Patent Publication Number: US-2009239273-A1

Title: Novel high-activity modified s-hydroxynitrile lyase

Description:
TECHNICAL FIELD 
     The present invention relates to a novel high-activity modified S-hydroxynitrile lyase (SHNL). More particularly, the present invention relates to SHNL that is obtained by modifying an amino acid sequence at a specific site and has improved activity compared with wild-type SHNLs. 
     BACKGROUND ART 
     S-Hydroxynitrile lyase (SHNL) catalyzes the reaction between hydrocyanic acid and aldehyde or ketone to generate optically active cyanohydrins. Optically active cyanohydrins are important intermediates for synthesizing medicines. Accordingly, it can be said that SHNL is an industrially important enzyme. 
     SHNLs derived from Cassava ( Manihot esculenta ), Pará rubber tree ( Hevea brasiliensis ), poaceous plants (i.e., sorghum ( Sorghum bicolor )), and the like have been known. At the industrial level, recombinant SHNL is preferably used in addition to wild-type SHNLs because separation of enzymes from organisms incurs a high cost. 
     Recombinant SHNL can be produced using  E. coli , yeast, or other hosts. From an industrial point of view, use of modified SHNL with improved activity is preferable in order to further improve cost performance. 
     In order to enhance enzyme activity, a modified SHNL in which tryptophan 128 of the SHNL amino acid sequence had been substituted with alanine has been developed (JP Patent Publication (kokai) No. 2000-125886 (A); Lauble et al., Protein Science, 2002, 11: pp. 65-71). However, mutation of SHNL for the purpose of enhancing activity has not yet been reported, except for amino acid 128 of the amino acid sequence. 
     DISCLOSURE OF THE INVENTION 
     An object of the present invention is to provide novel SHNL with significantly improved activity compared with wild-type SHNL. 
     The present inventors have conducted concentrated studies in order to attain the above object. As a result, they found that amino acid substitution in SHNL through genetic engineering could produce enzymes having significantly improved activity compared with the enzyme before modification. This has led to the completion of the present invention. 
     More specifically, the present invention provides modified SHNL, which is obtained by substituting at least one amino acid selected from amino acids 14, 44, 66, 94, 103, 118, 122, 125, 127, 129, 147, 148, 152, 212, and 216 in the amino acid sequence (SEQ ID NO: 2) of wild-type SHNL derived from cassava ( Manihot esculenta ) with another amino acid, modified SHNL, which is obtained by substituting at least one amino acid selected from amino acids 14, 44, 66, 94, 103, 118, 122, 125, 127, 129, 146, 147, 151, 211, and 215 in the amino acid sequence (SEQ ID NO: 4) of wild-type SHNL derived from the Pará rubber tree ( Hevea brasiliensis ) with another amino acid, or modified SHNL, which is obtained by inserting at least one amino acid between amino acids 128 and 129 of the amino acid sequence of the cassava- or Pará rubber tree-derived wild-type SHNL. 
     Specifically, the present invention provides modified SHNL comprising at least one amino acid substitution selected from among the substitutions shown below in the amino acid sequence of wild-type SHNL derived from cassava ( Manihot esculenta ): 
     a) substitution of histidine 14 with glutamine; 
     b) substitution of aspartic acid 44 with asparagine; 
     c) substitution of glutamic acid 66 with aspartic acid or lysine; 
     d) substitution of valine 94 with alanine, cysteine, histidine, lysine, phenylalanine, proline, serine, threonine, tryptophan, or tyrosine; 
     e) substitution of histidine 103 with glutamine; 
     f) substitution of valine 118 with leucine; 
     g) substitution of leucine 122 with isoleucine, valine, glycine, or serine; 
     h) substitution of phenylalanine 125 with asparagine, cysteine, histidine, glutamine, arginine, leucine, isoleucine, threonine, or proline; 
     i) substitution of aspartic acid 127 with glutamic acid, asparagine, or glycine; 
     j) substitution of arginine 129 with histidine; 
     k) substitution of methionine 147 with threonine, serine, tyrosine, or cysteine; 
     l) substitution of lysine 148 with asparagine; 
     m) substitution of valine 152 with alanine or glycine; 
     n) substitution of leucine 212 with proline; and 
     o) substitution of glutamine 216 with histidine; or 
     modified SHNL, which is obtained by inserting glycine or lysine between amino acids 128 and 129 of the amino acid sequence of the cassava- or Pará rubber tree-derived wild-type SHNL. 
     The aforementioned modified SHNL may further comprise substitution of glycine 165 with glutamic acid and substitution of valine 173 with leucine. 
     For example, modified SHNL that comprises at least one amino acid substitution selected from: 
     d) substitution of valine 94 with alanine, cysteine, histidine, lysine, phenylalanine, proline, serine, threonine, tryptophan, or tyrosine; 
     g) substitution of leucine 122 with isoleucine, valine, glycine, or serine; 
     h) substitution of phenylalanine 125 with asparagine, cysteine, histidine, glutamine, arginine, leucine, isoleucine, threonine, or proline; and 
     o) substitution of glutamine 216 with histidine, in addition to substitution of glycine 165 with glutamic acid and substitution of valine 173 with leucine in the amino acid sequence of the cassava-derived wild-type SHNL, has a high level of enzyme activity. 
     In particular, modified SHNL that comprises substitution of phenylalanine 125 with asparagine and substitution of leucine 122 with serine in the amino acid sequence of the cassava-derived wild-type SHNL or modified SHNL that comprises substitution of glutamine 216 with histidine, in addition to the 2 aforementioned substitutions, has a remarkably high level of enzyme activity. 
     The present invention also provides DNA encoding the amino acid sequence of the aforementioned modified SHNL. 
     The modified SHNL of the present invention can be easily produced by introducing the DNA into an adequate host, culturing the same therein, and recovering a protein having SHNL activity from the resulting culture product. 
     The resulting modified SHNL has higher enzyme activity than that of wild-type SHNL, and it can produce optically active cyanohydrin of high purity from a carbonyl compound and cyanide. The present invention also provides a method for producing such modified SHNL and a method for producing optically active cyanohydrin using the same. 
     The high-activity modified SHNL of the present invention has remarkably improved enzyme activity compared with the enzyme activity of conventional enzymes. Accordingly, industrial production of optically active cyanohydrin can be enhanced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an alignment diagram showing amino acid sequences of SHNL derived from cassava ( Manihot esculenta ) and SHNL derived from the Pará rubber tree ( Hevea brasiliensis ). 
     
    
    
