Preparation of Glycosyltransferase UGT76G1 Mutant and Use Thereof

Abstract

Provided are a glycosyltransferase UGT76G1 mutant, an isolated polynucleotide, a carrier, a production method, and a composition and a kit for the glycosylation of substrates. Further provided are a method for increasing the catalytic activity of the glycosyltransferase UGT76G1 mutant, and a method for promoting the glycosylation of flavone compounds and the use thereof. The glycosyltransferase UGT76G1 mutant is subjected to mutations at positions corresponding to positions 87, 199, 204 or 379 of SEQ ID NO: 1, and significantly improves the transformation activity of substrates of the flavone compounds.

Claims

1. A glycosyltransferase UGT76G1 mutant, comprising (1) a protein of amino acid sequence corresponds to SEQ ID NO: 1, with a mutation at residue 87, 199, 204 or 379; (2) the conservative variant of the protein of (1), in which the amino acids corresponding to residue 87, 199, 204 or 379 of SEQ ID NO: 1 are the same as those mutated at the corresponding position of the protein of (1).

2. The glycosyltransferase UGT76G1 mutant according to claim 1, wherein the conservative variant comprises: (a) a protein derived from the protein of (1), having one or more amino acids deleted, substituted, or inserted in the sequence, and still having the function of the protein of (1); (b) a protein derived from the protein of (1), having more than 80% sequence identity with the amino acid sequence of the protein of (1), and still having the function of the protein of (1); (c) the active fragment of the protein of (1), containing the structure interacting with the glycosyl donor and glycosyl receptor in the spatial structure of glycosyltransferase UGT76G1, and still having the function of the protein of (1).

3. The glycosyltransferase UGT76G1 mutant according to claim 1, wherein the residue at position 87, 199, 204 or 379 is mutated to a residue having a side chain group of aromatic ring.

4. An isolated polynucleotide, wherein the polynucleotide encodes the glycosyltransferase UGT76G1 mutant according to claim 1.

5. A vector, comprising the polynucleotide according to claim 4.

6. A genetically engineered host cell, comprising a vector comprising the polynucleotide according to claim 4 or has the polynucleotide integrated in the genome.

7. A method for preparing the glycosyltransferase UGT76G1 mutant according to claim 1, comprising the steps of: (1) culturing a host cell comprising the polynucleotide encoding the glycosyltransferase UGT76G1 mutant according to claim 1 to obtain a culture; and (2) isolating the glycosyltransferase UGT76G1 mutant from the culture.

8. A method of increasing the catalytic activity of glycosyltransferase UGT76G1, comprising: mutating the amino acid of its sequence, wherein the amino acid corresponds to residue 87, 199, 204 or 379 of SEQ ID NO: 1.

9. The method according to claim 8, wherein the method comprising: mutating the residue at position 87, 199, 204 or 379 to a residue having a side chain group of aromatic ring.

10. The method according to claim 8, wherein improving the catalytic activity of glycosyltransferase UGT76G1 is to improve the catalytic activity to glycosylate flavonoids.

11. A method of improving glycosylation of a flavonoid, comprising conducting catalyzation via the glycosyltransferase UGT76G1 mutant according to claim 1.

12. The method according to claim 7, wherein, the flavonoid is a compound having the following core structure: ##STR00003##

13. The method according to claim 12, wherein, the flavonoid includes: isoorientin, orientin, apigenin, luteolin and the like.

14. (canceled)

15. (canceled)

16. (canceled)

17. A composition for glycosylating a substrate, wherein, the composition comprises: the glycosyltransferase UGT76G1 mutant according to claim 1; or a host cell comprising the polynucleotide encoding the glycosyltransferase UGT76G1 mutant according to claim 1.

18. A kit for glycosylating a substrate, wherein, the kit comprises: the glycosyltransferase UGT76G1 mutant according to claim 1; or a host cell comprising the polynucleotide encoding the glycosyltransferase UGT76G1 mutant according to claim 1; or a composition comprising the glycosyltransferase UGT76G1 mutant according to claim 1, or a host cell comprising the polynucleotide encoding the glycosyltransferase UGT76G1 mutant according to claim 1.

