Abstract:
The present invention relates to the field of genetic engineering and hereditary engineering. The present invention discloses a β-galactosidase (β-D-galactoside galactohydrolase, EC 3.2.1.23) mutant with high transglycosidase activity, which is obtained by single-site-saturation mutation of amino acid sequences of β-galactosidase from  Aspergillus candidus  and  Aspergillus oryzae,  with own signal peptides removed. The transglycosidase activity of the mutant is over 15% higher than that of wild types. Meanwhile, the present invention also discloses a DNA molecule which encodes the mutant, a recombinant expression vector containing the DNA molecule, and a host cell expressing the DNA molecule. In addition, the present invention also provides a method for preparing β-galactosidase mutant with high transglycosidase activity by using the recombinant expression vector and applications of the mutant, the DNA molecule, the recombinant expression vector and the host cell in preparation of β-galactosidase.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a national phase entry and claims the priority of International Application No. PCT/CN2015/075942, filed on Apr. 07, 2015, which is based upon and claims priority to NO. CN201410148999.4, filed on Apr. 14, 2014, the entire contents of which are incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to the field of genetic engineering and hereditary engineering. The present invention discloses a β-galactosidase (β-D-galactoside galactohydrolase, EC 3.2.1.23) mutant with high transglycosidase activity, a preparation method thereof, and uses thereof. 
       BACKGROUND OF THE INVENTION 
       [0003]    Galactooligosaccharides (GOS) are a kind of oligosaccharides which cannot be digested and absorbed by the gastrointestinal tracts of human bodies, but directly enter the large intestine to be well utilized by various  Bifidobacterium,  and have special biological functions. GOS can improve the micro-ecological environment in the human body, aid multiplication of  Bifidobacterium  and other beneficial bacteria, and improve immunity of the human body. Meanwhile, GOS generate organic acids through metabolism to decline the pH value in tracts, to restrain the growth of salmonella and putrefying bacteria in the tracts, to reduce toxic fermented products and hazardous bacterial enzymes, to adjust gastrointestinal functions, thus reducing burdens on livers for decomposition of toxins. GOS have properties better than those of other functional oligosaccharides and therefore are more conveniently and easily applied to various fields as additives. GOS can be adapted to more food varieties and wider consumer groups, and have a huge application value and a huge market prospect. 
         [0004]    GOS are usually prepared by five methods, namely extraction from natural materials, acid hydrolysis of natural polysaccharides, chemical synthesis, fermentation and enzymatic synthesis. GOS merely exist in nature, are color-less, have no charge, and therefore are difficultly extracted and separated. Products converted from the natural polysaccharides have a low yield, are complicated in elements and hard to be purified. The chemical synthesis tends to generate a lot of toxins and residues, causing serious environmental pollution. The fermentation method for producing the GOS is rarely studied, is still in the laboratory stage, and fails to realize mass production. At present, the industrial production of the GOS is completed through β-galactosidase (β-D-galactoside galactohydrolase, EC 3.2.1.23). β-galactosidase, also called lactase, has dual activities, namely hydrolysis and transglycosylation. Previously, studies on the β-galactosidase mainly focus on utilization of the hydrolysis function thereof to produce low lactose milk products to relieve various side effects such as diarrhea and abdominal distension of lactose-intolerant patients caused by taking milk products. Since the special health-care functions of the GOS have been determined, production of the GOS by the transglycosidase effect of the β-galactosidase has become a study hotspot. The study mainly focuses on the three following aspects: 
         [0005]    1. Screening of Strains for Generating the β-galactosidase with High Transglycosidase Activity 