     BEST MODES FOR CARRYING OUT THE INVENTION 
     Hereafter, the present invention is described in detail. 
     1. Wild-Type S-Hydroxynitrile Lyase 
     In the present invention, the term “wild-type S-hydroxynitrile lyase (hereafter abbreviated to “SHNL”)” refers to SHNL isolated and purified from a plant or SHNL having an amino acid sequence identical to that of the former SHNL. The origin of wild-type SHNL is not particularly limited. Examples thereof include SHNL derived from poaceous plants such as sorghum ( Sorghum bicolor ), SHNL derived from Euphorbiaceae plants such as cassava ( Manihot esculenta ) or Pará rubber tree ( Hevea brasiliensis ), and SHNL derived from Olacaceae plants such as  Ximenia americana . The amino acid sequences and the nucleotide sequences of the genes of such SHNLs are already known and can be easily obtained from public databases such as GenBank. For example, the SHNL gene derived from the Pará rubber tree ( Hevea brasiliensis ), the SHNL gene derived from  Manihot esculenta , and the SHNL gene derived from sorghum are registered in GenBank under the accession nos. U40402 (SEQ ID NO: 3 is equivalent to CDS of U40402), Z29091, and AJ421152, respectively. 
       FIG. 1  shows the aligned amino acid sequences of the cassava ( Manihot esculenta )-derived SHNL and of the Pará rubber tree ( Hevea brasiliensis )-derived SHNL. Amino acid homology between these SHNLs is 74%, and amino acids of each SHNL are not completely identical. For example,  Hevea brasiliensis -derived SHNL lacks the amino acid that is equivalent to amino acid 139 of  Manihot esculenta -derived SHNL. Thus, the amino acid number in the helix D3′ domain is out of alignment by one residue. Specifically, amino acids 14, 44, 66, 94, 103, 118, 122, 125, 127, 128, 129, 147, 148, 152, 212, and 216 in the amino acid sequence (SEQ ID NO: 2) of SHNL derived from cassava ( Manihot esculenta ) are equivalent to amino acids 14, 44, 66, 94, 103, 118, 122, 125, 127, 128, 129, 146, 147, 151, 211, and 215 in the amino acid sequence (SEQ ID NO: 4) of the Pará rubber tree ( Hevea brasiliensis )-derived SHNL. SHNLs derived from cassava and derived from the Pará rubber tree belong to the α/β hydrolase superfamily and their conformations are very similar to each other. Based on the effects of modification of an amino acid sequence by SHNL derived from cassava, accordingly, similar effects can be expected concerning SHNL derived from the Pará rubber tree ( Hevea brasiliensis ) by modification of an amino acid sequence at a relevant site. 
     2. Modified S-Hydroxynitrile Lyase 
     The present invention relates to high-activity modified SHNL, which is obtained by modification (i.e., substitution or insertion) of an amino acid sequence at a given site of an amino acid sequence of wild-type SHNL derived from cassava or from Pará rubber tree. Specifically, the present invention relates to modified SHNL, which is obtained by substituting at least one amino acid selected from amino acids 14, 44, 66, 94, 103, 118, 122, 125, 127, 129, 147, 148, 152, 212, and 216 in the amino acid sequence (SEQ ID NO: 2) of wild-type SHNL derived from cassava ( Manihot esculenta ) with another amino acid, modified SHNL, which is obtained by substituting at least one amino acid selected from amino acids 14, 44, 66, 94, 103, 118, 122, 125, 127, 129, 146, 147, 151, 211, and 215 in the amino acid sequence (SEQ ID NO: 4) of wild-type SHNL derived from the Pará rubber tree ( Hevea brasiliensis ) with another amino acid, and modified SHNL, which is obtained by inserting at least one amino acid between amino acids 128 and 129 of the amino acid sequence of the cassava- or Pará rubber tree-derived wild-type SHNL (SEQ ID NO: 2 or 4). 
     A more specific example of modified SHNL is one comprising at least one amino acid substitution selected from the amino acid substitutions shown below in the amino acid sequence (SEQ ID NO: 2): 
     a) substitution of histidine 14 with glutamine; 
     b) substitution of aspartic acid 44 with asparagine; 
     c) substitution of glutamic acid 66 with aspartic acid, lysine, or arginine; 
     d) substitution of valine 94 with alanine, cysteine, histidine, lysine, phenylalanine, proline, serine, threonine, tryptophan, or tyrosine; 
     e) substitution of histidine 103 with glutamine; 
     f) substitution of histidine 118 with glutamine; 
     g) substitution of leucine 122 with isoleucine, valine, glycine, or serine; 
     h) substitution of phenylalanine 125 with asparagine, cysteine, histidine, glutamine, arginine, leucine, isoleucine, threonine, or proline; 
     i) substitution of aspartic acid 127 with glutamic acid, asparagine, or glycine; 
     j) substitution of arginine 129 with histidine; 
     k) substitution of methionine 147 with threonine, serine, tyrosine, or cysteine; 
     l) substitution of lysine 148 with asparagine; 
     m) substitution of valine 152 with alanine or glycine; 
     n) substitution of leucine 212 with proline; and 
     o) substitution of glutamine 216 with histidine; or 
     modified SHNL, which is obtained by inserting glycine or lysine between amino acids 128 and 129 of the amino acid sequence (SEQ ID NO: 2 or 4) of the cassava- or Pará rubber tree-derived wild-type SHNL. 
     Further, SHNL comprising combination of the aforementioned mutation sites has higher activity. For example, modified SHNL comprising amino acid substitutions at positions 94, 122, 125, and 216 in the amino acid sequence (SEQ ID NO: 2) of wild-type SHNL derived from cassava has high enzyme activity. Among them, modified SHNL comprising substitution of phenylalanine 125 with asparagine, that of leucine 122 with serine, or that of glutamine 216 with histidine has remarkably high enzyme activity. 
     Amino acid substitution and insertion may be carried out via site-directed mutagenesis into a gene that encodes the amino acid sequence in accordance with a conventional technique. Such site-directed mutagenesis can be easily performed using a commercially available kit (for example, QuikChange XL Site-Directed Mutagenesis Kit (Stratagene) or Transformer™ Site-Directed Mutagenesis Kit (Clontech.)). Since the modified SHNL of the present invention has enzyme activity higher than from that of wild-type SHNL, the modified SHNL of the present invention can be a very useful enzyme for industrial production of optically active cyanohydrin. 
     3. Production of High-Activity Modified S-Hydroxynitrile Lyase 
     3.1 DNA that Encodes High-Activity Modified SHNL 
     DNA that encodes the modified SHNL protein according to the present invention is obtained by introducing site-directed mutations into the gene of known wild-type SHNL. Specifically, a pair of primers that can modify a codon at a substitution site into a codon that encodes the amino acid of interest is designed. Next, the resulting pair of primers is used to perform an extension reaction utilizing DNA that encodes wild-type SHNL as a template. Site-directed mutagenesis can be easily carried out utilizing commercialized kits (for example, QuikChange XL Site-Directed Mutagenesis Kit (Stratagene) or Transformer™ Site-Directed Mutagenesis Kit (Clontech)). 
     3.2 Recombinant Vector 
     DNA that encodes the aforementioned high-activity modified SHNL is then ligated (inserted) to (into) a known vector such as a plasmid to prepare a recombinant vector. Such vector is not particularly limited as long as it can replicate the gene of interest in a host. Examples include plasmid DNA and phage DNA. 
     Examples of such plasmid DNA include  E. coli -derived plasmids (for example, a pET21 vector having a particularly potent T7 promoter, such as pBR322, pBR325, pUC18, pUC119, pHCEIIB, pTrcHis, or pBlueBacHis, is preferable),  Bacillus subtilis -derived plasmids (for example, pUB110 and pTP5), and yeast-derived plasmids (for example, YEp13, YEp24, YCp50, and pYE52). An example of phage DNA is λphage. 
     The gene of the present invention is inserted into the aforementioned vector by first cleaving purified DNA with an adequate restriction enzyme and inserting the cleaved DNA into an adequate restriction enzyme site or multi-cloning site of the vector DNA for ligation. 
     In order to express a foreign gene in a host, an adequate promoter needs to be positioned prior to the positioning of a structural gene. Such promoter is not particularly limited. Any promoter that is known to function in a host can be employed. Promoters will be described in detail concerning each host in sections concerning transformants below. If necessary, a cis element such as an enhancer, splicing signal, poly A additional signal, libosome binding sequence (SD sequence), terminator sequence, and the like may be positioned. 
     3.3 Modified SHNL Expression System (Transformant) 
     Subsequently, the aforementioned recombinant vector is introduced into a host in a manner such that the target gene can be expressed therein to prepare a modified SHNL expression system. A host cell is not particularly limited as long as the DNA of the present invention can be expressed therein. Examples thereof include: bacteria belonging to  Escherichia  such as  Escherichia coli, Bacillus  such as  Bacillus subtilis, Pseudomonas  such as  Pseudomonas putida , or  Rhizobium  such as  Rhizobium meliloti ; yeast such as  Saccharomyces cervisiae, Schizosaccharomyces pombe , and  Pichia pastoris ; animal cells such as COS cells and CHO cells; and insect cells such as Sf19 and Sf21. 