19. The glycosyltransferase UGT76G1 mutant according to claim 1, wherein the residue at position 87, 199, 204 or 379 is mutated to a residue having a side chain group of Phe or Trp.

20. The method according to claim 8, wherein the residue at position 87, 199, 204 or 379 is mutated to a residue having a side chain group of Phe or Trp.

21. The method according to claim 11 wherein the glycosylation comprises adding 1 to 3 glycosyls in the flavonoid.

22. The method according to claim 12, wherein, the flavonoid includes isoorientin, and glycosylation of isoorientin produces isoorientin-O-glucoside, isoorientin-O-diglucoside, or orientin-O-glucoside.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0027] FIG. 1. Conversion of isoorientin by UGT76G1 wild-type and mutant. ST, isoorientin standard; WT, conversion reaction by wild-type UGT76G1; G87F, conversion reaction by UGT76G1 mutant G87F; I199F, conversion reaction by UGT76G1 mutant I199F; L204F, conversion reaction by UGT76G1 mutant L204F.

[0028] FIG. 2. MS/MS spectrum of compound 2.

[0029] FIG. 3. MS/MS spectrum of compound 3.

[0030] FIG. 4. The relative yields of the glycosylated products converted from isoorientin by the mutants G87F, I199F and L204F (100% for the wild type).

[0031] FIG. 5. Conversion of orientin by UGT76G1 wild-type and mutant.

[0032] FIG. 6. LC-MS/MS spectrum of compound 5, 6. (A) [M-H].sup.− selective ion current spectra of compounds 5 and 6; (B) quasi-molecular ion peak m/z 609.15 [M-H].sup.− secondary fragmentation information.

[0033] FIG. 7. Conversion of apigenin by UGT76G1 wild-type and mutant.

[0034] FIG. 8. Conversion of luteolin by UGT76G1 wild-type and mutant.

DETAILED DESCRIPTION

[0035] Upon in depth research, the inventor mutated key amino acid residues in the substrate binding pocket of UGT76G1, and explore UGT76G1 mutants that improve the catalytic activity of flavonoids. Here the inventor revealed a group of glycosyltransferase UGT76G1 mutants which have residues at particular positions mutated to amino acid residues with aromatic ring side chain substituents, especially G87F, I199F, L204F. The mutants have significantly improved conversion activity for flavonoid substrates.

[0036] As used herein, unless otherwise specified, the “glycosyltransferase UGT76G1 mutant” and “mutated glycosyltransferase UGT76G1” can be used interchangeably, which refer to the polypeptide after mutation near the substrate binding pocket corresponding to the wild-type glycosyltransferase UGT76G1. Preferably, the polypeptide is formed after mutation at position 87, 199 or 204 of the sequence.

[0037] The wild-type glycosyltransferase UGT76G1 can be refered as “a protein of amino acid sequence of SEQ ID NO: 1”, or “a functional variant or active fragment of the protein”. Preferably, the wild-type glycosyltransferase UGT76G1 is derived from Stevia rebaudiana. However, it should be understood that the invention also encompasses UGT76G1 homologues from other plants with homology and the same function.

[0038] As used herein, “isolated glycosyltransferase UGT76G1” means that the glycosyltransferase UGT76G1 mutant substantially contains no other naturally related proteins, lipids, carbohydrates or other substances. The skilled in the art can purify the glycosyltransferase UGT76G1 mutant by standard protein purification technology. Substantially pure protein can produce a single main band on non-reducing SDS-PAGE.

[0039] As used herein, “substrate binding pocket” refers to the position where the glycosyltransferase UGT76G1 interacts (binds) with the substrate in the spatial structure.

[0040] The protein of the invention can be a recombinant protein, a natural protein, a synthetic protein, preferably a recombinant protein. The protein of the invention can be a naturally purified product, or a chemically synthesized product, or can be produced by prokaryotic or eukaryotic hosts (such as bacteria, yeast, higher plants, insects and mammalian cells) using recombination technology.