         [0006]    Various microorganisms including yeasts,  Bacillus, Aspergillus, Penicillium  and Bifidobacteria, all have β-galactosidase with transglycosidase activity. Studies show that, due to different enzymatic properties, β-galactosidase coming from different sources vary with reactions conditions for synthesizing the GOS. β-Galactosidase can be classified into acidic type and neutral type according to the optimum pH values. Usually, β-galactosidase sourced from mold is acidic enzyme, with the best performance at a pH value in a range of 2.5-5.5 and at a relatively high temperature (50-60° C.) β-Galactosidase generated by yeasts and bacteria is neutral enzyme, with the best performance at a pH value in a range of 6-7.5 and at a relatively low temperature (30-40° C). β-Galactosidase generated by different sources work on different substrates, and the types and ratios of oligosaccharide in the generated GOS are also diversified, so the new GOS verities emerge in endlessly. Even so, the screened β-galactosidase generally has low transglycosidase activity. Moreover, the highest yield of the GOS is usually 5-30%, failing to meet the demands of industrial production. 
         [0007]    2. Optimization of Reaction Conditions and Improvement of Production Process 
         [0008]    Some researchers tried to overcome the defect of low transglvcosidase activity through optimizing the production conditions and processes of the GOS to enhance the yield of the GOS, and have made some achievements. Main methods include: increase in concentration of the initial lactose, control over the water activity using organic solvents and use of the immobilization technology. The hydrolysis and transglycosylation reaction of the β-galactosidase are inversible. When the substrate (lactose) concentration is low, the concentration of the hydrolysis product, namely galactose, is low, and the galactose has a limit effect on restraining the hydrolysis enzymatic activity. In such circumstances, the β-galactosidase represents high hydrolysis activity, while the transglycosylation activity is low, so the content of monosaccharides contained in the product is relatively high. When the lactose concentration is relatively high, the concentration of the hydrolysis product, namely the galactose, is relatively high, and the galactose reaching a certain-value can restrain the hydrolysis enzymatic activity. The galactose is the substrate of the transglycosidase, and the high galactose concentration aids synthesis of galactooligosaccharide, and the product has a high content of oligosaccharide. Using organic solvents is good for composition of the oligosaccharide because organic solvents can reduce the water activity in the reaction system to affect the activity site and reaction mechanism of the enzyme, to induce the hydrolase to catalyze inverse transglycosylation, and to deviate the reaction balance from hydrolysis to oligosaccharide synthesis. Using the immobilization technology can greatly increase the pH and thermal stability of free enzymes, and can realize recycling and reduce production cost. Mozaffar was reported that β-galactosidase is absorbed to phenolic resin and then is cross-linked with glutaraldehyde, and then the yield of the oligosaccharide is enhanced by 20%. However, some studies find that when an immobilized enzyme is applied to the lactose solution with a relatively high concentration, the yield of the oligosaccharide is smaller than that the yield of the oligosaccharide generated when the free enzyme is used. Thus it can be seen that problems cannot always be radically solved simply by optimizing conditions. 
         [0009]    3. Genetic Engineering to Enhance Expression of the β-galactosidase and to Improve its Properties 
         [0010]    In the natural world, the yield of GOS by wild β-galactosidase is generally maintained in the range of 20-45%. The low yield fails to meet production demands, screening excellent transglycosidase mutant enzyme through molecular modification has become a research hotspot. Hansen O. (2001) found that after Bifidobacteria β-galactosidase BIF3 is deleted with 580 amino acids at a C-terminal, the protein is converted into an efficient transglycosidase which can generate GOS by using almost 90% of lactose, while hydrolysis elements account for 10%. When the lactose concentration is in the range of 10%-40%, the ratio of the transglycosidase activity to the hydrolysis activity is always maintained at 9:1. In 2009, Planer G. carried out directed revolution on the β-galactosidase sourced from  Geobacillus stearothermophillus  KVE39, and successfully obtained three strains of mutants R109W, R109V and R109K on the screen strategy of enhancing the transglycosidase activity while reducing the hydrolysis activity. In 18% (w/v) lactose, the yield of oligosaccharide generated by three mutants was 23%, 11.5% and 21%, respectively. In wild enzyme, the oligosaccharide yield was 2% only. Wu Y. (2013) modified molecules of the β-galactosidase sourced from  Sulfolobus acidocaldarius  to study the most appropriate generation conditions of GOS. Under respective most appropriate conditions, the GOS yield of the mutant F441Y was 61.7%, F359Q was 58.3%, and the wild enzyme was 50.9%. 
