Abstract:
The present invention provides a novel psicose epimerase derived from  Flavonifractor plautii  and capable of converting fructose to psicose. The novel psicose epimerase according to the present invention possesses an activity producing psicose by epimerizing the carbon-3 position of fructose, and has maximal activity for the conversion of fructose to psicose at a relatively high temperature and a pH less than or equal to neutral, has excellent thermal stability, and can mass-produce psicose from fructose in a high yield for a short amount of time. Therefore, the psicose epimerase according to the present invention is advantageous in the industrial production of psicose, and it is expected that the psicose produced thereby can be usefully utilized in the functional sugar industry and also as materials for health food, medicine, cosmetics, and the like using the psicose.

Description:
FIELD 
       [0001]    The present invention relates to a novel psicose epimerase capable of converting fructose into D-psicose, a method of producing the psicose epimerase from a recombinant strain, a method of producing D-psicose from fructose using the same, and the like. 
       BACKGROUND 
       [0002]    D-psicose as an epimer of a carbon-3 position of fructose has 70% sweetness as compared with sugar (Oshima 2006), but is functional monosaccharide applicable as a low-calorie sweetener of diet food in which energy is only 0.3% (Matsuo et al. 2002). Further, the D-psicose serves to inhibit glucose by suppressing absorption of glucose to be applied to food for diabetic patients, receiving food, and the like and serves to suppress the enzyme activity associated with lipid synthesis in the liver to suppress the accumulation of abdominal fat to be used in various functional foods such as health foods (Matsuo et al. 2001; Iida et al. 2008; Hayashi et al. 2010; Hossain et al. 2011). 
         [0003]    As the above features, the psicose is a good source capable of replacing sugar, but the psicose belongs to a rare saccharide as a monosaccharide that is rarely present in the nature, and thus, a method for efficiently producing the psicose is required so as to be applied to food industry. The psicose is mostly produced through a chemical process as an existing method of producing the psicose. Bilik, et al. propose a method of converting fructose into psicose by using a catalytic action of molybdate ions. McDonald produced the psicose by a 3-step chemical treatment process from 1,2:4,5-di-δ-isopropylidene-beta-D-fructopyranose. Further, Doner produced the psicose by heating the fructose together with ethanol and trimethylamine. However, there are disadvantages in that a lot of costs are consumed in the chemical production methods, whereas the efficiencies thereof are low and a lot of byproducts are generated. 
         [0004]    As a biological production method of psicose, a method of producing psicose from galactitol, D-tagatose, D-talitol, or the like by using a cell reaction of a microorganism is proposed (Ken Izumori). However, the method is difficult to be applied to the industrial production because the substrate is the rare saccharide. The most efficient method for industrialization is a method of finding an enzyme for converting fructose into psicose in a D-ketose 3-epimerase group. By the existing reported contents, the psicose was produced from the fructose by using a D-tagatose 3-epimerase which is expressed in  E. coli  transformed by inserting and transforming the D-tagatose 3-epimerase derived from  Clostridium celluloticum  H (10) (Mu et al. 2011),  Agrobacterium tumefaciens  (Kim et al. 2006),  Pseudomonas cichorii  (Itoh et al. 1994),  Rhizobium spheroides  (Zhang et al. 2009) in  E. coli . With relation to a technique of producing psicose from fructose by using an enzyme, in Korea Patent Registration No. 10-0744479, a method of producing psicose by a psicose epimerase derived from  Agrobacterium tumefaciens  is disclosed, in Korea Patent Registration No. 10-0832339,  Sinorhizobium  YB-58 KCTC 10983BP having an activity of converting fructose into psicose and a method for converting fructose into psicose by using the same are disclosed, in Korea Patent Registration No. 10-1106253,  E. coli  including a polynucleotide coding a psicose 3-epimerase of  Agrobacterium tumefaciens  C58 having an activity of catalyzing conversion of fructose into psicose and a method of producing psicose from fructose by using the same are disclosed, in Korea Patent Registration No. 10-1339443 (Korea Patent Application Publication No. 10-2008-0071176), a ketose 3-epimerase derived from microorganisms belonging to genus  Rhizobium  and a method for converting fructose into psicose by using the same are disclosed, and in Korea Patent Registration No. 10-1318422, a D-psicose 3-epimerase derived from  Clostridiuim scindens  and a method for producing psicose from fructose by using the same are disclosed. 
         [0005]    However, according to an existing enzymatic method of which functions are known, a method of producing psicose is best at a medium temperature and pH of an alkaline condition. In the alkaline condition, the reaction induces non-specific reaction and browning of sugar, and thus is not suitable for the industrialization. Further, there is a problem in that the existing enzymes have factors of increasing producing costs for producing psicose which is applied to the industrialization due to deteriorated stability or a slow reaction rate at a high temperature. Therefore, development of a novel D-psicose 3-epimerase in which a product yield, a temperature, a pH and a reaction rate of the psicose are suitable for industrialization is required. With relation to this, in Korea Patent Application Publication No. 10-2014-0021974, a D-psicose 3-epimerase derived from  Treponema primitia  ZAS-1 having a rapid psicose conversion rate and stability at a high temperature by inducing mutation at a gene level is disclosed, and in Korea Patent Registration No. 10-1203856, psicose epimerase variants having improved thermal stability obtained by mutation of a wild-type psicose epimerase derived from  Agrobacterium tumefaciens  is disclosed. 
       DISCLOSURE OF INVENTION 
     Technical Problem 
       [0006]    A first object of the present invention is to provide a novel D-psicose 3-epimerase having an activity of converting fructose into psicose, a maximum activity at a relatively high temperature or a pH less than or equal to neutral, and excellent thermal stability. 
         [0007]    A second object of the present invention is to provide a method of producing a novel D-psicose 3-epimerase or various elements required for producing the novel D-psicose 3-epimerase. 
         [0008]    A third object of the present invention is to provide a method of producing psicose from fructose or various elements required for producing the psicose from the fructose. 
       Solution to Problem 
       [0009]    The inventors of the present invention discovered that an enzyme derived from  Flavonifractor plautii  has a high activity for converting fructose into psicose, excellent thermal stability, and an optimal activity in a high temperature range and a pH range corresponding to neutral or weak acid, and thus, completed the present invention. 
         [0010]    In order to solve the first object, an aspect of the present invention provides a psicose epimerase consisting of an amino acid sequence of SEQ ID NO: 1. 