[0041] The invention also includes fragments, derivatives and analogues of the glycosyltransferase UGT76G1 mutant. As used herein, the terms “fragments”, “derivatives” and “analogues” refer to proteins that basically maintain the same biological function or activity of the natural glycosyltransferase UGT76G1 mutant of the invention. Functional fragments, derivatives or analogs in the disclosure may be (i) proteins with one or more conservative or non-conservative amino acid substitution (preferably conservative), where the substituted amino acid residues may or may not be one encoded by the genetic code, or (ii) a polypeptide having substituent groups in one or more amino acid residues, or (iii) proteins formed by having said protein fused with additional amino acid sequence (such as leader sequence or secretory sequence, or sequence used for purification of the protein or proprotein sequence, or fusion protein). In accordance with the teachings provided herein, these fragments, derivatives and analogs are well known to a person skilled in the art. However, the mutation disclosed herein, the amino acid mutation near the substrate binding pocket in protein's spatial structure, must exist in the amino acid sequence of the glycosyltransferase UGT76G1 mutant and its fragments, derivatives and analogues; preferably, the amino acid corresponding to residue 87, 199, 204 or 379 of SEQ ID NO: 1 is mutated.

[0042] As used herein, the “glycosyltransferase UGT76G1 mutant” further comprises but is not limited to: deletion, insertion and/or substitution of several (usually 1-20, preferably 1-10, more preferably 1-8, 1-5, 1-3, 1-2) amino acids, and addition or deletion of one or several (usually within 20, preferably within 10, more preferably within 5) amino acids at the C-terminal and/or N-terminal. For example, substitution with amino acids of comparable or similar properties usually does not change protein function in the art. As another example, addition or deletion of one or more amino acids to the C-terminus and/or N-terminus usually does not change the function of a protein either. The term also includes the active fragments and active derivatives of the glycosyltransferase UGT76G1 mutant. However, these variants should comprise the mutation described herein, i.e., the amino acid mutation near the substrate binding pocket in protein's spatial structure; preferably, the mutation occurs at amino acid corresponding to residue 87, 199, 204 or 379 of SEQ ID NO: 1.

[0043] In the present invention, the term “glycosyltransferase UGT76G1 mutant” also includes (but is not limited to): a derived protein having more than 80%, more preferably more than 85%, more preferably more than 90%, and further more preferably more than 95%, such as more than 98% and more than 99% sequence identity with the amino acid sequence of the glycosyltransferase UGT76G1 mutant and retaining the activity of the mutant. Similarly, these derived proteins should comprise the mutation described herein, i.e., the amino acid mutation near the substrate binding pocket in protein's spatial structure; preferably, the mutation occurs at amino acid corresponding to residue 87, 199, 204 or 379 of SEQ ID NO: 1.

[0044] The invention also provides a polynucleotide sequence encoding a glycosyltransferase UGT76G1 mutant of the invention or a conservative variant protein thereof.

[0045] The polynucleotide sequences herein can be in the form of DNA or RNA. Forms of DNA include cDNA, genomic DNA or artificially synthesized DNA. DNA can be single-stranded or double-stranded. The DNA may be coding strand or non-coding strand.

[0046] The polynucleotide encoding the mature protein of the mutant disclosed herein includes: the coding sequence only encoding the mature protein; the coding sequence encoding the mature protein and a various additional coding sequence; the coding sequence encoding the mature protein (and an optional additional coding sequence) and a noncoding sequence.

[0047] The term “polynucleotide encoding a/the protein” can include a polynucleotide encoding the protein, or a polynucleotide that further includes additional coding and/or non-coding sequences.

[0048] The disclosure also relates to vectors comprising the polynucleotide of the disclosure, as well as host cells genetically engineered using the vectors or coding sequences of the glycosyltransferase UGT76G1 mutant disclosed herein, and a method for producing the protein of the invention by recombination technology.