         [0011]    However, so far, the screening and separation as well as process optimization of the natural enzymes, and genetic engineering to enhance the expression of the β-galactosidase and improve properties both fail to change the current states of low transglycosidase activity and low yield of the β-galactosidase, resulting in low synthesis yield of the GOS and extremely high production cost which seriously restrain the low-cost production, promotion and application of the GOS. 
         [0012]    Therefore, creating a novel β-galactosidase with high transglycosidase activity and low-cost production are two of main problems to be solved urgently in the current research and production. 
       SUMMARY OF THE INVENTION 
       [0013]    Aiming at the defects mentioned above, on the one hand, the present invention provides a β-galactosidase mutant with high transglycosidase activity, which is obtained by single-site-saturation mutation of β-galactosidase from  Aspergillus candidus  or  Aspergillus oryzae  preferably obtained through single-site-saturation mutation of an amino acid sequence as shown in sequence 2 or sequence 4, wherein transglycosidase activity of the mutant is over 15% higher than that of the wild types, preferably over 20%, and more preferably over 30%. 
         [0014]    In an optimal implementation scheme of the present invention, the sites where mutation occurs include an amino acid at site 219, an amino acid at site 245 or an amino acid at site 785. 
         [0015]    In a further optimal implementation scheme of the present invention the single-site-saturation mutation is respectively involved in replacement of serine residues (S219G) at site 219 by glycine residues, replacement of serine residues (S219E) at site 219 by glutamic acid residues, replacement of serine residues (S219F) at site 219 by phenylalanine residues, replacement of serine residues (S219V) at site 219 by valine residues, replacement of serine residues (S219A) at site 219 by alanine residues, replacement of phenylalanine residues (F245R) at site 245 by Arginine residues, replacement of phenylalanine residues (F245K) at site 245 by lysine residues, replacement of phenylalanine residues (F245G) at site 245 by lysine residues, replacement of phenylalanine residues (F245S) at site 245 by serine residues, or replacement of glutamic acid residues (E785V) at site 785 by value residues. 
         [0016]    On another hand, the present invention provides a DNA molecule encoding the mutant. 
         [0017]    On another hand, the present invention provides a recombinant expression vector containing the DNA molecule, preferably a recombinant yeast expression vector. 
         [0018]    On another hand, the present invention provides a host cell expressing the DNA molecule, preferably strains of saccharomyces, kluyveromyces, schizosaccharomyces and methylotrophic yeasts, wherein the methylotrophic yeast strains are preferably  Pichia pastoris  strains. 
         [0019]    On another hand, the present invention provides a method for preparing β-galactosidase mutant with high transglycosidase activity, including the following steps: 
         [0020]    1. transformation the host cell by using the recombinant expression vector according to the claims above to obtain recombinant strains; 
         [0021]    2. culturing the recombinant strains to induce expression of the recombinant β-galactosidase protein; 
         [0022]    3. gathering and purifying the expressed β-galactosidase mutant with high transglycosidase activity. 
         [0023]    At last, the present invention provides applications of the mutant, the DNA molecule, the recombinant expression vector and the host cell in preparation of β-galactosidase. 
         [0024]    The present invention adopts the single-site-saturation mutation technology to perform site-saturation mutation on the β-galactosidase gene lacb′, with its own signal peptides removed of the  Aspergillus candidus  and the β-galactosidase gene laco′, with own signal peptides removed, of the  Aspergillus oryzae,  and obtains the β-galactosidase mutant with high transglycosidase activity, so the transglycosidase activity of the mutant is over 15% higher than that of wild types, and even over 30% higher. Therefore, the preparation of the β-galactosidase mutant with high transglycosidase activity becomes true, which lays a good foundation for applications of the β-galactosidase GOS production. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1 : Process of molecular docking between the β-galactosidase of  Aspergillus candidus, Aspergillus oryzae  with substrates. 
           [0026]      FIG. 2 : Position relationship among the mutation sites of the β-galactosidase of the  Aspergillus candidus  and the  Aspergillus oryzae  with lactose in molecular space. 