     When a bacterium such as  E. coli  is used as a host, it is preferable that the recombinant vector of the present invention be capable of self-replicating in the bacterium and, at the same time, also be comprised of a promoter, a ribosome binding sequence, the gene of the present invention, and a transcription termination sequence. Further, it may also comprise a gene for regulating a promoter. Examples of  E. coli  include  Escherichia coli  HMS174 (DE3), K12, DH1, and B strains, and examples of  Bacillus subtilis  include  Bacillus subtilis  MI 114 and 207-21. A promoter is not particularly limited as long as it can express the gene of interest in a host such as  E. coli . For example,  E. coli -derived or phage-derived promoters can be employed, such as: trp promoter, lac promoter, P L  promoter, and P R  promoter. Alternatively, an artificially designed and modified promoter, such as a tac promoter, may also be employed. A method for introducing a recombinant vector into a bacterium is not particularly limited. For example, a method involving the use of calcium ions (Cohen, S, N. et al., Ploc. Natl. Acad. Sci., U.S.A., 69: 2110-2114, 1972) and electroporation can be employed. 
     Where yeast is used as a host, for example,  Saccharomyces cerevisiae, Schizosaccharomyces pombe , or  Pichea pastris , is used. A promoter is not particularly limited as long as it can express the gene of interest in yeast. For example, gall promoter, gal10 promoter, heat shock protein promoter, MFα1 promoter, PHO5 promoter, PGK promoter, GAP promoter, ADH promoter, or AOX1 promoter can be employed. A method for introducing a vector into yeast is not particularly limited. Examples of such methods include electroporation (Becker, D. M. et al., Methods. Enzymol., 194: 182-187, 1990), the spheroplast method (Hinnen, A. et al., Proc. Natl. Acad. Sci., U.S.A., 75: 1929-1933, 1978), and the lithium acetate method (Itoh, H., J. Bacteriol., 153: 163-168, 1983). 
     3.4 Culture of Transformant 
     The modified SHNL of the present invention can be obtained by culturing the transformant according to the present invention in an adequate medium and recovering a protein having enzyme activity from the culture product. A method for culturing the transformant according to the present invention is adequately determined in accordance with the host. In the case of a transformant where a microorganism such as  E. coli  or yeast is employed as a host, for example, either a natural or synthetic medium may be used as long as it contains carbon sources, nitrogen sources, and inorganic salts assimilable by the microorganism and is capable of efficiently culturing the transformant. 
     During the culture, an antibiotic such as ampicillin or tetracycline may be added to the medium, if necessary. When a microorganism transformed with an expression vector containing an inducible promoter is cultured, an inducer may be added to the medium, if necessary. When a microorganism transformed with an expression vector containing lac promoter is cultured, for example, isopropyl-β-thiogalactopyranoside (IPTG) may be added to the medium. When culturing a microorganism transformed with an expression vector containing trp promoter, indoleacrylic acid (IAA) or the like may be added to the medium. 
     If the enzyme protein of the present invention is produced in the relevant microorganism or cell after the culture, the cultured microorganism or cell is homogenized. If the protein of the present invention is secreted outside of the microorganism or cell, the culture broth may be used in such state or may be subjected to centrifugation or another procedure to remove the microorganism or cell. 
     Ammonium sulfate precipitation, SDS-PAGE, gel filtration, ion exchange chromatography, affinity chromatography, or other means are employed independently or in an appropriate combination to isolate and purify proteins. 
     The enzyme activity of the modified SHNL according to the present invention can be confirmed by adding the enzyme to a reaction solution that contains a substrate, i.e., adequate cyanide, and aldehyde or ketone and detecting the generated optically active cyanohydrin. Optically active cyanohydrin can be confirmed by, for example, gas chromatography or high-performance liquid chromatography. Alternatively, an antibody that specifically binds to the modified SHNL of the present invention can be prepared to confirm the expression via Western blotting using the resulting antibody. For example, the enzyme activity of SHNL can be confirmed by assaying the amount of aldehyde generated per unit time (calculated based on absorbance at 249.6 nm) upon degradation of mandelonitrile by SHNL. 
     The modified SHNL of the present invention can be produced in accordance with the methods disclosed in JP Patent Publication (Kokai) Nos. 10-373246 A (1998), 10-373248 A (1998), or 11-367251 A (1999). 
     4. Synthesis of Optically Active Cyanohydrin Using High-Activity Modified SHNL 
     The modified SHNL of the present invention can synthesize optically active cyanohydrin with production efficiency and optical purity higher than those of wild-type SHNL. Optically active cyanohydrin can be synthesized with the use of the present invention&#39;s high-activity modified SHNL in the same manner as wild-type SHNL. 
     Specifically, the modified SHNL of the present invention and a reaction substrate are added to a reaction solvent, and the reaction is carried out at 10° C. to 50° C. for 20 minutes to 24 hours. Thus, optically active cyanohydrin can be synthesized. The reaction time is adequately determined in accordance with the conversion rate of the substrate. Examples of a reaction substrate that can be employed include a carbonyl compound and cyanide. A carbonyl compound is an aldehyde or ketone represented by COR1R2 wherein R1 and R2 each independently represent: a hydrogen atom, substituted or non-substituted, linear or branched, and saturated alkyl having 1 to 18 carbon atoms, or a substituted or non-substituted cyclic 5-22-membered aromatic group, provided that R1 and R2 do not simultaneously represent a hydrogen atom. Cyanide is not particularly limited as long as it generates cyanide ions (CN − ). Examples thereof that can be employed include hydrogen cyanides such as sodium cyanide or potassium cyanide and cyanohydrins such as acetone cyanohydrin. 
     Use of a reaction solvent mainly composed of an organic solvent that is hardly soluble or insoluble in water is preferable from the viewpoint described below. That is, when a large quantity of water is present in a reaction system, racemization of optically active cyanohydrin generated via enzyme reaction is likely to occur. When aldehyde or ketone having a low degree of water solubility is used as a starting material, the production efficiency deteriorates. Such organic solvent is not particularly limited if it does not affect the synthesis of optically active cyanohydrin via enzyme reaction. An organic solvent can be adequately selected in accordance with properties of aldehyde or ketone used as a starting material for synthesis or properties of cyanohydrin, which is a generated product. Specific examples include: aliphatic or aromatic, linear, branched, or cyclic, and saturated or unsaturated hydrocarbon solvents that may be optionally halogenated, such as pentane, hexane, toluene, xylene, and methylene chloride; aliphatic or aromatic, linear, branched, or cyclic, and saturated or unsaturated alcohol solvents that may be optionally halogenated, such as isopropyl alcohol, n-butanol, isobutanol, t-butanol, hexanol, cyclohexanol, and n-amyl alcohol; aliphatic or aromatic, linear, branched, or cyclic, and saturated or unsaturated ether solvents that may be optionally halogenated, such as diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, and methyl t-butyl ether; and aliphatic or aromatic, linear, branched, or cyclic, and saturated or unsaturated ester solvents that may be optionally halogenated, such as methyl formate, methyl acetate, ethyl acetate, butyl acetate, and methyl propionate. They may be used alone or in combinations of two or more. An aforementioned solvent that comprises or is saturated with water or an aqueous buffer can also be employed. 
     In the process of industrial production, the modified SHNL may be utilized as an enzyme immobilized on an adequate inorganic carrier (see, for example, JP Patent Publication (Kokai) No. 2002-176974 A). Examples of preferable methods for synthesizing cyanohydrin with the use of the modified SHNL of the present invention include those disclosed in JP Patent Publication (Kokai) Nos. 2002-355085 A, 2002-176974 A, 2001-363840 A, 2001-346596 A, 2001-190275 A, 2000-245286 A, 2001-120289 A, and 2000-217590 A. 
     EXAMPLES 
     Hereafter, the present invention is described in greater detail with reference to examples, although the technical scope of the present invention is not limited thereto. 
     Example 1 
     Screening for of High-Activity Mutant SHNL from Library of Random Mutant SHNL 
     [Material and Method] 
     1) Library of Random Mutant SHNL 
     The SHNL gene used in the present invention comprised a sequence (SEQ ID NO: 1: JP Patent Application No. 2002-365675 (hereafter this SHNL gene is referred to as “SHNL-Wild”) resulting from conversion of the gene sequence of SHNL cloned from cassava ( Manihot esculenta ) into an  E. coli - type  codon. With the use of such gene as a template, mutation was introduced randomly via Error prone PCR using the GeneMorph™PCR Mutagenesis Kit (Stratagene). PCR was carried out using the primers having the sequences shown below. 
     Forward primer: 5′-GGG GGG GAA TTC ATG GTT ACT GCA CAC TTC GTT CTG ATT CAC-3′ (SEQ ID NO: 5)
 