[0049] Through conventional recombinant DNA technology, the polynucleotide sequence of the invention can be used to express or produce the recombinant glycosyltransferase UGT76G1 mutant. Generally, there are the following steps:

[0050] (1) Transforming or transducing a suitable host cell with a polynucleotide (or variant) encoding a glycosyltransferase UGT76G1 mutant of the present invention, or with a recombinant expression vector containing the polynucleotide;

[0051] (2) culturing the host cell in a suitable medium;

[0052] (3) isolating and purifying proteins from the medium or cell.

[0053] In the invention, the polynucleotide sequence of glycosyltransferase UGT76G1 mutant can be inserted into the recombinant expression vector. The term “recombinant expression vector” refers to bacterial plasmid, phage, yeast plasmid, plant cell virus, mammalian cell virus or other vectors well known in the art. In short, any plasmid or vector can be used, provided that it can replicate and be stable in the host. An important charecterastic of an expression vector is that it usually contains an origin of replication, a promoter, a marker gene and a translation control element.

[0054] Suitable methods for constructing expression vector which comprises the coding DNA sequence of the glycosyltransferase UGT76G1 mutant and appropriate transcriptional/translational control signals are well known to the person skilled in the art. These methods include in vitro recombinant DNA technology, DNA synthesis technology, in vivo recombinant technology and so on. Said DNA sequence may be effectively linked to a proper promoter in the expression vector to direct mRNA synthesis. Expression vector further comprises a ribosome binding site for the initation of translation, and a transcription terminator. The expression vector preferably contains one or more selective marker genes to provide phenotypic traits for the selection of transformed host cells.

[0055] Vectors containing the above appropriate DNA sequences and appropriate promoters or regulatory sequences can be used to transform appropriate host cells so that they can express proteins.

[0056] In this disclosure, the host cells can be prokaryotic cells, such as bacterial cells; or lower eukaryotic cells, such as yeast cells; or higher eukaryotic cells, such as plant cells. Examples include Escherichia coli, Bacillus subtilis, Streptomyces, Agrobacterium; eukaryotic cells, such as yeast, plant cells, etc. In a specific embodiment of the invention, Escherichia coli is used as the host cells.

[0057] The choice of appropriate carrier, promoter, enhancer and host cells is evident to a person of ordinary skills in the art.

[0058] In a specific example of the present invention, the UGT76G1 mutant was expressed and purified in E. coli: the wild-type UGT76G1 gene was amplified from the cloning vector through PCR and subcloned into the protein expression vector pETDuet-1. Using the UGT76G1 gene expression vector as the template, the mutation was introduced by PCR primers, and the mutant protein expression vector was constructed using PCR amplification. The mutants that are sequenced to be correct were heterologously expressed in E. coli BL21(DE3). The protein was purified by Ni-NTA affinity chromatography and tested for enzyme activity in vitro, or applied to other aspects.

[0059] In a preferred embodiment, the flavonoid (substrate) is a compound having the following core structure:

##STR00002##

[0060] In a preferred embodiment, the flavonoid (substrate) includes: isoorientin, orientin, apigenin, luteolin and the like.

[0061] Based on the information of the mutant glycosyltransferase UGT76G1 described herein, those skilled in the art know how to use the mutant to perform glycosylation of the compound having the core structure of flavonoid.

[0062] The enzyme activity of the glycosyltransferase UGT76G1 mutant can be detected by a technique known in the art. In a specific example of the present invention, the enzyme reaction system with the same conditions is used to detect the enzyme activity of different mutants in vitro. After the reaction, the product was detected by HPLC. The change of a mutant's catalytic activity was analyzed by comparing the relative conversion ratio of the flavonoid isoorientin by the mutant and the wild-type protein.

[0063] Compared with the prior art, the improved effect of the present invention is that the disclosed glycosyltransferase UGT76G1 mutants can efficiently and specifically catalyze the glycosylation of flavonoids in vitro. The glycosylation efficiency is significantly improved over the wide-type protein.

[0064] The glycosyltransferase UGT76G1 mutant of the present invention can be prepared in a pharmaceutically or industrially acceptable carrier to obtain a composition suitable for catalytic reaction or suitable for storage.