         [0011]    In order to solve the second object, another aspect of the present invention provides a polynucleotide coding a psicose epimerase consisting of an amino acid sequence of SEQ ID NO: 1. Further, the present invention provides a primer pair for synthesizing the polynucleotide coding the psicose epimerase consisting of the amino acid sequence of SEQ ID NO: 1. Further, the present invention provides a recombinant vector including the polynucleotide coding the psicose epimerase consisting of the amino acid sequence of SEQ ID NO: 1. Further, the present invention provides a recombinant strain which is transformed by the polynucleotide coding the psicose epimerase consisting of the amino acid sequence of SEQ ID NO: 1 or the recombinant vector including the polynucleotide coding the psicose epimerase consisting of the amino acid sequence of SEQ ID NO: 1. Further, the present invention provides a method for producing psicose epimerase, in which the method includes steps of: expressing the psicose epimerase by culturing a recombinant strain which is transformed by a polynucleotide coding the psicose epimerase consisting of the amino acid sequence of SEQ ID NO: 1 or a recombinant vector including the polynucleotide coding the psicose epimerase consisting of the amino acid sequence of SEQ ID NO: 1; and isolating the psicose epimerase from a lysate of the recombinant strain with the expressed psicose epimerase. 
         [0012]    In order to solve the third object, yet another aspect of the present invention provides a composition for producing psicose including a psicose epimerase consisting of an amino acid sequence of SEQ ID NO: 1. Further, the present invention provides the composition for producing psicose including a recombinant strain which is transformed by a polynucleotide coding the psicose epimerase consisting of the amino acid sequence of SEQ ID NO: 1 or a recombinant vector including the polynucleotide coding the psicose epimerase consisting of the amino acid sequence of SEQ ID NO: 1, a culture of the recombinant strain, or a lysate of the recombinant strain. Further, the present invention provides the method for producing psicose, in which the method includes reacting fructose with the psicose epimerase consisting of an amino acid sequence of SEQ ID NO: 1 or the composition including the psicose epimerase. Further, the present invention provides the method for producing psicose, in which the method includes reacting fructose with the recombinant strain which is transformed by the polynucleotide coding the psicose epimerase consisting of the amino acid sequence of SEQ ID NO: 1 or the recombinant vector including the polynucleotide coding the psicose epimerase consisting of the amino acid sequence of SEQ ID NO: 1, a culture of the recombinant strain, a lysate of the recombinant strain, or the composition including one or more thereof. 
       Advantageous Effects 
       [0013]    The novel psicose epimerase, according to the present invention, possesses an activity producing psicose by epimerizing the carbon-3 position of fructose, and has maximal activity for the conversion of fructose into psicose at a relatively high temperature and a pH less than or equal to neutral, has excellent thermal stability, and can mass-produce psicose from fructose in a high yield for a short amount of time. Therefore, the psicose epimerase according to the present invention is advantageous in the industrial production of psicose, and the psicose produced thereby can be usefully utilized in the functional sugar industry and as materials for health food, medicine, cosmetics, and the like using the psicose. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0014]      FIG. 1  is a cleavage map of pET-FDPE which is a recombinant expression vector. 
           [0015]      FIG. 2  is a result of performing SDS-PAGE with respect to a D-psicose 3-epimerase purified through a His tag affinity chromatography in Example 2 of the present invention. 
           [0016]      FIG. 3  is a graph illustrating an activity of the D-psicose 3-epimerase of the present invention according to a kind of added metal ion. In  FIG. 3 , an enzyme activity in the case of treating each metal ion is relatively represented by setting an enzyme activity in a control group to 100. 
           [0017]      FIG. 4  is a graph illustrating an activity of the D-psicose 3-epimerase of the present invention for each kind of added metal ion and each treated concentration. 
           [0018]      FIG. 5  is a graph illustrating an activity of the D-psicose 3-epimerase of the present invention for each reaction pH. In  FIG. 5 , the enzyme activity is relatively represented by setting a maximum activity to 100. 
           [0019]      FIG. 6  is a graph illustrating an activity of the D-psicose 3-epimerase of the present invention for each reaction temperature. In  FIG. 6 , the enzyme activity is relatively represented by setting a maximum activity to 100. 
           [0020]      FIG. 7  is a graph illustrating an activity of the D-psicose 3-epimerase of the present invention for each substrate. In  FIG. 7 , the enzyme activity is relatively represented by setting an enzyme activity of a reaction using psicose as a substrate to 100. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    Hereinafter, the present invention will be described in detail. 
         [0022]    The present invention relates to a novel D-psicose 3-epimerase (hereinafter, referred to as a psicose epimerase) capable of converting fructose into psicose. The psicose epimerase is derived from  Flavonifractor plautii , and has maximal activity for the conversion of the fructose into the psicose at a relatively high temperature and a pH less than or equal to neutral, has excellent thermal stability, and can mass-produce the psicose from the fructose in a high yield for a short amount of time. The psicose epimerase may be obtained by a method of amplifying a specific DNA in many DNAs present in a  Flavonifractor plautii  genomic DNA by a polymerase reaction, preparing a recombinant expression vector by inserting the amplified specific DNA to an expression vector, producing a recombinant strain by transforming a host strain with the recombinant expression vector, and then culturing and expressing the recombinant strain. The psicose epimerase according to the present invention preferably has a molecular weight of 30 to 34 kDa, an optimal activity temperature in a range of 55 to 67° C., and an optimal activity pH in a range of 6.5 to 8. The psicose epimerase according to the present invention consists of an amino acid sequence of SEQ ID NO: 1, but an equivalent range of the psicose epimerase according to the present invention is not limited thereto. For example, in the equivalent range of the psicose epimerase according to the present invention, so long as the activity of converting the fructose into the psicose is maintained, some of amino acids of SEQ ID NO: 1 may be replaced, inserted, and/or deleted. Preferably, the replacement of the amino acid may be performed by conservative amino acid replacement in which a characteristic of a protein is not changed. Further, the modification of the amino acid may be performed by glycosylation, acetylation, phosphorylation, and the like. Further, the equivalent range of the psicose epimerase according to the present invention may include a protein with increased structural stability to heat, pH, and the like by mutation or modification of the amino acid sequence or increased activity for the conversion of fructose into psicose. Further, the equivalent range of the psicose epimerase according to the present invention may include amino acid sequences having homology of 70% or more, 80% or more, 90% or more, 95% or more, or 99% or more with the amino acid sequence of SEQ ID NO: 1. The following Table 1 lists amino acids which may replace amino acids in the protein by the conservative amino acid replacement. 
         [0000]    
       