Reverse primer: 5′-GGG GGG AAG CTT TTA AGC GTA TGC ATC AGC AAC TTC TTG CAG-3′ (SEQ ID NO: 6)
 
     The resulting randomly-mutated SHNL gene was inserted into a multicloning site of a pKK223-3 vector plasmid (Amersham BioSciences) and transformed into  Escherichia coli  DH5α to prepare a library of random mutant SHNLs. The prepared library of random mutant SHNL was cryopreserved by the glycerol stock method. 
     2) Screening for of High-Activity Mutant Enzyme 
     Clones obtained from the library of random mutant SHNL were inoculated on a deep-well plate into which NS-1 medium shown in Table 1 had been dispensed, and shake culture was carried out at 37° C. and 900 rpm. When culturing clones obtained from the library of random mutant SHNL, several  E. coli  strains with recombinant SHNL-Wild genes were inoculated on the same plate as the control. 
     
       
         
           
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Composition of NS-1 medium 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Glycerol 
                 20 
                 g 
               
               
                   
                 (NH 4 ) 2 SO 4   
                 5 
                 g 
               
               
                   
                 KH 2 PO 4   
                 1 
                 g 
               
               
                   
                 K 2 HPO 4   
                 3 
                 g 
               
               
                   
                 Yeast ext. (Ebios P2G) 
                 20 
                 g 
               
               
                   
                 MgSO 4 •7H 2 O 
                 0.5 
                 g 
               
               
                   
                 Ampicillin 
                 1 
                 g 
               
               
                   
                 IPTG 
                 0.238 
                 g 
               
               
                   
                 Distilled water 
                 1 
                 liter 
               
               
                   
                   
               
            
           
         
       
     
     After the cells were thoroughly grown, part of the culture solution was sampled using the screening robotic system (Beckman coulter), the culture solution was adequately diluted with distilled water, and the absorbance at 660 nm was assayed to determine the cell density. Similarly, 10 μl of the culture solution was applied to the deep-well plate comprising 300 μl each of the substrate solution dispensed in each well using the screening robotic system, and enzyme reaction was performed by shaking. Phosphoric acid (30 μl) was added 10 minutes later to terminate the reaction, and the reaction solution was separated from the cells via centrifugation. The separated reaction solution was transferred to a UV plate, the absorbance at 250 nm was assayed using a plate reader (GENios; TECAN), and the determined value was designated as the SHNL activity value. Based on this activity value, the activity value per unit cell density was determined. The obtained value was compared with that of  E. coli  strains of recombinant SHNL-Wild gene to select clones exhibiting an enhanced activity value per unit cell density. 
     The clones selected via the present screening were subjected to plasmid extraction. Using the resultant as a template, sequence analysis was carried out to identify the mutation site. 
     [Results] 
     The results of evaluating the clones obtained from the library of random mutant SHNL selected by the screening robotic system are shown in Table 2. The results demonstrate that substitution of at least one amino acid selected from amino acids 14, 44, 66, 94, 103, 118, 125, 127, 129, 147, 148, 152, 212, and 216 of the SHNL amino acid (SEQ ID NO: 2) with another amino acid results in improvement in SHNL activity by 1.2 to 6.7 times. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Evaluation of clones obtained from the library of random mutant SHNL 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Amino acid 
                 Nucleotide 
                   
                   
                 Activity value/ 
               
               
                 Strain No. 
                 No. 
                 No. 
                 Before mutation 
                 After mutation 
                 average of wild-type 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 021A1 
                 12 
                 42 
                 His 
                 CAC 
                 Gln 
                 CAA 
                 2.69 
               
               
                 021C4 
                 44 
                 130 
                 Asp 
                 GAC 
                 Asn 
                 AAC 
                 3.09 
               
               
                 025D3 
                 66 
                 198 
                 Glu 
                 GAA 
                 Asp 
                 GAT 
                 5.17 
               
               
                 020A4 
                 66 
                 196 
                 Glu 
                 GAA 
                 Lys 
                 AAA 
                 2.82 
               
               
                 011H7 
                 94 
                 281 
                 Val 
                 GTT 
                 Ala 
                 GCT 
                 4.69 
               
               
                 043G10 
                 94 
                 281 
                 Val 
                 GTT 
                 Ala 
                 GCT 
                 2.03 
               
               
                 048H6 
                 94 
                 281 
                 Val 
                 GTT 
                 Ala 
                 GCT 
                 2.39 
               
               
                 022B3 
                 103 
                 309 
                 His 
                 CAC 
                 Gln 
                 CAG 
                 3.98 
               
               
                 052G5 
                 103 
                 309 
                 His 
                 CAC 
                 Gln 
                 CAA 
                 2.24 
               
               
                 042H9 
                 118 
                 352 
                 Val 
                 GTT 
                 Leu 
                 CTT 
                 1.30 
               
               
                 050C12 
                 125 
                 374 
                 Phe 
                 TTC 
                 Ser 
                 TCC 
                 1.55 
               
               
                 028B3 
                 127 
                 380 
                 Asp 
                 GAC 
                 Glu 
                 GAA 
                 5.08 
               
               
                 005D6 
                 127 
                 380 
                 Asp 
                 GAC 
                 Gly 
                 GGC 
                 1.27 
               
               
                 030C7 
                 127 
                 379 
                 Asp 
                 GAC 
                 Asn 
                 AAC 
                 6.27 
               
               
                   
                 129 
                 386 
                 Arg 
                 CGT 
                 His 
                 CAT 
               
               
                 002A7 
                 127 
                 380 
                 Asp 
                 GAC 
                 Gly 
                 GGC 
                 1.50 
               
               
                   
                 152 
                 455 
                 Val 
                 GTT 
                 Ala 
                 GCT 
               
               
                 012F7 
                 129 
                 386 
                 Arg 
                 CGT 
                 His 
                 CAT 
                 3.43 
               
               
                   
                 152 
                 455 
                 Val 
                 GTT 
                 Gly 
                 GGT 
               
               
                 020B10 
                 129 
                 386 
                 Arg 
                 CGT 
                 His 
                 CAT 
                 2.77 
               
               
                 039B4 
                 147 
                 439 
                 Met 
                 ATG 
                 Val 
                 GTG 
                 1.41 
               
               
                 012F2 
                 148 
                 444 
                 Lys 
                 AAA 
                 Asn 
                 AAT 
                 2.88 
               
               
                 012E10 
                 212 
                 635 
                 Leu 
                 CTG 
                 Pro 
                 CCG 
                 3.83 
               
               
                 018C12 
                 216 
                 648 
                 Gln 
                 CAA 
                 His 
                 CAT 
                 6.70 
               
               
                 005F6 
                 128 
                 383 
                 Trp 
                 TGG 
                 Leu 
                 TTG 
                 1.79 
               
               
                 033D10 
                 128 
                 384 
                 Trp 
                 TGG 
                 Cys 
                 TGT 
                 2.03 
               
               
                 046D9 
                 128 
                 383 
                 Trp 
                 TGG 
                 Ser 
                 TCG 
                 1.08 
               
               
                   
               
            
           
         
       
     
     Example 2 
     Substitution of Amino Acid 125 in SHNL Amino Acid Sequence 
     Based on the experiment of Example 1, a high-activity mutant in which amino acid 125 of the amino acid sequence of Wild-SHNL (SEQ ID NO: 2), i.e., phenylalanine, had been substituted with tyrosine was obtained. Site-directed mutagenesis was applied to the amino acid 125 of the SHNL amino acid sequence to substitute the amino acid 125 with any of various types of amino acids. Thus, production of a high-activity mutant SHNL was further attempted. 
     [Material and Method] 
     Site-directed mutagenesis was performed using the QuikChange XL Site-Directed Mutagenesis Kit (Stratagene) to substitute the amino acid 125 with any of various types of amino acids. As a template, 10 ng of a vector plasmid, SHNL-G165E,V173L/pKK223-3, comprising the SHNL-G165E,V173L gene incorporated therein (i.e., the SHNL gene modified by substituting Gly 165 with Glu and Val 173 with Leu of the amino acid sequence of Wild-SHNL: shown in SEQ ID NO: 25, hereafter this gene is referred to as “Ver.0”) was used (this plasmid is hereafter referred to as “Ver.0/pKK”) (see Reference Examples). Primers shown below were used. 
     
       
         
           
               
               
            
               
                 (SEQ ID NO: 7) 
                   
               
            
           
           
               
               
            
               
                 Forward primer: 5′-GTT GAA AAG CTG CTG GAA TCG 
                   
               
               
                 NNN CCG GAC TGG CGT GAC ACA G-3′ 
               
               
                   
               
            
           
           
               
               
            
               
                 (SEQ ID NO: 8) 
                   
               
            
           
           
               
               
            
               
                 Reverse primer: 5′-CTG TGT CAC GCC AGT CCG GNN 
                   
               
               
                 NCG ATT CCA GCA GCT TTT CAA C-3′ 
               
            
           
         
       
     
     The resulting extension product was digested with the DpnI restriction enzyme and transformed into DHα a competent cells to prepare  E. coli  strains comprising mutant genes.  E. coli  strains comprising the mutant genes were cultured, cells were then homogenized, and the supernatant of the homogenized cells was used to assay the SHNL activity and the protein concentration. SHNL activity was determined using DL-mandelonitrile as a substrate and determining the rate of benzaldehyde generation, which is generated by degrading mandelonitrile by an enzyme, based on changes in the absorbance at 249.6 nm. The protein concentration was determined using the BCA protein assay kit (Pierce) and BSA as a standard. Based on such results, the specific activity was determined, and the determined value was compared with the specific activity value obtained by culturing the  E. coli  Ver.0/pKK223-3/DH5α strains containing the Ver.0/pKK223-3 vector plasmid before mutagenesis. As a result of comparison, strains exhibiting improved specific activity were subjected to sequence analysis to identify mutated amino acids. 
     [Results] 
     As a result of sequence analysis, SHNL in which amino acid 125 had been substituted with asparagine, cysteine, histidine, glutamine, arginine, leucine, isoleucine, threonine, or proline was found to exhibit significantly improved specific activity compared with the SHNL before mutagenesis (amino acid 125: phenylalanine) (Table 3). 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Results of mutation of amino acid 125 
               