           [0027]      FIG. 3 : Construction process of a recombinant expression vector containing the gene of the mutant, namely the β-galactosidase. 
           [0028]      FIG. 4 : Yield of GOS generated by typical mutants at site S219. 
           [0029]      FIG. 5 : Yield trend of the GOS in a mutant library at site F245. 
           [0030]      FIG. 6 : Yield of GOS generated by typical mutants at site F245. 
           [0031]      FIG. 7 : Yield trend of the GOS in a mutant library at site E785. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0032]    The present invention is described in further detail in conjunction with embodiments, which are used for describing the present invention, not limiting the present invention. It should be pointed out that, for those skilled in this field, various improvements and modifications can be made on the present invention according to the principle of the present invention. Those improvements and modifications shall also fall within the protective scope of the present invention. 
       Embodiment 1 
     Tertiary Structure of the β-galactosidase and the Prediction of Mutation Sites 
       [0033]    β-Galactosidase gene lacb′ with own signal peptide removed was obtained by the laboratory through cloning from  Aspergillus condidus.  The gene with own signal peptide sequence removed consists of 2,958 nucleotides, with a specific sequence as shown in sequence 1. A protein encoded by the gene consists of 986 amino acids, with a specific sequence as shown in sequence 2. 
         [0034]    β-Galactosidase gene laco′ with own signal peptide removed was obtained by the laboratory through cloning from  Aspergillus oryzae.  The gene also consists of 2,958 nucleotides, with a specific sequence is as shown in sequence 3. A protein encoded by the gene also consists of 986 amino acids, with a specific sequence as shown in sequence 4. The sequence of the amino acids was different from the protein encoded by the gene lacb′ in only three amino acids at site 231: lacb′ (Gly), laco′(Ser); site 401: lacb′(Met), laco′(Ile); and site 970: lacb′ (Asp), lacb′(Asn). 
         [0035]    The β-galactosidase of the  Aspergillus candidus  and the  Aspergillus oryzae  was used as the research material. Crystal structure of β-galactosidase (PDB login No.: 1TG7) from  Penicillium  crystal structure of β-galactosidase (PDB login No.: 4IUG) from  Aspergillus oryzae  and a protein crystal structure of β-galactosidase (PDB login No: 30G2) from  Trichoderma reesei  were used as homologous models to predict the 3D structure of the β-galactosidase and its docking areas with substrates. The predicted structures were highly similar to the prediction results reported in literatures (See The crystal structure of acidic β-galactosidase from  Aspergillus oryzae,  Mirko M. Maksimainen, International Journal of Biological Macromolecules, 2013, 109-115). The protein consisted of five structural domains: structural domain 1 (amino acids 1-394) close to terminal N was the activity center of the enzyme, wherein the activity center was a TIM barrel-type structure. Structural domain 2 (amino acids 395-573) consisted of 16 reverse parallel β-pleated sheets and 1 α-helix, containing a structural sub-domain similar to the immune globulin; structural domain 3 (amino acids 574-661) consisted of a β-interlayer, which was comprised of 8 reverse parallel β-pleated sheets and was shaped like a “Greek key”, and 1 α-helix; structural domain 4 (amino acids 662-857) and structural domain 5 (amino acids 858-1005) were consisted of “spring roll” shaped topological structures. Through focused analysis of the activity center, it was found that, Glu160 and Glu258, respectively on the fourth and the seventh β-pleated sheets of the TIM barrel-type activity center might be acid acids which were necessary for catalytic reaction, while Asn140 and Tyr96 might be used for immobilizing lactose naolecules. 
         [0036]    According to the 3D structure of the obtained β-galactosidase, software Discovery Studio was used to simulate the molecular docking of the enzyme and the substrate (see  FIG. 1 ), and by analyzing the docking results, the amino acids interacting with the substrate might be known (see  FIG. 2 ). Computational biological software was used to evaluate the evolution entropies of the amino acids one by, one, and six amino acid sites, S219, D239, S240, V741, F245 and E785 (see table 1), with large changes in the evolution entropy were finally screened and determined to carry out site-saturation mutation. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Entropies of typical amino acids of the β-galactosidase 
               