         
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Amino acid 
                 Conservative 
               
               
                 residues in peptide 
                 replacement group 
               
               
                   
               
             
             
               
                 Ala 
                 Ser 
               
               
                 Arg 
                 Lys 
               
               
                 Asn 
                 Gln, His 
               
               
                 Asp 
                 Glu 
               
               
                 Gln 
                 Asn 
               
               
                 Cys 
                 Ser 
               
               
                 Glu 
                 Asp 
               
               
                 Gly 
                 Pro 
               
               
                 His 
                 Asn, Gln 
               
               
                 Ile 
                 Leu, Val 
               
               
                 Leu 
                 Ile, Val 
               
               
                 Lys 
                 Arg, Gln 
               
               
                 Met 
                 Leu, Ile 
               
               
                 Phe 
                 Met, Leu, Tyr 
               
               
                 Ser 
                 Thr, Gly 
               
               
                 Thr 
                 Ser, Val 
               
               
                 Trp 
                 Tyr 
               
               
                 Tyr 
                 Trp, Phe 
               
               
                 Val 
                 Ile, Leu 
               
               
                   
               
             
          
         
       
     
         [0023]    Another exemplary embodiment of the present invention relates to a method of producing a novel psicose epimerase or various elements required for producing the novel psicose epimerase. The various elements required for producing the novel psicose epimerase include polynucleotide, a primer pair, a recombinant vector, a recombinant strain, and the like. 
         [0024]    The polynucleotide is polynucleotide coding an epimerase consisting of an amino acid sequence of SEQ ID NO: 1 and preferably consists of a base sequence of SEQ ID NO: 2. In the present invention, the term “polynucleotide” means all of non-modified or modified polyribonucleotides (RNA) or polydeoxyribonucleotides (DNA). The polynucleotide includes single- or double-stranded DNA, DNA which is a mixture of single- and double-stranded regions, single- and double-stranded RNA, RNA which is a mixture of single- and double-stranded regions, or hybrid molecules thereof, but is not limited thereto. Further, the equivalent range of the polynucleotide coding the epimerase includes a sequence having the substantial identity with a base sequence of SEQ ID NO: 2. The substantial identity means that any other sequence is arranged to maximally correspond to the base sequence of SEQ ID NO: 2 and the any other sequence has the sequence homology of 70% or more, 90% or more, or 98% or more with the base sequence of SEQ ID NO: 2 by analyzing the sequence thereof. It can be easily understood to those skilled in the art that one or more bases of the base sequence of the polynucleotide are replaced, added, or deleted by using a gene recombination technique known in the art to produce polynucleotide coding an enzyme having the same activity in a range having the substantial homology. A comparison of the homologies may be performed by calculating homology between two or more sequences in percentage (%) by using a computer program on the market. 
         [0025]    Further, the primer pair is to synthesize polynucleotide coding a psicose epimerase consisting of an amino acid sequence of SEQ ID NO: 1, and preferably constituted by a forward primer having a base sequence of SEQ ID NO: 3 and a reverse primer having a base sequence of SEQ ID NO: 4. 
         [0026]    Further, the recombinant vector includes polynucleotide coding the psicose epimerase consisting of the amino acid sequence of SEQ ID NO: 1. The recombinant vector may be provided in a form of inserting the polynucleotide coding the psicose epimerase into a cloning vector or an expression vector by using a known standard method. In the present invention, the term “cloning vector” is defined as a material which can transport a DNA fragment into a host cell and reproduce the DNA fragment. In the present invention, the cloning vector may further include a polyadenylation signal, a transcription termination sequence, and a multiple cloning site. In this case, the multiple cloning sites include at least one endonuclease restriction site. Further, the cloning vector may further include a promoter. As an example, in the present invention, the polynucleotide coding the psicose epimerase may be located at the upstream of the polyadenylation signal and the transcription termination sequence, and at least one endonuclease restriction site may be located at the upstream of the polyadenylation signal and the transcription termination sequence. Further, in the present invention, the term “expression vector” is defined as a DNA sequence required for transcription and translation of the cloned DNA in an appropriate host. Further, in the present invention, the term “expression vector” means a gene construct including a required regulation element which is operably connected to an insertion so as to express the insertion in cells of an object. The expression vector may be produced and purified by using a standard recombinant DNA technique. A kind of expression vector is not particularly limited as long as the expression vector expresses a desired gene in various host cells, such as, prokaryotic and eukaryotic cells and performs a function of producing a desired protein, but is preferably a vector which may mass-produce a foreign protein in a similar form to a natural state, while possessing a promoter representing a strong activity and strong expression force. The expression vector preferably includes at least a promoter, an initiation codon, a gene coding a desired protein, and