recombinant D-amino acid oxidase characterized by the proline residue or serine residue at position 53 of the wild-type D-amino acid oxidase having the amino acid sequence as shown in SEQ ID NO: 2, and characterized by its catalytic activity for oxidizing cephalosporin C which is at least 25% higher than that of the wild-type D-amino acid oxidase having the amino acid sequence as shown in SEQ ID NO: 2.

2. The recombinant D-amino acid oxidase according to claim 1, wherein the oxidase has the amino acid sequence of SEQ ID NO: 4.

3. The recombinant D-amino acid oxidase according to claim 2, wherein the amino acid sequence is encoded by the nucleotide sequence of SEQ ID NO: 3.

4. The recombinant D-amino acid oxidase according to claim 1, wherein the oxidase has the amino acid sequence of SEQ ID NO: 6.

5. The recombinant D-amino acid oxidase according to claim 4, wherein the amino acid sequence is encoded by the nucleotide sequence of SEQ ID NO: 5.

6. An isolated DNA molecule comprising the nucleotide sequence encoding the recombinant D-amino acid oxidase of claim 1.

7. The isolated DNA sequence according to claim 6, wherein said DNA sequence encodes the amino acid sequence of SEQ ID NO: 4.

8. The isolated DNA sequence according to claim 7, wherein said DNA sequence comprises the nucleotide sequence of SEQ ID NO: 3.

9. The isolated DNA sequence according to claim 6, wherein said DNA sequence encodes the amino acid sequence of SEQ ID NO: 6.

10. The isolated DNA sequence according to claim 9, wherein said DNA sequence comprises the nucleotide sequence of SEQ ID NO: 5.

11. A vector comprising the DNA sequence according to claims 6.