             
          
           
               
                 Amino acid 
                 Amino acid site 
                 Evolution entropy 
               
               
                   
               
             
          
           
               
                 S 
                 219 
                 0.8114 
               
               
                 D 
                 239 
                 0.6678 
               
               
                 S 
                 240 
                 0.8581 
               
               
                 Y 
                 241 
                 0.8374 
               
               
                 F 
                 245 
                 0.8594 
               
               
                 E 
                 785 
                 0.288 
               
               
                   
               
             
          
         
       
     
       Embodiment 2 
     Construction of Single-Site-Saturation Mutant Library of  Pichia pastoris    
       [0037]    1. Materials and Method 
         [0038]    (1) Strains and Plasmid 
         [0039]    Wild genes were sourced from β-galactosidase gene lacb′, with signal peptide removed, of  Aspergillus candidus,  and the β-galactosidase gene laco′ of the  Aspergillus oryzae,  which were obtained by previous cloning in the laboratory. The specific sequences can be seen in sequence 1 and sequence 3, respectively. The wild genes were connected to pPIC9 expression vectors, and were expressed in the  Pichia pastoris  GS1.15.  Escherichia coli  Trans1-T1 competent cells were brought from TransGen Corporation Ltd. pPIC9 expression vectors and  Pichia pastoris  GS115 were brought from Invitrogen Corporation Ltd. 
         [0040]    (2) Culture Mediums and Preparation of Related Solutions 
         [0041]    For transformation, culture and screening of the  Pichia pastoris,  conventional culture mediums and reagents, refer to the Pichia Expression Kit manual of Invitrogen Corporation Ltd. 
         [0042]    PTM trace salts: 0.6% CuSO 4 , 0.008% NaI 2 , 0.3% MnSO 4 , 0.02% Na 2 MoO 4 , 0.002% H 3 BO 3 , 0.05% CoCl 2 , 2% ZnCl 2 , 6.5% FeSO 4  and 0.5% sulfuric acid (v/v). 
         [0043]    Fermentation base salt medium (FBSM): 0.5% KH 2 PO 4 , 5% NH 4 H 2 PO 4 , 1.485% MgSO 4 , 1.82% K 2 SO 4 , 0.093% CaSO 4 , 0.15% KOH, 0.00011% Biotin, 0.44% PTM trace salts, and 2% glucose. 
         [0044]    Fermentation base induction medium (FBIM): 0.5% KH 2 PO 4 , 5% NH 4 H 2 PO 4 , 1.485% MgSO 4 , 1.82% K 2 SO 4 , 0.093% CaSO 4 , 0.15% KOH, 0.00011% Biotin, 0.44% PTM trace salts and 0.5% methanol. 
         [0045]    Na 2 HPO 4−  citric acid buffer solution (0.1mol/L, pH5.2): 536 mL of 0.2 mol/L Na 2 HPO 4 , 464 niL of 0.1 mol/L citric acid were mixed uniformly, with a pH value adjusted to 5.2. 
         [0046]    (3) Oligonucleotides Primer 
         [0047]    Specific primers used in the gene mutation can be seen in table 2. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 List of primers used in gene mutation 
               
             
          
           
               
                   
                   
                 Primer sequence 
               
               
                 Primer position 
                 Primer name 
                 (5′-3′, mutated basic group underlined) 
               
               
                   
               
               
                 5′ terminal and 
                 Bgl-down(A) 
                 CGCGAGGCAGAGATCTTGAGATAAATTTCACG 
               
               
                 3′ terminal of gene 
                 Bgl-down-Com(D) 
                 ACGTGAAATTTATCTCAAGATCTCTGCCTCGCG 
               
               
                   
               
               
                 Library S219 
                 219-up(B 219 ) 
                 ACTTCCAGGAGCATTGTGCCCAGAAGG MNN GGCATCGTTG 
               
               
                   
                 219-down(C 219 ) 
                 TTCTGGGCACAATGCTCCTGGAAGTGGAACG 
               
               
                   
               
               
                 Library D239 
                 239-up(B 239 ) 
                 TGCGCAATCAAAGCCAAGGGGATAGCT MNN GTGACC 
               
               
                   
                 239-down(C 239 ) 
                 TCCCCTTGGCTTTGATTGCGCAAACCC 
               
               
                   
               
               
                 Library S240 
                 240-up(B 240 ) 
                 TGCGCAATCAAAGCCAAGGGGATA MNN ATCGTGACC 
               
               
                   
               
               
                 Library Y241 
                 241-up(B 241 ) 
                 TTTGCGCAATCAAAGCCAAGGGG MNN GCTATC 
               
               
                   