a stop codon terminator. In addition, the expression vector may appropriately include a DNA coding a signal peptide, an additional expression regulating sequence, non-translation regions of 5′-terminal and 3′-terminal of a desired gene, a selection marker region, a replicable unit, or the like. The “promoter” means a minimal sequence enough to indicate the transcription. Further, a promoter configuration enough to express a regulatory promoter-dependent gene induced by a cell type-specific or external signal or agent may be included, and the configurations may be located at the 5′ or 3′ portion of the gene. Both a conservative promoter and an induction promoter are included. The promoter sequence may be derived from prokaryote, eukaryote, or virus. The term “operably connected” means that one function is regulated by another thing by association with a polynucleotide sequence on a single polynucleotide. For example, in the case where the promoter may control expression of the coding sequence (that is, the coding sequence is under the transcription regulation of the promoter), if the promoter is connected with the coding sequence to operate or a ribosome binding site is located to promote the translation, the ribosome binding site is connected to the coding sequence and operates. The coding sequence may be connected to a regulation sequence and operate in a sense direction or an antisense direction. In the expression vector according to the present invention, the recombinant vector is preferably an expression vector and the expression vector preferably has a cleavage map of  FIG. 1   
         [0000]    Further, the recombinant strain is transformed by the polynucleotide coding the psicose epimerase consisting of the amino acid sequence of SEQ ID NO: 1 or transformed by a recombinant vector including the polynucleotide coding the psicose epimerase consisting of the amino acid sequence of SEQ ID NO: 1. In the present invention, the term “recombinant strain” means cells transformed by introducing a polynucleotide coding one or more target proteins or an expression vector having the polynucleotide to a host cell. A method for producing a transformant by introducing the expression vector to the host cell includes a chemical treating method including transient transfection, microinjection, transduction, cell fusion, calcium phosphate precipitation, liposemmediated transfection, DEAE Dextran-mediated transfection, polybrene-mediated transfection, electroporation, electroinjection, PEG, and the like, a method using a gene gun and the like, and the like, but is not limited thereto. In the present invention, as long as a host cell which may be transformed by the expression vector is known in the art, such as prokaryotic cells, plant cells, insect cells, animal cells, and the like, a kind thereof is largely not limited thereto. Preferably, a host having high introduction efficiency of DNA and high expression efficiency of the introduced DNA is generally used. For example, the host cell may be  E. coli . The  E. coli  includes BL21, JM109, K-12, LE392, RR1, DH5α, W3110, or the like, but is not limited thereto. In addition, the host cell may be a strain selected from the group consisting of  Bacillus  strains such as  Bacillus subtilis  and  Bacillus thuringiensis , Coryne bacterial strains such as  Corynebacterium glutamicum, Salmonella  strains such as  Salmonella typhimurium , other  Serratia marcescens , and Enterobacteriaceae strains such as various  Pseudomonas  species. 
         [0027]    Further, a method of producing the psicose epimerase includes the steps of expressing psicose epimerase by culturing a recombinant strain which is transformed by a polynucleotide coding the psicose epimerase consisting of the amino acid sequence of SEQ ID NO: 1 or a recombinant vector including the polynucleotide coding the psicose epimerase consisting of the amino acid sequence of SEQ ID NO: 1; and isolating the psicose epimerase from a lysate of the recombinant strain in which the psicose epimerase is expressed. The expression of the protein by the host cell may be induced by using isopropyl-1-thio-β-D-galactopyranoside (IPTG) as an inducing factor and an inducing time may be adjusted to maximize the amount of the protein. In the present invention, the psicose epimerase may be collected from a lysate of the recombinant strain. The cells used in protein expression may be lysed by various physical or chemical means such as freeze-thawing repetition, sonication, mechanical breakage, or a cell lytic agent and can be isolated or purified by a general biochemical isolation technique (Sambrook et al., Molecular Cloning: A laboratory Manual, 2nd Ed., Cold Spring Harbor Laboratory Press, 1989; Deuscher, M., Guide to Protein Purification Methods Enzymology, Vol. 182. Academic Press. Inc., San Diego, Calif., 1990). For example, the method of isolating or purifying the protein expressed by the host cell includes electrophoresis, centrifugation, gel filtration, precipitation, dialysis, chromatography (ion exchange chromatography, affinity chromatography, immunosorbent affinity chromatography, reverse-phase HPLC, and gel permeation HPLC), isoelectricity focus, and various modified or complex methods thereof, but is not limited thereto. Meanwhile, in the present invention, the isolating of the psicose epimerase from the lysate of the recombinant strain may be preferably performed by affinity chromatography using a peptide tag. As the peptide tag, various known tags such as a HA tag, a FLAG tag, a His tag, a biotin carboxyl carrier protein (BCCP), a c-myc tag, a V5 tag, a glutathione-S-transferase (GST), or a maltose binding protein (MBP) may be used and among them, the His tag is preferably used. The His-tagging protein is specifically trapped on a column of a nickel-nitrilotriacetic acid (Ni-NTA) resin and may be released by EDTA or imidazole. 
         [0028]    Yet another exemplary embodiment of the present invention relates to a method of producing psicose from fructose or various elements required for producing the psicose from the fructose. As various elements required for producing the psicose from the fructose, there is a composition for producing the psicose. 