[0065] The glycosyltransferase UGT76G1 mutant of the present invention can also be prepared in a kit for ease of use or sale. Generally, the kit also includes instructions for use.

[0066] The disclosure is further illustrated by the specific examples described below. It should be understood that these examples are merely illustrative, and do not limit the scope of the present disclosure. The experimental methods without specifying the specific conditions in the following examples generally used the conventional conditions, such as those described in J. Sambrook, Molecular Cloning: A Laboratory Manual (3rd ed. Science Press, 2002) or followed the manufacturer's recommendation.

Materials and Instruments

[0067] PCR gel extraction kit and plasmid preparation kit were available from Axygen (U.S.).

[0068] PCR high-fidelity polymerase PrimeSTAR Max DNA Polymerase was purchased from Takara (Japan).

[0069] Restriction endonuclease and T4 ligase were purchased from New England Biolabs (NEB).

[0070] Seamless cloning kit was purchased from Vazyme Biotech Co., Ltd.

[0071] E. coli DH10B was used for cloning construction, BL21(DE3) was used for protein expression.

[0072] Vector pETDuet-1 (Novagen) was used for gene cloning and protein expression.

[0073] Wild-type UGT76G1 and EUGT11 were synthesized by Genscript Biotechnology Co., Ltd. (NanJing) and optimized with E. coli codon.

[0074] Ni NTA was purchased from Qiagen (Germany).

[0075] Isoorientin standard was purchased from Dalian Meilun Biotechnology Co., Ltd. Other reagents are analytical grade reagent or chromatographic grade reagent, purchased from Sinopharm Chemical Reagent Co., Ltd.

[0076] IPTG, MgCl.sub.2, PMSF and ampicillin were purchased from Sangon Biotech (Shanghai) Co., Ltd.

[0077] DNase I (10 mg/ml) was purchased from Shanghai yanye biotechnology service center.

[0078] PMSF was purchased from Sigma China.

[0079] Arktik Thermal Cycler (Thermo Fisher Scientific) was used for PCR.

[0080] Concentrator plus concentrator (Eppendorf) was used for vacuum concentration.

[0081] OD.sub.600 was detected using UV-1200 spectrophotometer (Shanghai Mapada Instrument Co., Ltd.).

[0082] C3 high pressure cell crusher (Sunnybay Biotech Co., Canada) was used for cell broken.

[0083] Dionex UltiMate 3000 Liquid Chromatography System (Thermo Fisher Scientific) was used for HPLC.

[0084] Thermo Fisher Scientific electrostatic field orbitrap combined mass spectrometry Q Exactive was used for high-resolution mass spectra.

EXAMPLE 1. CONSTRUCTION OF WILD-TYPE UGT76G1 EXPRESSION VECTOR pQZ11

[0085] Amino acid sequence of wild-type UGT76G1:

TABLE-US-00001 >SrUGT76G1 (SEQ ID NO: 1) MENKTETTVRRRRRIILFPVPFQGHINPILQLANVLYSKGFSITIFHTNF NKPKTSNYPHFTFRFILDNDPQDERISNLPTHGPLAGMRIPIINEHGADE LRRELELLMLASEEDEEVSCLITDALWYFAQSVADSLNLRRLVLMTSSLF NFHAHVSLPQFDELGYLDPDDKTRLEEQASGFPMLKVKDIKSAYSNWQIL KEILGKMIKQTKASSGVIWNSFKELEESELETVIREIPAPSFLIPLPKHL TASSSSLLDHDRTVFQWLDQQPPSSVLYVSFGSSSEVDEKDFLEIARGLV DSKQSFLWVVRPGFVKGSTWVEPLPDGFLGERGRIVKWVPQQEVLAHGAI GAFWTHSGWNSTLESVCEGVPMIFSDFGLDQPLNARYMSDVLKVGVYLEN GWERGEIANAIRRVMVDEEGEYIRQNARVLKQKADVSLMKGGSSYESLES LVSYISSL*

[0086] The target gene was amplified with specific primer pairs (harboring BamHI and HindIII, Table 1) and with the codon-optimized UGT76G1 gene cloning vector as the template. The PCR product was ligated using T4 ligase into the vector pETDuet1 double digested by BamHI/HindIII. The resulted expression vector was verified as pQZ11 by sequencing.