            
           
           
               
               
               
            
               
                   
                 Mutated 
                   
               
               
                   
                 amino acid 
                 Specific activity* 
               
               
                   
                   
               
               
                   
                 Asn 
                 6.9 
               
               
                   
                 Cys 
                 6.7 
               
               
                   
                 His 
                 6.7 
               
               
                   
                 Gln 
                 4.7 
               
               
                   
                 Arg 
                 4.4 
               
               
                   
                 Leu 
                 3.7 
               
               
                   
                 Ile 
                 3.2 
               
               
                   
                 Thr 
                 2.5 
               
               
                   
                 Pro 
                 1.5 
               
               
                   
                   
               
               
                   
                 *Specific activity = specific activity of mutant SHNL (U/mg-protein)/specific activity of SHNL before mutagenesis (U/mg-protein) 
               
            
           
         
       
     
     Example 3 
     Substitution of Amino Acid 216 of SHNL Amino Acid Sequence 
     The results of Example 1 demonstrated that a high-activity mutant SHNL could be obtained by substituting glutamine 216 of the SHNL amino acid sequence with isoleucine. Accordingly, Site-directed mutagenesis was applied to the amino acid 216 of the SHNL amino acid sequence to substitute the amino acid 216 with any of various types of amino acids. Thus, further improvement in activity was attempted. 
     [Material and Method] 
     In the same manner as in Example 2, amino acid 216 of SHNL was substituted with any of various types of amino acids. Similarly, Ver.0/pKK223-3 was used as a template. Primers shown below were used. 
     
       
         
           
               
               
            
               
                 (SEQ ID NO: 9) 
                   
               
            
           
           
               
               
            
               
                 Forward primer: 5′-CAA AAT ATT CCT GCC GGA CTT 
                   
               
               
                 CNN NCG CTG GCA AAT TGC AAA CTA CA-3′ 
               
               
                   
               
            
           
           
               
               
            
               
                 (SEQ ID NO: 10) 
                   
               
            
           
           
               
               
            
               
                 Reverse primer: 5′-TGT AGT TTG CAA TTT GCC AGC 
                   
               
               
                 GNN NGA AGT CCG GCA GGA ATA TTT TG-3′ 
               
            
           
         
       
     
     The resulting DpnI-digested extension products were transformed into DH5α competent cells to prepare  E. coli  strains comprising mutant genes. The  E. coli  strains comprising the mutant genes were cultured, and specific activity thereof was compared with that of the  E. coli  Ver.0 pKK223-3/DH5α strains comprising the genes before mutagenesis. As a result of comparison, strains exhibiting improved specific activity were subjected to sequence analysis to identify mutated amino acids. 
     [Results] 
     All the clones selected as high-activity mutants were found to be SHNLs in which amino acid 216 had been substituted with histidine. Such clones exhibited significantly enhanced specific activity compared with glutamine (i.e., wild-type). 
     Example 4 
     Substitution of Amino Acid 94 of SHNL Amino Acid Sequence 
     The results of Example 1 demonstrate that a high-activity mutant SHNL could be obtained by substituting valine 94 of the SHNL amino acid sequence with alanine. Site-directed mutagenesis was applied to the amino acid 94 of the SHNL amino acid sequence to substitute the amino acid 94 with any of various types of amino acids. Thus, production of high-activity mutant SHNL was further attempted. 
     [Material and Method] 
     In the same manner as in Example 2, amino acid 94 of SHNL was substituted with any of various types of amino acids. Similarly, Ver.0/pKK223-3 was used as a template. Primers shown below were used. 
     
       
         
           
               
               
            
               
                 (SEQ ID NO: 11) 
                   
               
            
           
           
               
               
            
               
                 Forward primer: 5′-TGC TAT TGC TGC TGA TCG TTA 
                   
               
               
                 CNN NGA CAA AAT TGC AGC TGG CGT TT-3′ 
               
               
                   
               
            
           
           
               
               
            
               
                 (SEQ ID NO: 12) 
                   
               
            
           
           
               
               
            
               
                 Reverse primer: 5′-AAA CGC CAG CTG CAA TTT TGT 
                   
               
               
                 CNN NGT AAC GAT CAG CAG CAA TAG CA-3′ 
               
            
           
         
       
     
     The resulting extension product was digested with the DpnI restriction enzyme and transformed into DH5α competent cells to prepare  E. coli  strains comprising mutant genes.  E. coli  strains comprising the mutant genes were cultured, the specific activity thereof was compared with that of the  E. coli  Ver.0/pKK223-3/DH5α strains comprising the genes before mutagenesis. As a result of comparison, strains exhibiting improved specific activity were subjected to sequence analysis to identify mutated amino acids. 
     [Results] 
     As a result of sequence analysis, SHNL in which amino acid 94 had been substituted with alanine, cysteine, histidine, lysine, phenylalanine, proline, serine, threonine, tryptophan, or tyrosine was found to exhibit significantly improved specific activity compared with the SHNL before mutagenesis (amino acid 94: valine) (Table 4). 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Results of mutation of amino acid 94 
               
            
           
           
               
               
               
            
               
                   
                 Mutated 
                   
               
               
                   
                 amino acid 
                 Specific activity* 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 Ala 
                 11.9 
               
               
                   
                 Cys 
                 6.4 
               
               
                   
                 His 
                 8.4 
               
               
                   
                 Lys 
                 7.9 
               
               
                   
                 Phe 
                 4.6 
               
               
                   
                 Pro 
                 14.3 
               
               
                   
                 Ser 
                 7.4 
               
               
                   
                 Thr 
                 11.8 
               
               
                   
                 Trp 
                 6 
               
               
                   
                 tyr 
                 5.3 
               
               
                   
                   
               
               
                   
                 *Specific activity = specific activity of mutant SHNL (U/mg-protein)/specific activity of SHNL before mutagenesis (U/mg-protein) 
               
            
           
         
       
     
     Example 5 
     Substitution of Amino Acid 147 of SHNL Amino Acid Sequence 
     The results of Example 1 demonstrate that a high-activity mutant SHNL could be obtained by substituting methionine 147 of the SHNL amino acid sequence with valine or isoleucine. Site-directed mutagenesis was applied to the amino acid 147 of the SHNL amino acid sequence to substitute the amino acid 147 with any of various types of amino acids. Thus, production of high-activity mutant SHNL was further attempted. 
     [Material and Method] 
     In the same manner as in Example 2,  E. coli  strains comprising mutant genes in which amino acid 147 of SHNL had been substituted with any of various types of amino acids were prepared. As a template, 10 ng of the SHNL-Wild/pET21 a vector plasmid comprising the SHNL-Wild genes incorporated into the pET21 a vector (Novagen) was used, and extension reaction was carried out using the following primers. 
     Forward primer: 5′-GGC GAA ACC ATC ACT ACC NNN AAA CTG GGT TTC GTT CTG CTG-3′ (SEQ ID NO: 13)
 
Reverse primer: 5′-CAG CAG AAC GAA ACC CAG TTT NNN GGT AGT GAT GGT TTC GCC-3′ (SEQ ID NO: 14)
 
     The vector plasmid after mutagenesis was digested with the DpnI restriction enzyme and transformed into BL21 (DE3) cells. In the same manner as in Example 2, the specific activity thereof was compared with that of the  E. coli  SHNL-Wild/pET21a/BL21(DE3) strains comprising the genes before mutagenesis. As a result of comparison, strains exhibiting improved specific activity were subjected to sequence analysis to identify mutated amino acids. 
     [Results] 
     As a result of sequence analysis, SHNL in which amino acid 147 had been substituted with threonine, serine, tyrosine, or cysteine was found to exhibit significantly enhanced specific activity compared with SHNL before mutagenesis (amino acid 147: methionine) (Table 5). 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Results of mutation of amino acid 147 
               
            
           
           
               
               
               
            
               
                   
                 Mutated 
                   
               
               
                   
                 amino acid 
                 Specific activity* 
               
               
                   
                   
               
               
                   
                 Thr 
                 2.6 
               
               
                   
                 Ser 
                 2.4 
               
               
                   
                 Tyr 
                 1.6 
               
               
                   
                 Cys 
                 2.2 
               
               
                   
                   
               
               
                   