         [0029]    An example of the composition for producing the psicose includes a psicose epimerase consisting of the amino acid sequence of SEQ ID NO: 1. Further, another example of the composition for producing the psicose includes a recombinant strain which is transformed by the polynucleotide coding the psicose epimerase consisting of the amino acid sequence of SEQ ID NO: 1 or a recombinant vector including the polynucleotide coding the psicose epimerase consisting of the amino acid sequence of SEQ ID NO: 1, a culture of the recombinant strain, or a lysate of the recombinant strain. In this case, preferably, the composition for producing the psicose may further include one or more kinds selected from the group consisting of manganese ions, nickel ions, and cobalt ions, and more preferably, may further include nickel ions or cobalt ions. The novel psicose epimerase according to the present invention has a metalloenzyme characteristic in which activation is adjusted by a metal ion and performs the reaction by the enzyme in a presence of a specific metal ion such as nickel ions or cobalt ions to increase a production yield of the psicose. 
         [0030]    Further, an example of the method of producing the psicose from the fructose includes reacting the fructose with the psicose epimerase consisting of the amino acid sequence of SEQ ID NO: 1 or the composition including the psicose epimerase. Further, another example of the method of producing the psicose from the fructose includes reacting the fructose with a recombinant strain which is transformed by a polynucleotide coding the psicose epimerase consisting of the amino acid sequence of SEQ ID NO: 1 or a recombinant vector including the polynucleotide coding the psicose epimerase consisting of the amino acid sequence of SEQ ID NO: 1, a culture of the recombinant strain, a lysate of the recombinant strain, or a composition including one or more thereof. Further, the method of producing the psicose from the fructose may additionally include adding metal ions, and a kind of metal ion is as described above. As an example, the metal ion may be added to the fructose that is a substrate or added to a mixture of the enzyme and the fructose. Further, as another example, the metal ion may be added to a carrier immobilized with the enzyme (before adding the fructose), added to a mixture of the carrier immobilized with the enzyme and the fructose (after adding the fructose), or added in a form of the mixture with the fructose when the fructose is added. In the case of using the recombinant strain, the metal ion may be added in the culture or the culturing may be performed in a culture medium added with the metal ion. Further, in the method of producing the psicose from the fructose, the psicose epimerase or the recombinant strain is preferably immobilized in the carrier. The carrier may create an environment in which the activity of the immobilized enzyme may be maintained for a long amount of time, and may be selected from all known carriers which may be used for enzyme immobilization. For example, sodium alginate may be used as the carrier. The sodium alginate is a natural colloidal polysaccharide abundant in the cell wells of algae and consists of β-D-mannuronic acid and α-L-gluronic acid. In terms of the content thereof, the sodium alginate is formed by randomly forming a β-1,4 bond and the strain or the enzyme is stably immobilized, and thus it is advantageous to have excellent psicose yield. As an example, in order to further promote the yield of the psicose, a sodium alginate solution at a concentration of 1.5 to 4.0% (w/v) (for example, an aqueous sodium alginate solution), preferably a sodium alginate solution at a concentration of about 2.5% (w/v) may be used for immobilizing the recombinant strain. Further, in the method of producing the psicose from the fructose, the reaction temperature is in the range of 55 to 67° C., preferably 55 to 65° C., and more preferably 55 to 60° C. when considering stability of the enzyme, and the reaction pH is in the range of 6.5 to 8, preferably 6.5 to 7.5, and more preferably 6.5 to 7. Further, in the method of producing the psicose from the fructose, the concentration of the fructose is not particularly limited thereto, but preferably 35 to 75% (w/w) and more preferably 40 to 70% (w/w) based on the entire reactant when considering productivity and economics. Further, in the method of producing the psicose from the fructose, the amount of used enzyme may be 0.001 to 0.1 mg/ml, preferably 0.01 to 0.1 mg/ml, and more preferably 0.02 to 0.05 mg/ml based on the entire reactant. Further, in the case of producing the psicose from the fructose by using the recombinant strain, the host strain of the recombinant strain is preferably a cytologically safe strain. The cytologically safe strain means a generally accepted as safe (GRAS) strain which is generally accepted as safe and for example, may be a  corynebacterium  strain. The  corynebacterium  strain is an industrial microorganism which produces chemical materials having various uses in fields of feed, medicines, and food, and the like including L-lysine, L-threonine, and various nucleic acids. The  corynebacterium  strain is the GRAS strain and has a strain characteristic which is easily used for gene manipulation and mass culture. Further, the  corynebacterium  strain is a strain having high stability in various process conditions and has a relatively hard cell membrane structure as compared with other bacteria to have a biological characteristic in which the strain is present in a stable state even under high osmotic pressure caused by high sugar concentration. A particular example of the  corynebacterium  strain includes  Corynebacterium glutamicum  and the like. 
       Example 1: Preparation of Recombinant Strain Producing D-Psicose 3-Epimerase 
       [0031]    A genomic DNA was extracted from  Flavonifractor plautii  KCTC 5970 received from Korean Collection for Type Cultures and used as a template, and a polymerase chain reaction (PCR) was performed by using a primer and an Ex-Taq (TAKARA) polymerase for cloning a gene (a polynucleotide of SEQ ID NO: 2) coding D-psicose 3-epimerase. The following Table 2 lists a primer used for cloning the gene coding the D-psicose 3-epimerase from the genomic DNA of  Flavonifractor plautii . The primers listed in the following Table 2 were prepared by Bioneer co., KR. 
         [0000]    
       