TABLE-US-00002 TABLE 1 Primers used in the construction of wild-type UGT76G1 expression vector Primer Name Sequence Primer_F ATTCTGGATCCATGGAAAACAAAAC(SEQ ID NO: 6) Primer_R CGCAAGCTTTTAACTTTACAGAGAA(SEQ ID NO: 7)

EXAMPLE 2. CONSTRUCTION OF MUTANTS G87F, I199F, L204F, L379W

[0087] After extensive and in-depth screening, the inventors focus on the following mutations:

[0088] Mutation of G at 87th to F;

[0089] Mutation of I at 199th to F;

[0090] Mutation of L at 204th to F;

[0091] Mutation of L at 379th to W.

[0092] The amino acid sequences of the key mutant enzymes are as follows:

TABLE-US-00003 >SrUGT76G1_G87F (SEQ ID NO: 2) MENKTETTVRRRRRIILFPVPFQGHINPILQLANVLYSKGFSITIFHTNF NKPKTSNYPHFTFRFILDNDPQDERISNLPTHGPLAFMRIPIINEHGADE LRRELELLMLASEEDEEVSCLITDALWYFAQSVADSLNLRRLVLMTSSLF NFHAHVSLPQFDELGYLDPDDKTRLEEQASGFPMLKVKDIKSAYSNWQIL KEILGKMIKQTKASSGVIWNSFKELEESELETVIREIPAPSFLIPLPKHL TASSSSLLDHDRTVFQWLDQQPPSSVLYVSFGSSSEVDEKDFLEIARGLV DSKQSFLWVVRPGFVKGSTWVEPLPDGFLGERGRIVKWVPQQEVLAHGAI GAFWTHSGWNSTLESVCEGVPMIFSDFGLDQPLNARYMSDVLKVGVYLEN GWERGEIANAIRRVMVDEEGEYIRQNARVLKQKADVSLMKGGSSYESLES LVSYISSL* >SrUGT76G1_I199F (SEQ ID NO: 3) MENKTETTVRRRRRIILFPVPFQGHINPILQLANVLYSKGFSITIFHTNF NKPKTSNYPHFTFRFILDNDPQDERISNLPTHGPLAGMRIPIINEHGADE LRRELELLMLASEEDEEVSCLITDALWYFAQSVADSLNLRRLVLMTSSLF NFHAHVSLPQFDELGYLDPDDKTRLEEQASGFPMLKVKDIKSAYSNWQFL KEILGKMIKQTKASSGVIWNSFKELEESELETVIREIPAPSFLIPLPKHL TASSSSLLDHDRTVFQWLDQQPPSSVLYVSFGSSSEVDEKDFLEIARGLV DSKQSFLWVVRPGFVKGSTWVEPLPDGFLGERGRIVKWVPQQEVLAHGAI GAFWTHSGWNSTLESVCEGVPMIFSDFGLDQPLNARYMSDVLKVGVYLEN GWERGEIANAIRRVMVDEEGEYIRQNARVLKQKADVSLMKGGSSYESLES LVSYISSL* >SrUGT76G1_L204F (SEQ ID NO: 4) MENKTETTVRRRRRIILFPVPFQGHINPILQLANVLYSKGFSITIFHTNF NKPKTSNYPHFTFRFILDNDPQDERISNLPTHGPLAGMRIPIINEHGADE LRRELELLMLASEEDEEVSCLITDALWYFAQSVADSLNLRRLVLMTSSLF NFHAHVSLPQFDELGYLDPDDKTRLEEQASGFPMLKVKDIKSAYSNWQIL KEIFGKMIKQTKASSGVIWNSFKELEESELETVIREIPAPSFLIPLPKHL TASSSSLLDHDRTVFQWLDQQPPSSVLYVSFGSSSEVDEKDFLEIARGLV DSKQSFLWVVRPGFVKGSTWVEPLPDGFLGERGRIVKWVPQQEVLAHGAI GAFWTHSGWNSTLESVCEGVPMIFSDFGLDQPLNARYMSDVLKVGVYLEN GWERGEIANAIRRVMVDEEGEYIRQNARVLKQKADVSLMKGGSSYESLES LVSYISSL* >SrUGT76G1_L379W (SEQ ID NO: 5) MENKTETTVRRRRRIILFPVPFQGHINPILQLANVLYSKGFSITIFHTNF NKPKTSNYPHFTFRFILDNDPQDERISNLPTHGPLAGMRIPIINEHGADE LRRELELLMLASEEDEEVSCLITDALWYFAQSVADSLNLRRLVLMTSSLF NFHAHVSLPQFDELGYLDPDDKTRLEEQASGFPMLKVKDIKSAYSNWQIL KEILGKMIKQTKASSGVIWNSFKELEESELETVIREIPAPSFLIPLPKHL TASSSSLLDHDRTVFQWLDQQPPSSVLYVSFGSSSEVDEKDFLEIARGLV DSKQSFLWVVRPGFVKGSTWVEPLPDGFLGERGRIVKWVPQQEVLAHGAI GAFWTHSGWNSTLESVCEGVPMIFSDFGWDQPLNARYMSDVLKVGVYLEN GWERGEIANAIRRVMVDEEGEYIRQNARVLKQKADVSLMKGGSSYESLES LVSYISSL*