                 *Specific activity = specific activity of mutant SHNL (U/mg-protein)/specific activity of SHNL before mutagenesis (U/mg-protein) 
               
            
           
         
       
     
     Comparative Example 1 
     Substitution of Amino Acid 128 of SHNL Amino Acid Sequence 
     Mutant SHNLs with improved substrate receptivity of an enzyme (JP Patent Publication (kokai) No. 2000-125886 (A)) were prepared, and enzyme activity thereof was compared with that of Wild-SHNL. 
     [Material and Method] 
     In the same manner as in Example 2,  E. coli  strains comprising mutant genes in which tryptophan 128 of SHNL had been substituted with alanine were prepared. As a template, 10 ng of the SHNL-Wild/pET21a vector plasmid comprising the SHNL-Wild genes incorporated into the pET21a vector (Novagen) was used. For mutagenesis, primers having the sequences shown below were used. The vector plasmid after mutagenesis was digested with the DpnI restriction enzyme and then transformed into BL21(DE3). In the same manner as in Example 2, the specific activity of the resultant was compared with that of the  E. coli  SHNL-Wild/pET21a/BL21(DE3) strain comprising the gene before mutagenesis. 
     Forward primer: 5′-CTG CTG GAA TCG TTC CCG GAC GCA CGT GAC ACA GAA TAT TTC ACG-3′ (SEQ ID NO: 15)
 
Reverse primer: 5′-CGT GAA ATA TTC TGT GTC ACG TGC GTC CGG GAA CGA TTC CAG CAG-3′ (SEQ ID NO: 16)
 
     [Results] 
     The activity value of the Trp128Ala-SHNL enzyme with improved substrate receptivity of an enzyme was 25.5 U/ml-broth, and the specific activity value thereof was 8.9 U/mg-protein. The specific activity value of the Wild-SHNL enzyme that had been cultured under the same conditions was 9.8 U/mg-protein. Specifically, under the assay conditions employed in the experiment, the activity value of the Trp128Ala-SHNL enzyme was found to be equivalent to that of the Wild-SHNL enzyme. Also, improvement in the activity value of the mutants obtained in Example 1 in which tryptophan 128 had been substituted with leucine, cysteine, or serine (i.e., 005F6, 033D10, and 046D9 strains shown in Table 2) was about 1.1 to 2 times that of the Wild-SHNL enzyme. According to the present invention, mutant enzymes with activity 2 to 10 times higher than that a wild-type strain were obtained, as demonstrated in Examples 1 to 5. Accordingly, the high-activity mutant SHNL of the present invention was demonstrated to be very useful. 
     Example 6 
     Substitution of Amino Acid 122 of SHNL Amino Acid Sequence 
     The results of Examples 1 to 5 demonstrate that a high-activity mutant SHNL could be obtained by substituting amino acid at a given site of the SHNL with another amino acid. Based on the conformational data of SHNL registered at the Protein Data Bank (PDB), Site-directed mutagenesis was applied to amino acid 122 located in the vicinity of the mutation site of the high-activity mutant SHNL that had been already obtained to substitute an amino acid of interest with any of various types of amino acids. Thus, production of high-activity mutant SHNL was further attempted. 
     [Material and Method] 
     In the same manner as in Example 2, recombinant  E. coli  strains comprising mutant genes in which amino acid 122 of SHNL had been substituted with any of various types of amino acids were prepared. As a template, 10 ng of the Ver.0/pET21a plasmid comprising the SHNL-Wild genes incorporated into the pET21a vector (Novagen) was used. Primers shown below were used. 
     
       
         
           
               
               
            
               
                 (SEQ ID NO: 17) 
                   
               
            
           
           
               
               
            
               
                 Forward primer: 5′-GTC TTA CAC TGT TGA AAA GCT 
                   
               
               
                 GNN NGA ATC GTT CCC GGA CTG GCG TG-3′ 
               
               
                   
               
            
           
           
               
               
            
               
                 (SEQ ID NO: 18) 
                   
               
            
           
           
               
               
            
               
                 Reverse primer: 5′- CAC GCC AGT CCG GGA ACG ATT 
                   
               
               
                 CNN NCA GCT TTT CAA CAG TGT AAG AC-3′ 
               
            
           
         
       
     
     The plasmid after mutagenesis was transformed into BL21(DE3). In the same manner as in Example 2, the specific activity of the resultant was compared with that of the SHNL-Wild/pET21a/BL21(DE3) strain before mutagenesis. The strains exhibiting improved specific activity were subjected to sequence analysis to identify the mutated amino acid. 
     [Results] 
     SHNL in which amino acid 122 had been substituted with isoleucine, valine, glycine, or serine exhibited improved specific activity compared with wild-type SHNL (amino acid 122: leucine) before mutagenesis (Table 6). 
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 Results of mutation of amino acid 122 
               
            
           
           
               
               
               
            
               
                   
                 Mutated 
                   
               
               
                   
                 amino acid 
                 Specific activity* 
               
               
                   
                   
               
               
                   
                 Ile 
                 2.1 
               
               
                   
                 Ser 
                 2.4 
               
               
                   
                 Val 
                 1.5 
               
               
                   
                 Gly 
                 1.6 
               
               
                   
                   
               
               
                   
                 *Specific activity = specific activity of mutant SHNL (U/mg-protein)/specific activity of SHNL before mutagenesis (U/mg-protein) 
               
            
           
         
       
     
     Example 7 
     Insertion of Amino Acid Between Amino Acids 128 and 129 of SHNL Amino Acid Sequence 
     [Material and Method] 
     In the same manner as in Example 2,  E. coli  strains comprising mutant genes in which a new amino acid had been inserted between amino acids 128 and 129 of SHNL were prepared. As a template, 10 ng of the SHNL-Wild/pET21a vector plasmid comprising the SHNL-Wild genes incorporated into the pET21a vector was used. Primers shown below were used. 
     
       
         
           
               
               
            
               
                 (SEQ ID NO: 19) 
                   
               
            
           
           
               
               
            
               
                 Forward primer: 5′-GAA TCG TTC CCG GAC TGG NNN 
                   
               
               
                 CGT GAC ACA GAA TAT TTC-3′ 
               
               
                   
               
            
           
           
               
               
            
               
                 (SEQ ID NO: 20) 
                   
               
            
           
           
               
               
            
               
                 Reverse primer: 5′-GAA ATA TTC TGT GTC ACG NNN 
                   
               
               
                 CCA GTC CGG GAA CGA TTC-3′ 
               
            
           
         
       
     
     The vector plasmid after mutagenesis was digested with the DpnI restriction enzyme and then transformed into BL21(DE3). In the same manner as in Example 2, the specific activity of the resultant was compared with that of the  E. coli  SHNL-Wild/pET21a/BL21(DE3) strain comprising the gene before mutagenesis. The strains exhibiting improved specific activity were subjected to sequence analysis to identify the inserted amino acid. 
     [Results] 
     As a result of sequence analysis, SHNL in which glycine or lysine had been inserted between amino acid 128 and 129 exhibited improved specific activity compared with wild-type SHNL before mutagenesis (Table 7). 
     
       
         
           
               
             
               
                 TABLE 7 
               
             
            
               
                   
               
               
                 Results of amino acid insertion between amino acids 128 and 129 
               
            
           
           
               
               
               
            
               
                   
                 Inserted amino 
                   
               
               
                   
                 acid 
                 Specific activity* 
               
               
                   
                   
               
               
                   
                 Lys 
                 1.6 
               
               
                   
                 Gly 
                 1.4 
               
               
                   
                   
               
               
                   
                 *Specific activity = specific activity of mutant SHNL (U/mg-protein)/specific activity of SHNL before mutagenesis (U/mg-protein) 
               
            
           
         
       
     
     Example 8 
     Construction of High-Activity Mutant SHNL Via Combination of Mutation Sites and Culture Using Jar Fermenter 
     [Material and Method] 
     In the same manner as in Example 2, site-directed mutagenesis was applied to the Ver.0 gene using the QuikChange XL Site-Directed Mutagenesis Kit to obtain a high-activity mutant SHNL gene comprising combined mutation sites encoding SHNL in which amino acids 94, 122, 125, and 216 of SHNL had been substituted with any of various types of amino acids. A pET21a vector plasmid comprising the high-activity mutant SHNL gene comprising combined mutation sites obtained by this method was transformed into BL21(DE3) or BL21(DE3)Ster (Novagen) competent cells to prepare  E. coli  strains comprising high-activity mutant SHNL gene comprising combined mutation sites. 
     The resulting  E. coli  strains comprising the high-activity mutant SHNL gene comprising combined mutation sites were inoculated in 5 ml of LB medium, and reactivation culture was carried out at 37° C. for 16 hours. The reactivation culture medium (2 ml) was inoculated on 50 ml of NS-2 medium shown in Table 8 to perform seed culture at 15° C. for 48 hours. 
     