         
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                 Restriction 
               
               
                   
                   
                 Base 
                 enzyme 
               
               
                 SEQ 
                   
                 sequence 
                 recognition site 
               
               
                 ID NO 
                 Primer type 
                 (5′→3′) 
                 included in primer 
               
               
                   
               
             
             
               
                 3 
                 Forward primer 
                 CGG CAT ATG 
                 Nde I 
               
               
                   
                 for cloning 
                 AAC CCG ATT 
               
               
                   
                 psicose 
                 GGA ATG CAC 
               
               
                   
                 epimerase 
                 TAC 
               
               
                   
               
               
                 4 
                 Reverse primer 
                 CGG CTC GAG 
                 Xho I 
               
               
                   
                 for cloning 
                 TTA CGC GGT 
               
               
                   
                 psicose 
                 CAG CTC CTT 
               
               
                   
                 epimerase 
                 GAG G 
               
               
                   
               
             
          
         
       
     
         [0032]    Thereafter, only a desired target DNA was isolated from a PCR product by using a gel extraction kit (Qiagen) and then bound to an easy T-vector (Promega). A base sequence analysis of the isolated target DNA was entrusted to Bioneer Co., KR. As a result, it was verified that the target DNA amplified through the PCR corresponded to the polynucleotide of SEQ ID NO: 2. Thereafter, pET-FDPE as a recombinant expression vector was prepared by inserting the target DNA amplified by the PCR reaction to the same restriction recognition site of a pET15b vector (Novagen) as the expression vector by using Nde I and Xho I which were restrictive enzymes.  FIG. 1  is a cleavage map of pET-FDPE which is a recombinant expression vector. Thereafter, a recombinant strain was prepared by transforming BL21 (manufacturer: RBC, Taipei, Taiwan)  E. coli  as a competent cell to the pET-FDPE as the recombinant expression vector by using electroporation. 
       Example 2: Expression and Purification of D-Psicose 3-Epimerase 
       [0033]    A single colony of the transformed recombinant strain was inoculated in 15 ml of a LB-ampicillin medium (Difco) and then pre-cultured for about 6 hrs in a condition of 37° C. and 200 rpm. Thereafter, the pre-culture solution was inoculated in 500 ml of a LB-ampicilline medium and shaking-cultured in a condition of 37° C. and 200 rpm. Thereafter, when the absorbance (at 600 nm) of the culture solution was 0.5, IPTG was added to be a concentration of 0.1 mM to induce overexpression of a target enzyme. In this case, the culture from the overexpression inducing time was converted to a condition of 16° C. and 150 rpm and maintained for about 16 hrs. Thereafter, the culture solution of the recombinant strain was centrifuged for 2 mins at 13,000 rpm and a supernatant was removed to collect strain cells of the recombinant strain. 
         [0034]    The collected strain cells of the recombinant strain were suspended in a lysis buffer (50 mM Tris_HCl 300 mM NaCl pH 8.0, 10 mM imidazole) and then lysed by ultrasonic treatment. The cell lysate was centrifuged for 10 mins at 13,000 rpm and only the supernatant was gathered and then applied to a Ni-NTA column (Bio-Rad, Profinia) pre-equilibrated with a lysis buffer, and buffer solutions including 20 mM imidazole and 200 mM imidazole were sequentially flowed in 50 mM Tris_HCl 300 mM NaCl pH 8.0. Finally, 50 mM Tris_HCl 300 mM NaCl pH 8.0 and 200 mM imidazole were flowed to elute a target protein. It was verified from an experimental result to be described below that eluted protein was the D-psicose 3-epimerase. Thereafter, the eluted protein was stored in a buffer solution (PIPES, pH 7.5) for measuring an enzyme activity so as to be used for a next experiment. Further, the SDS-PAGE for the eluted protein was performed, and thus, it was verified that the size of the eluted protein was 32 kDa.  FIG. 2  is a result of performing SDS-PAGE for a D-psicose 3-epimerase purified through a His tag affinity chromatography in Example 2 of the present invention. 
       Example 3: Verification of Characteristics of D-Psicose 3-Epimerase 
     (1) Analysis of Metal Ion Requirement of D-Psicose 3-Epimerase 
       [0035]    Whether the metal ion had an effect on the D-psicose 3-epimerase obtained in Example 2 was examined. 