                 241-down(C 241 ) 
                 TGGCTTTGATTGCGCAAACCCATCCGTATG 
               
               
                   
               
               
                 Library F245 
                 245-up(B 245 ) 
                 ATACGGATGGGTTTGCGCAATC MNN GCCAAG 
               
               
                   
                 245-down(C 245 ) 
                 TGCGCAAACCCATCCGTATGGCCC 
               
               
                   
               
               
                 Library E785 
                 785-up(B 785 ) 
                 TTTCCTCGCCGACCGTCCAATT AAC GTCG 
               
               
                   
                 785-down(C 785 ) 
                 TGGACGGTCGGCGAGGAAACCATGAAG 
               
               
                   
               
             
          
         
       
     
         [0048]    2. Amplification of Mutation Sites Using Overlap PCR 
         [0049]    Overlap PCR was adopted to perform saturation imitationn single site. This means that two segments were respectively amplified by PCR, and then extended in an overlapped way to be fused. A pair of compatible primers was designed where was overlapped in a certain extent nearby a target point (refer to primers B, C in  FIG. 3 ). The primers were respectively combined with primers at terminals 5′ and 3′ of the gene (refer to primers A, D in.  FIG. 3 ). Upstream segments and downstream segments containing the target point were amplified. Those primers were complementary, so the produced PCR product chains were mutually overlapped. The Upstream segments and downstream segments were crossed at the target point, and extended in an overlapped way with template to each other to obtain full-length genes. 1 μL of pPIC9-lacb′ plasmids were taken as template, primer pairs A and C, B and D were respectively amplified by using TransStart FastPfu DNA polymerase. PCR products were tested with agarose gel electrophoresis. Segments with correct sizes were recycled (for the method, refer to the agarose gel DNA extraction kit of TIANGEN Corporation Ltd). 
         [0050]    3. In-vivo Homologous Recombination for Construction of the Expression Vector 
         [0051]    Two PCR segments with homologous arms are mixed with an equal molar weight, and then added with homologous recombinase to perform in-vivo recombination. The mixture reacted for 30 min at the temperature of 25° C. and then placed on ice for 5 min. Then the transformation shall occur immediately or the product shall be stored at a temperature of −20° C. 10 μL of the homologous recombination product was taken and chemically transferred into 100 μL of Trans1-T1 competent cells of the  Escherichia coli  the mixed substance was coated on a LB plate containing ampicillin. Then the plate was culture overnight at a temperature of 37° C. 
         [0052]    Mutation in an amount of 3-5 times the theoretical quantity (mutation codon was MNN; the theoretical value of the mutant library was 4×4×2=32: 32 single-site-saturation mutated clones could cover all mutants) growing from the LB plate could coverage mutation sites. 6-8 individual clones were randomly selected from the LB plate of each mutation library to measure the DNA sequence. The measurement was entrusted to MedicalBio Corporation Ltd. The mutant librarys were respectively named as library S219, library D239, library S240, library Y241, library F245 and library E785 according to mutation sites. 
         [0053]    4. Expression of the β-galactosidase Mutant and Screening Method of Strains with High Transglycosidase Activity 
         [0054]    (1) Expression of the Recombinant Plasmitis in the  Pichia pastoris    
         [0055]    Mixed plasmids (about 200-230 μg) in the  Escherichia coli  were extracted from each mutant library, completely digested by using sufficient restriction enzymes, then settled with propanol, washed by 70% ethanol, and then dissolved in de-ionized water to transform the  Pichia pastoris.  The transformed  Pichia pastoris  was coated on an MM plate containing x-gal. Blue strains on the MM plate were positive clones of the β-galactosidase, and corresponding strains on an MD plate were transferred into a 48-hole culture plate. Different mutants were cultured with the FBSM first, then grew quickly for 48 h in the 48-hole plate and next cultured in an inducing manner with the FBIM. The supernatant of the cultured product was taken to measure the β-galactosidase activities of the positive strains. 