       
         
           
               
             
               
                 TABLE 8 
               
               
                   
               
               
                 Composition of NS-2 medium 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Glycerol 
                 40 
                 g 
               
               
                   
                 (NH 4 ) 2 SO 4   
                 10 
                 g 
               
               
                   
                 KH 2 PO 4   
                 2 
                 g 
               
               
                   
                 K 2 HPO 4   
                 6 
                 g 
               
               
                   
                 Yeast ext. (Ebios P2G) 
                 40 
                 g 
               
               
                   
                 MgSO 4 •7H 2 O 
                 1 
                 g 
               
               
                   
                 Ampicillin 
                 1 
                 g 
               
               
                   
                 IPTG 
                 0.238 
                 g 
               
               
                   
                 Distilled water 
                 1 
                 liter 
               
               
                   
                   
               
            
           
         
       
     
     The obtained seed culture medium (24 ml) was inoculated in 1.2 1 of NS-2 medium, and culture was carried out using a 2-l-jar fermenter. Culture was carried out at 15° C., an aeration rate of 1 vvm (1.2.1/min), and an agitation rate of 680 rpm. Culture was terminated when the stationary phase was attained, and the obtained culture medium was centrifuged at 6,000×G for 30 minutes to recover cells. The recovered cells were weighed, 0.2 M Na-citrate buffer (pH 5.5) was added thereto in an amount twice by weight the cells to allow the cells to suspend therein, and cells were subjected to homogenization three times using a pressurized homogenizer at 700 to 800 bar. The solution of homogenized cells were heated at 50° C. for 2 hours, and centrifuged at 6,000×G for 30 minutes to obtain a supernatant of homogenized cells. The SHNL activity and the protein concentration were assayed using the supernatant of homogenized cells. Based on the results, the specific activity was determined, and the determined value was compared with the specific activity attained by culturing the  E. coli  Ver.0/pET21a/BL21(DE3) strains comprising the genes before mutagenesis. As a result of comparison, strains exhibiting improved specific activity were subjected to sequence analysis to identify the substituted amino acid. For comparison, SHNL-W128A/pET21a/BL21(DE3) constructed in Comparative Example 1 was cultured in the same manner, and the specific activity was determined. 
     [Results] 
     The  E. coli  strains comprising the high-activity mutant SHNL gene comprising combined mutation sites exhibited higher specific activity than  E. coli  strains comprising a vector plasmid before mutagenesis and  E. coli  strains comprising the high-activity SHNL genes comprising a single amino acid substitution. The  E. coli  strains exhibiting the highest specific activity were: the  E. coli  Ver.1+L122S+Q216H-2/pET21a/BL21(DE3) strains comprising a mutant SHNL plasmid vector obtained by combining an additional 3 high-activity mutations (i.e., substitution of phenylalanine 125 with asparagine, that of leucine 122 with serine, and that of glutamine 216 with histidine) to Ver.0/pET21a; and the  E. coli  Ver.1+L122S-2/pET21a/BL21(DE3) strains comprising a mutant SHNL plasmid vector obtained by combining an additional 2 high-activity mutations (i.e., substitution of phenylalanine 125 with asparagine and that of leucine 122 with serine) to Ver.0/pET21a (Table 9). The specific activity levels thereof exceeded 50 U/mg-protein, which was at least 5 times higher than that of the  E. coli  SHNL-Gly165E,V173L/pET21a/BL21(DE3) strains comprising a vector plasmid before mutagenesis. However, the results attained from the SHNL-W128A/pET21a/BL21(DE3) constructed in Comparative Example 1 were lower than the results attained from the  E. coli  strain comprising a vector plasmid before mutagenesis. 
     
       
         
           
               
             
               
                 TABLE 9 
               
             
            
               
                   
               
               
                 Comparison of activity of mutant enzymes cultured in jar fermenter 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                 Duration 
                   
                   
                   
                   
               
               
                   
                   
                 of 
                 Cell 
                   
                   
                 Specific 
               
               
                   
                   
                 culture 
                 density 
                 Activity 
                 Protein 
                 activity 
               
               
                 Strains 
                 Genotype 
                 (h) 
                 660 nm 
                 u/ml 
                 mg/ml 
                 u/mg 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Ver.0/pET21a/BL21(DE3) 
                 G165E 
                 V173L 
                   
                   
                   
                   
                 52 
                 58.24 
                 211.7 
                 22.8 
                 9.3 
               
               
                 Ver.0 + F125N-1/pET21a/BL21(DE3)Star 
                 G165E 
                 V173L 
                 F125N 
                   
                   
                   
                 60 
                 43.3 
                 802.7 
                 20.3 
                 39.6 
               
               
                 Ver.0 + F125H-2/pET21a/BL21(DE3)Star 
                 G165E 
                 V173L 
                 F125H 
                   
                   
                   
                 72 
                 42.8 
                 836.6 
                 21.1 
                 39.7 
               
               
                 Ver.0 + F125L-1/pET21a/BL21(DE3)Star 
                 G165E 
                 V173L 
                 F125L 
                   
                   
                   
                 78 
                 41.43 
                 772.0 
                 20.2 
                 38.2 
               
               
                 Ver.0 + F125R-1/pET21a/BL21(DE3)Star 
                 G165E 
                 V173L 
                 F125R 
                   
                   
                   
                 78 
                 44.31 
                 823.0 
                 21.4 
                 38.5 
               
               
                 Ver.0 + F125Q-1/pET21a/BL21(DE3)Star 
                 G165E 
                 V173L 
                 F125Q 
                   
                   
                   
                 78 
                 25.5 
                 635.0 
                 22.0 
                 28.8 
               
               
                 Ver.0 + F125C-1/pET21a/BL21(DE3)Star 
                 G165E 
                 V173L 
                 F125C 
                   
                   
                   
                 78 
                 32.46 
                 788.0 
                 20.6 
                 38.3 
               
               
                 Ver.0 + F125N-1/pET21a/BL21(DE3) 
                 G165E 
                 V173L 
                 F125N 
                   
                   
                   
                 54.5 
                 45.66 
                 640.5 
                 24.2 
                 26.5 
               
               
                 Ver.1 + L122I-2/pET21a/BL21(DE3) 
                 G165E 
                 V173L 
                 F125N 
                 L122I 
                   
                   
                 60 
                 39.6 
                 594.0 
                 22.4 
                 26.5 
               
               
                 Ver.1 + L122S-2/pET21a/BL21(DE3) 
                 G165E 
                 V173L 
                 F125N 
                 L122S 
                   
                   
                 60 
                 41.94 
                 1148.0 
                 21.6 
                 53.1 
               
               
                 Ver.1 + L122I + Q216H-1/pET21a/ 
                 G165E 
                 V173L 
                 F125N 
                 L122I 
                 Q216H 
                   
                 80 
                 40.08 
                 789.0 
                 19.8 
                 39.9 
               
               
                 BL21(DE3) 
               
               
                 Ver.1 + L122S + Q216H-2/pET21a/ 
                 G165E 
                 V173L 
                 F125N 
                 L122S 
                 Q216H 
                   
                 54 
                 41.28 
                 1053.0 
                 19.6 
                 53.8 
               
               
                 BL21(DE3) 
               
               
                 Ver.3 + V94A-1/pET21a/BL21(DE3) 
                 G165E 
                 V173L 
                 F125N 
                 L122S 
                 Q216H 
                 V94A 
                 60 
                 40.8 
                 875.0 
                 23.0 
                 38.0 
               
               
                 Ver.3 + V94P-4/pET21a/BL21(DE3) 
                 G165E 
                 V173L 
                 F125N 
                 L122S 
                 Q216H 
                 V94P 
                 70 
                 38.34 
                 643.0 
                 18.0 
                 35.7 
               
               
                 Ver.3 + V94T-1/pET21a/BL21(DE3) 
                 G165E 
                 V173L 
                 F125N 
                 L122S 
                 Q216H 
                 V94T 
                 67 
                 38.52 
                 806.0 
                 24.7 
                 32.6 
               
               
                 Ver.3 + V94S-1/pET21a/BL21(DE3) 
                 G165E 
                 V173L 
                 F125N 
                 L122S 
                 Q216H 
                 V94S 
                 48 
                 49.8 
                 1064.0 
                 22.6 
                 47.1 
               