         [0036]    ZnSO 4 , CaCl 2 , MnSO 4 , NiSO 4 , CoCl2, MgSO 4 , CuSO 4 , CaCO 3 , and FeSO 4  were put in the purified enzyme buffer solution (PIPES, pH 7.5) to be 1 mM, respectively and then the metal ions were bound to the enzymes for about 1 hr. Thereafter, the enzymes bound with the metal ions were mixed with a 100 mM aqueous fructose solution as a substrate at a weight ratio of 1:1 and the mixture in which the enzyme concentration was 0.025 ml/mg and the fructose concentration was 50 mM was made and reacted for 10 mins at 60° C. Then, a hydrochloric acid solution was added thereto to stop the reaction. Further, in a control group, the same experiment was performed by using an enzyme (None) without treating the metal ion. Thereafter, an amount (mM) of produced psicose was measured and divided into an enzyme amount and a reaction time to calculate an enzyme activity. The psicose amount was analyzed by HPLC. The HPLC analysis was performed while water 100% (v/v) as a mobile phase flowed at a flow velocity of 0.6 ml/min at 80° C. by using a 87C (BIO-RAD) column and the psicose was detected with a refractive index detector (Agilent 1260 TID) to analyze the produced amount of psicose.  FIG. 3  is a graph illustrating an activity of the D-psicose 3-epimerase of the present invention according to a kind of added metal ion. In  FIG. 3 , an enzyme activity in the case of treating each metal ion is relatively represented by setting an enzyme activity in a control group to 100. As illustrated in  FIG. 3 , in the D-psicose 3-epimerase obtained in Example 2, an activity of converting the fructose into the psicose was increased by adding manganese ions, nickel ions, and cobalt ions, and particularly, an increase in activity of the nickel ions and cobalt ions was significant. 
         [0037]    Thereafter, MnSO 4 , NiSO 4 , and CoCl 2  were treated at various concentrations in the purified D-psicose 3-epimerase solution and then the same experiments were performed.  FIG. 4  is a graph illustrating an activity of the D-psicose 3-epimerase of the present invention for each kind of added metal ion and each treated concentration. As illustrated in  FIG. 4 , the manganese ions, the nickel ions, and the cobalt ions increased the activity of the D-psicose 3-epimerase in various concentration ranges. 
       (2) Analysis of Activity of D-Psicose 3-Epimerase According to Change in pH or Temperature 
       [0038]    In order to examine an optimal pH of the D-psicose 3-epimerase obtained in Example 2, test solutions of various pHs were made by using an MES buffer, a PIPES buffer, an EPPS buffer, and a CHES buffer. In detail, NiSO 4  was treated at a concentration of 1 mM in a buffer solution of the purified enzyme and then mixed with a 100 mM aqueous fructose solution at a weight ratio of 1:1 to prepare the test solutions of various pHs having the buffer concentration of 50 mM, the enzyme concentration of 0.025 ml/mg, and the fructose concentration of 50 mM. Thereafter, the test solutions reacted for 10 mins at 60° C. and a hydrochloric acid solution was added thereto to stop the reaction. Thereafter, the amount (mM) of produced psicose was analyzed and measured by the HPLC and divided into an enzyme amount and a reaction time to calculate an enzyme activity.  FIG. 5  is a graph illustrating an activity of the D-psicose 3-epimerase of the present invention for each reaction pH. In  FIG. 5 , the enzyme activity is relatively represented by setting a maximum activity to 100. As illustrated in  FIG. 5 , the D-psicose 3-epimerase of the present invention had a high activity at pH of 6.5 to 8 and preferably pH of 6.5 to 7.5 and had a maximal activity at pH 7. 
         [0039]    Further, NiSO 4  was treated at a concentration of 1 mM in a buffer solution (PIPES, pH 7.5) of the purified enzyme and then mixed with a 100 mM aqueous fructose solution at a weight ratio of 1:1 to prepare a test solution having pH 7.0, the enzyme concentration of 0.025 ml/mg, and the fructose concentration of 50 mM. Thereafter, the test solution reacted for 10 mins at various temperatures and then a hydrochloric acid solution was added thereto to stop the reaction. Thereafter, the amount (mM) of produced psicose was analyzed and measured by the HPLC and divided into an enzyme amount and a reaction time to calculate an enzyme activity.  FIG. 6  is a graph illustrating an activity of the D-psicose 3-epimerase of the present invention for each reaction temperature. In  FIG. 6 , the enzyme activity is relatively represented by setting a maximum activity to 100. As illustrated in  FIG. 6 , the D-psicose 3-epimerase of the present invention had a high activity at a temperature of 55 to 67° C. and had a maximal activity at 65° C. 
       (3) Analysis of Substrate Specificity of D-Psicose 3-Epimerase 
       [0040]    Reaction activities for various substrates such as psicose, fructose, and tagatose of the D-psicose 3-epimerase obtained in Example 2 were analyzed. 