         [0056]    (2) Method for Measuring Activity of the β-galactosidase Adopting oNPG Substrate 
         [0057]    0.1 g of o-nitrophenyl-β-D-galactopyranoside (oNPG) substrate was accurately weighed and dissolved in 40 mL of Na 2 HPO 4 -citric acid buffer solution (pH 5.2, 0.1 mol/L). Then, oNPG solution with a concentration of 0.25% (W/V) was obtained. The crude enzyme solution to be measured was diluted with the 0.1 mol/L Na 2 HPO 4 -citric acid buffer solution to a proper factor. 800 μL of substrate solution was added into a test tube and then was preheated for 2 min in a water bath at a temperature of 60° C. 200 μL of diluted enzyme solution was added and then mixed uniformly 1 mL of 10% trichloroacetic acid (ICA) was added to end the reaction after the reaction proceeded, for 15 min first; then 2 mL of 1 mol/L Na 2 CO 3  was added for coloration; and next the optical density (OD 420 nm ) was measured at 420 nm. The Na 2 HPO 4 -citric acid buffer solution (pH 5.2, 0.1 mol/L) was used, as a blank reference, and a standard curve was employed to calculate the amount of the generated oNP. Then, the activity of the β-galactosidase could be calculated. Definition of enzymatic activity unit: one unit of the β-galactosidase activity is defined as the enzyme amount required that can catalyze oNPG to generate 1 μmol o-nitrophenol (oNP) every minute at the temperature of 60° C. and at the pH of 5.2. 
         [0058]    According to the result obtained from the standard curve of the β-galactosidase, the calculation formula of the enzymatic activity is: 
         [0000]      Enzymatic activity ( U/ mL)=5* N* (0.9472 X+ 0.0046)/15 
         [0000]    X: optical density (OD 420 nm ) of the reaction; N: dilution factor of the enzyme solution; 15; 15 min reaction time; 5: converting the enzymatic activity in 200 μL of dilated enzyme solution into 1 mL. 
         [0059]    (3) Basic Reaction System and Reaction Conditions for Measurement of the Transglycosidase Activity of the Mutated-Protein 
         [0060]    The crude enzyme solution of each mutant was diluted by 0.1 mol/L Na 2 HPO 4 -citric acid buffer solution with a pH value of 5.2 to reach an equivalent protein concentration, meaning that every 60 μL of enzyme solution contained 5. μg of protein (with a concentration of about 0.08 mg/mL), 60 μL of the enzyme solution was taken, diluted, then placed in a tube, and added with 440 μL, of 30% (w/v) lactose substrate. The substrate and the enzyme solution were mixed as quickly as possible. All reactants were placed in a 200 rpm constant-temperature shaker to react for 6 h at a temperature of 50° C. After the reaction ended, the reaction product was boiled for 10 min, in a 100° C. water bath to end the reaction, and then centrifuged for 10 min at 12,000 r/min. 
         [0061]    The reaction product was diluted by 16 folds with ultra-purified water and then centrifuged for 10 min at 12,000 r/min, and 700 μL, of the centrifuged product was taken to do the HPLC test. 
         [0062]    Test conditions of the high performance liquid chromatograph (HPLC): 
         [0063]    Before carrying out the quantitative test with the HPLC, standard curves of the glucose, galactose and lactose were drawn. The test ranges of the glucose, galactose and lactose were all 0-25.6 mg/mL. Test conditions included: Waters e2695 Separations Module; mobile phase: pure water with 50 mM EDTA calcium sodium salt; column temperature: 85° C.; flow rate: 0.5 mL/min, time: 12 min/sample. 
         [0000]      Yield of oligosaccharide (mg/mL)=Initial amount of lactose (mg/mL)−Residual amount of lactose (mg/mL)−Amount of glucose (mg/mL)−Amount of galactose (mg/mL) (Jorgensen F et al, 2001).
 
         [0000]      GOS Conversion rate=Amount of oligosaccharide (mg/mL)/Initial amount of lactose (mg/mL). 
         [0000]      Rate of GOS obtained by consuming lactose=Amount of oligosaccharide (mg/mL)/(Initial amount of lactose (mg/mL)−Residual amount of lactose(mg/mL)).
 