               
                 Ver.0 + L122I-1/pET21a/BL21(DE3)Star 
                 G165E 
                 V173L 
                   
                 L122I 
                   
                   
                 72 
                 51.84 
                 645.8 
                 36.1 
                 17.9 
               
               
                 Ver.0 + L122S-1/pET21a/BL21(DE3)Star 
                 G165E 
                 V173L 
                   
                 L122S 
                   
                   
                 72 
                 33.54 
                 569.1 
                 26.7 
                 21.3 
               
               
                 Ver.0 + L122G-2/pET21a/BL21(DE3)Star 
                 G165E 
                 V173L 
                   
                 L122G 
                   
                   
                 70 
                 35.58 
                 269.0 
                 18.7 
                 14.4 
               
               
                 Ver.0 + L122V-1/pET21a/BL21(DE3)Star 
                 G165E 
                 V173L 
                   
                 L122V 
                   
                   
                 76.45 
                 41.52 
                 454.6 
                 20.7 
                 22.0 
               
               
                 W128A/pET21a/BL21(DE3) 
                   
                   
                   
                   
                 W128A 
                   
                 61 
                 33.66 
                 123.8 
                 24.8 
                 5.0 
               
               
                   
               
            
           
         
       
     
     Example 9 
     Production of Optically Active Cyanohydrin Using High-Activity Mutant Comprising Combined Mutations 
     Whether or not the high-activity mutant SHNL comprising combined mutation sites could produce optically active cyanohydrin as with common SHNL was inspected. 
     [Material and Method] 
     1) Preparation of Immobilized Enzyme 
     0.2 M Na-citrate buffer (pH 5.5) was added to the supernatant of homogenized Ver.1+L122I-2/pET21a/BL21(DE3) cells obtained in Example 8, and the activity level was adjusted at 500 U/ml. Silica gel (300 mg) was mixed with 0.3 ml of the enzyme solution to obtain immobilized enzymes. The supernatant of homogenized  E. Coli  Ver.0/pET21a/BL21(DE3) cells comprising the SHNL gene before mutagenesis was subjected to the same procedure to prepare immobilized enzymes. 
     2) Enzyme Reaction 
     To t-butylmethylether (4.492 ml) comprising 1.5 M HCN dissolved therein, 0.337 ml of a 0.2 M citrate buffer (pH 5.5) was added, the resulting mixture was agitated for 30 minutes and allowed to stand thereafter, and the separated aqueous phase was removed. The solution was added to a 9-ml screw vial containing 300 mg of the immobilized enzymes prepared above. 2-Chlorobenzaldehyde was added to a final concentration of 1.0 M therein, and an enzyme reaction was carried out by stirring. After the completion of the reaction, the reaction solution was recovered, and the substrate conversion rate and the optical purity were assayed. 
     [Results] 
     The supernatant of the homogenized Ver.1+L 122I-2/pET21a/BL21 (DE3) cells exhibited the conversion rate and the optical purity substantially the same as those of the strain before mutagenesis (Table 10). Thus, the Ver.1+L122I-2 SHNL strains, i.e., the high-activity enzymes comprising combined mutations, were found to be capable of producing optically active cyanohydrin as with the mutant SHNL that the present inventors had produced in the past. 
     
       
         
           
               
             
               
                 TABLE 10 
               
             
            
               
                   
               
               
                 Synthesis of S-2-chloromandelonitrile using mutant enzyme 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Conversion 
                 Optical 
               
               
                   
                   
                 rate 
                 purity 
               
               
                 Enzyme strain used 
                 Genotype 
                 (mol %) 
                 (% ee) 
               
               
                   
               
               
                 Ver.0/pET21a/BL21 (DE3) 
                 G165E V173L 
                 97.2 
                 95.5 
               
               
                 Ver.1 + L122I-2/pET21a/ 
                 G165E V173L 
                 91.5 
                 95.8 
               
               
                 BL21 (DE3) 
                 F125N L122I 
               
               
                   
               
            
           
         
       
     
     Reference Example 
     Preparation of Ver.0/pKK223-3 
     1) Substitution of Val 173 with Leu 
     SHNL in which amino acid 173 had been substituted with Leu was prepared using the QuikChange XL Site-Directed Mutagenesis kit (Stratagene). 
     As a template, 10 ng of the SHNL-Wild/pKK223-3 vector plasmid was used, and extension reaction was carried out using the following oligo DNAs as the primers. 
     
       
         
           
               
               
            
               
                 (SEQ ID NO: 21) 
                   
               
            
           
           
               
               
            
               
                 Forward primer: 5′-GGC GAA TAT GAA CTG GCA AAA 
                   
               
               
                 ATG CTG ATG CGC AAG GGC TCT CTG-3′ 
               
               
                   
               
            
           
           
               
               
            
               
                 (SEQ ID NO: 22) 
                   
               
            
           
           
               
               
            
               
                 Reverse primer: 5′-CAG AGA GCC CTT GCG CAT CAG 
                   
               
               
                 CAT TTT TGC CAG TTC ATA TTC GCC-3′ 
               
            
           
         
       
     
     Subsequently, the obtained reaction product was digested with the DpnI restriction enzyme included in the kit, the resulting restriction enzyme-digested reaction product was transformed into DH5a competent cells to prepare the SHNL-V173L/pKK223-3/DH5α strain. 
     2) Substitution of Glycine 165 with Glutamine 
     The SHNL-V173L/pKK223-3 plasmid was prepared from the resulting SHNL-V173L/pKK223-3/DH5α strain, and site-directed mutagenesis was performed again using the resulting strain as a template. Extension reaction was carried out using the following primers, and the resulting reaction product was digested with the DpnI restriction enzyme. 
     
       
         
           
               
               
            
               
                 (SEQ ID NO: 23) 
                   
               
            
           
           
               
               
            
               
                 Forward primer: 5′-CGT GAA AAC CTG TTC ACC AAA 
                   
               
               
                 TGC ACT GAT GAA GAA TAT GAA CTG GCA AAA ATG-3′ 
               
               
                   
               
            
           
           
               
               
            
               
                 (SEQ ID NO: 24) 
                   
               
            
           
           
               
               
            
               
                 Reverse primer: 5′-CAT TTT TGC CAG TTC ATA TTC 
                   
               
               
                 TTC ATC AGT GCA TTT GGT GAA CAG GTT TTC ACG-3′ 
               
            
           
         
       
     
     The resulting restriction enzyme-digested reaction product was transformed into DHα a competent cells to prepare the SHNL-V173L/pKK223-3/DH5α strain. The plasmid was prepared from the culture medium, and the Ver.0/pKK223-3 plasmid comprising the SHNL gene having 2 amino acid mutations, Gly165Glu and Val173Leu, was obtained. 
     All publications, patents, and patent applications cited herein are incorporated herein by reference in their entirety. 
     INDUSTRIAL APPLICABILITY 
     The high-activity modified SHNL of the present invention has higher enzyme activity than wild-type SHNL, and it can effectively synthesize optically active cyanohydrin with high optical purity. Accordingly, the high-activity modified SHNL of the present invention is very useful as an enzyme for producing optically active cyanohydrin at industry levels. 
     Sequence Listing Free Text 
     SEQ ID NO: 5; description of an artificial sequence: primer
 
SEQ ID NO: 6; description of an artificial sequence: primer
 
SEQ ID NO: 7; description of an artificial sequence: primer
 
SEQ ID NO: 8; description of an artificial sequence: primer
 
SEQ ID NO: 9; description of an artificial sequence: primer
 
SEQ ID NO: 10; description of an artificial sequence: primer
 
SEQ ID NO: 11; description of an artificial sequence: primer
 
SEQ ID NO: 12; description of an artificial sequence: primer
 
SEQ ID NO: 13; description of an artificial sequence: primer
 
SEQ ID NO: 14; description of an artificial sequence: primer
 
SEQ ID NO: 15; description of an artificial sequence: primer
 
SEQ ID NO: 16; description of an artificial sequence: primer
 
SEQ ID NO: 17; description of an artificial sequence: primer
 
SEQ ID NO: 18; description of an artificial sequence: primer
 
SEQ ID NO: 19; description of an artificial sequence: primer
 
SEQ ID NO: 20; description of an artificial sequence: primer
 
SEQ ID NO: 21; description of an artificial sequence: primer
 
SEQ ID NO: 22; description of an artificial sequence: primer
 
SEQ ID NO: 23; description of an artificial sequence: primer
 
SEQ ID NO: 24; description of an artificial sequence: primer
 
SEQ ID NO: 25; DNA encoding mutant SHNL in which glycine 165 and valine 173 had been substituted with glutamine and leucine, respectively