         [0041]    NiSO 4  was treated at a concentration of 1 mM in a buffer solution (PIPES, pH 7.5) of the purified enzyme and then mixed with a 100 mM aqueous substrate solution at a weight ratio of 1:1 to prepare a test solution having pH 7.0, the enzyme concentration of 0.025 ml/mg, and the substrate concentration of 50 mM. Thereafter, the test solution reacted for 10 mins at 65° C. and then a hydrochloric acid solution was added thereto to stop the reaction. Thereafter, an amount (mM) of epimer corresponding to the substrate was analyzed and measured by the HPLC and divided into an enzyme amount and a reaction time to calculate an enzyme activity.  FIG. 7  is a graph illustrating an activity of the D-psicose 3-epimerase of the present invention for each substrate. In  FIG. 7 , the enzyme activity is relatively represented by setting an enzyme activity of a reaction of using psicose as a substrate to 100. As illustrated in  FIG. 7 , it was determined that the D-psicose 3-epimerase of the present invention had a high activity for psicose and fructose, and particularly, the activity of converting the fructose into the D-psicose was very high as compared with the existing D-psicose 3-epimerase. Therefore, the D-psicose can be produced from the fructose in a high yield by using the D-psicose 3-epimerase of the present invention. 
       (4) Analysis of Thermal Stability of D-Psicose 3-Epimerase 
       [0042]    The D-psicose 3-epimerase obtained in Example 2 was prepared in a 50 mM PIPES buffer solution to be a concentration of 0.05 mg/ml and then, the obtained solutions were put and stored in water bathes set at temperatures of 55° C., 60° C., 65° C., and 70° C., respectively, to heat. A buffer solution of the purified enzyme was ejected for each stored time and treated with NiSO 4  at a concentration of 1 mM, and then mixed with a 100 mM aqueous substrate solution at a weight ratio of 1:1 to prepare a test solution having pH 7.0, the enzyme concentration of 0.025 ml/mg, and the substrate concentration of 50 mM. Thereafter, the test solution reacted for 10 mins at 65° C. and then a hydrochloric acid solution was added thereto to stop the reaction. Thereafter, the amount (mM) of produced psicose was analyzed and measured by the HPLC and divided into an enzyme amount and a reaction time to calculate an enzyme activity. As a result, even though the buffer solution of the purified enzyme was heated for 250 mins at 55° C., the enzyme activity was maintained as it is. Further, when the buffer solution of the purified enzyme was heated for about 200 mins at 60° C., the enzyme activity was reduced by about 80% as compared with before heat treatment. Generally, when it is considered that most of D-psicose 3-epimerase has a half-life (a heat treatment time when the enzyme activity is 50% as compared with before heat treatment) of approximately one hour at 50° C., the D-psicose 3-epimerase of the present invention has very excellent thermal stability. 
       (5) Analysis of Activity of D-Psicose 3-Epimerase in High Concentration of Fructose 
       [0043]    700 μl of fructose at a concentration of 70 wt % was preheated at 60° C. and added with 280 μl of a PIPES buffer solution at 0.1 mg/ml of a concentration of the D-psicose 3-epimerase obtained in Example 2 and then, the obtained reactant was reacted at 60° C. The reaction product was taken by 10 μl for each reaction time and diluted 25 times. A hydrochloric acid solution was added to the diluted product to stop the reaction. Thereafter, the amount (mM) of produced psicose was analyzed and measured by the HPLC and divided into an amount of the fructose used as the substrate to calculate a conversion rate. A conversion rate of fructose into psicose for each reaction time is listed in the following Table 3. As listed in Table 3, when the D-psicose 3-epimerase of the present invention reacted with the high-concentrated fructose, the conversion rate reached a maximum conversion rate exceeding 33% only for about 18 hrs. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                   
                 Conversion rate (%) of 
               
               
                   
                 Reaction elapse time (hr) 
                 fructose into psicose 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 1 
                 9.04 
               
               
                   
                 2 
                 15.20 
               
               
                   
                 3 
                 19.52 
               
               
                   
                 4 
                 22.36 
               
               
                   
                 5 
                 24.66 
               
               
                   
                 6 
                 26.75 
               
               
                   
                 7 
                 27.94 
               
               
                   
                 18 
                 33.62 
               
               
                   
                 28 
                 33.67 
               
               
                   
                   
               
             
          
         
       
     
         [0044]    As described above, the present invention has been described through the Examples, but is not always limited thereto, and various modified embodiments can be made within the range not departing from the scope and spirit of the present invention. Therefore, it should be interpreted that the scope of the present invention includes all embodiments belonging to claims accompanied in the present invention.