       Embodiment 3 
     Screening of Saturation Mutation Library S219 and Synthesis of Oligosaccharides 
       [0064]    2.00 positive  Pichia pastoris  clones were selected from the mutant library S219 to measure the transglycosidase activity and measure the nucleotide sequence. The sequence measurement result shows that, the mutants were respectively eight different types of amino acids all of which could enhance the transglycosidase activity of the mutant enzyme (see Table 3), in particular amino acids with smaller side chains, such as Gly, Ala, Val, and polarity amino acid Glu with negative charges, wherein the mutant Gly was most prominent (see  FIG. 4 ). with an oligosaccharide yield increased by 26.6%. After the S219 was mutated into Glu with small side chains and negative charges, the oligosaccharide yield was increased by 25.7%. After the S219 was mutated into Ala and Val, the oligosaccharide yields were increased by 15.0% and 15.5% after the S219 was mutated into Asp, Arg and Leu, the oligosaccharide yields were respectively increased by 10.4%, 7.9% and 8.2%. After the S219 was mutated into aromatic amino acid Phe with large side chains, the transglycosylation was also greatly enhanced by 16.4%. However, if the S219 was mutated into Pro and Trp, the transglycosylation was obviously reduced by 16.7% and 28.7%, respectively. Thus it can be seen that, after the S219 was mutated into other aminoacids, the transglycosylation varied a lot, representing this site is an important site related to the transglycosylation of the β-galactosidase. In terms of protein structure, the site was positioned in the activity center-TIM barrel of the β-galactosidase. By the effect of charges and polarity, the site together with the lactose substrate generates a certain role, and the site is a non-conservation amino acid in the activity center. 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Oligosaccharides generated by different amino acid mutants of S219 
               
             
          
           
               
                   
                   
                 Mean amount of 
                   
               
               
                   
                 Different 
                 oligosaccharides 
                 Increase by percentage 
               
               
                   
                 amino acid 
                 generated 
                 (%) in comparison with 
               
               
                   
                 mutants 
                 (mg/mL) 
                 the wild enzymes 
               
               
                   
                   
               
             
          
           
               
                   
                 WT (Ser) 
                 43.61 
                 0 
               
               
                   
                 Gly 
                 54.95 
                 26.6 
               
               
                   
                 Glu 
                 54.54 
                 25.7 
               
               
                   
                 Phe 
                 50.57 
                 16.4 
               
               
                   
                 Val 
                 49.93 
                 15.5 
               
               
                   
                 Ala 
                 50.17 
                 15.0 
               
               
                   
                 Asp 
                 47.96 
                 10.4 
               
               
                   
                 Leu 
                 46.92 
                 8.2 
               
               
                   
                 Arg 
                 47.02 
                 7.9 
               
               
                   
                 Pro 
                 36.34 
                 −16.7 
               
               
                   
                 Trp 
                 31.17 
                 −28.7 
               
               
                   
                   
               
               
                   
                 Note: 
               
               
                   
                 WT represents wild enzyme. 
               
             
          
         
       
     
       Embodiment 4 
     Screening of the Saturation Mutation Library F245 and Synthesis of Oligosaccharides 
       [0065]    200 positive mutants were selected from the library F245 to measure the transglycosidase activity. From the transglycosidase activity measurement result and the sequence measurement result show that, the mutants varied a lot in the aspect of oligosaccharide yield. (see  FIG. 5 ). The oligosaccharide yields of some mutants were greatly lower than those of the wild enzymes, and some were about 30% higher. Thus it can be seen that, F245 is also an important site related to the generation of the oligosaccharide by the β-galactosidase. 
         [0066]    Specifically, after the site S245 was mutated into Arg, the oligosaccharide yield was maximum increased by about 35%, followed by Lys and Gly, increased by 30% and 24.7%, respectively. After the site F245 was mutated into other amino acids such as Set, Glu, Ala, Thr and Met, the yield of the oligosaccharides was also increased (see  FIG. 6 ). 
       Embodiment 5 
     Screening of the Saturation Mutation Library E785 and Synthesis of Oligosaccharides 
       [0067]    Compared with the wild types, in the E785 saturation mutant library, only a few mutants were increased in the yield of the oligosaccharides, and the increase was not greater than 20% (see  FIG. 7 ). Among the mutants with increase in the oligosaccharide yield in the mutant library E785. After Glu was mutated to Val, the increase in the oligosaccharide yield was 15%, and the majority of the rest mutants had an oligosaccharide yield similar to that of the wild enzymes.