[0093] The mutant gene was amplified by PCR, with primers containing point mutation(s) (Table 2) and with wild-type UGT76G1 expression vector pQZ11 as a template. After digesting the template plasmid with Dpn I enzyme, the mutant gene was transformed into DH10B. Clones were selected and verified by sequencing.

TABLE-US-00004 TABLE 2 Primers used to amplify mutants Primer Name Sequence Primer_G87F_F GGTCCGCTGGCGTTCATGCGTATCCCGATC (SEQ ID NO: 8) Primer_G87F_R GAACGCCAGCGGACCGTGGGTCGG (SEQ ID NO: 9) Primer_I199F_F GCGTACTCTAACTGGCAGTTCCTGAAAGAAATCCTGGG (SEQ ID NO: 10) Primer_I199F_R CTGCCAGTTAGAGTACGCAGATTTGATGTCTTTAAC (SEQ ID NO: 11) Primer_L204F_F GGCAGATCCTGAAAGAAATCTTCGGTAAAATGATCAAAC AGACC (SEQ ID NO: 12) Primer_L204F_R CTTTCAGGATCTGCCAGTTAGAGTACGCAG (SEQ ID NO: 13) Primer_L379W_F CTTCTCTGACTTCGGTTGGGACCAGCCGCTGAACG (SEQ ID NO: 14) Primer_L379W_R ACCGAAGTCAGAGAAGATCATCGGAACACCTTCGC (SEQ ID NO: 15)

EXAMPLE 3. MUTANT PROTEIN EXPRESSION AND PURIFICATION

[0094] The expression vector containing the mutant that was verified to be correct was transformed into E. coli expression host BL21(DE3). BL21 (DE3) harboring mutant expression vector was cultured overnight and transferred to 1 L LB (Ampicillin=100 μg/L) at 1% (v/v) and cultured at 37° C. and 200 rpm for 1 h to 2 h. Then, the bacteria were cultured at 16° C. and 160 rpm until OD.sub.600=1.0. IPTG of final concentration 0.1 mM was used for induction, and the cells were collected after 18 to 20 hours of overnight culture. The cells were resuspended with Buffer A (20 mm Tris HCl, pH 8.0, 100 mM NaCl), added with 1 mM PMSF, 2 mM MgCl.sub.2 and 5 μg/mL DNaseI and mixed well, than held on ice for 30 minutes. The cells were lysed by high-pressure cell crusher, and centrifuged at high speed (10000 rpm, 99 min). The supernatant was spin-incubated with 1 ml Ni-NTA purification resin (4° C., 1 h), and then eluted with 6-10 column volumes of 25 mM imidazole. Finally, 1 mL of 250 mM imidazole was used to incubate at 4° C. for 10-30 minutes to elute the target protein. The BSA method was used to determine the concentration of the target protein, which was stored in 50% glycerol (−20° C.). The mutant protein was used for enzyme activity test in vitro later.

EXAMPLE 4. ENZYMIC REACTION OF MUTANT IN VITRO

[0095] The enzymic reaction system (50 μL) includes: 5 μg protein, 1.5 mM UDP-glucose, 250 μM glycosyl acceptor substrate isoorientin, orientin, apigenin or luteolin, and Buffer A (20 mM Tris-HCl, pH=8.0, 100 mM NaCl). The reaction of each mutant protein for the same substrate was repeated three times. Reaction conditions: 37° C. overnight. The reaction was quenched with an equal volume of methanol. After vigorous shaking, the reaction was centrifuged at 12000 rpm for 30 min. The supernatant was used for HPLC detection. Detection method: mobile phase A (acetonitrile, containing 0.1% formic acid)—mobile phase B (water, containing 0.1% formic acid) gradient elution. Chromatographic conditions: gradient elution from initial 5% A to 50% A within 11 min, elution at 100% A for 4 min, and back to 5% A within 1 min for equilibration for 4 min. The peak area of the catalytic product of the mutant was calculated and compared with the peak area of the catalytic product of wild-type UGT76G1.

EXAMPLE 5. CATALYTIC ACTIVITY OF MUTANT

[0096] HPLC analysis of FIG. 1 shows the in vitro functional verification results for the substrate isoorientin. The conversion efficiency of isoorientin (compound 1) by wild-type UGT76G1 is low, and the glycosylation products mainly include compound 2 and compound 3. The mutants had a significant promoting effect on the conversion of isoorientin, among which the conversion efficiency of G87F was relatively the highest. The secondary MS fragmentation information of compounds 2 and 3 (FIG. 2, FIG. 3) suggested that they are isoorientin-O-glucoside and isoorientin-O-diglucoside, respectively.

[0097] The glycosylation efficiency by mutants G87F, I199F, L204F was significantly increased, and the yield of compound 2 was greatly increased to 1381%, 799% and 285% as compared with the wild type; the yield of compound 3 was greatly increased to 1235%, 278% and 159% of the wild type (FIG. 4).

[0098] HPLC analysis of FIG. 5 indicates the functional verification results for the substrate orientin (compound 4) in vitro. Wild-type UGT76G1 shows a certain conversion rate for orientin (compound 4), and the glycosylated products mainly include compound 5 and 6. The mutant L204F can increase the production efficiency of product 5, and the mutant L379W can increase the production efficiency of product 6. The LC-MS results of compounds 5 and 6 (FIG. 6) suggested that they are the two isomers of orientin-O-glucoside.

[0099] HPLC analysis of FIG. 7 indicates the in vitro functional verification of the substrate apigenin (compound 7). The glycosylation products of apigenin (compound 7) converted by mutants G87F, L204F are significantly different from those converted by wild-type (WT) or non-aromatic side chain mutants (I90L, I199L), suggesting that the glycosylation of the products occurs at different position.

[0100] HPLC analysis of FIG. 8 indicates the in vitro functional verification of the substrate luteolin (compound 8). The glycosylation products of luteolin (compound 8) converted by mutants G87F, L204F are significantly different from those converted by wild-type (WT) or non-aromatic side chain mutants (I90L, I199L), which is presumed to be caused by the different glycosylation sites of the products.

[0101] Each reference provided herein is incorporated by reference to the same extent as if each reference was individually incorporated by reference. In addition, it should be understood that based on the above teaching content of the disclosure, those skilled in the art can practice various changes or modifications to the disclosure, and these equivalent forms also fall within the scope of the appended claims.