PHYTASE VARIANTS YKAPPA HAVING IMPROED PEPSIN RESISTANCE AND INCREASED CATALYTIC EFFICIENCY

Abstract

The present invention relates to the field of genetic engineering, particularly to phytase variants YkAPPA having amino acid sequence substituting Leucine at the 162.sup.th site of the sequence set forth in SEQ ID NO.1 with glycine or proline, or having amino acid sequence substituting glutamic acid at the 230.sup.th site of the sequence set forth in SEQ ID NO.1 with glycine, alanine, serine, threonine, aspartic acid, proline, or arginine, and having improved pepsin resistance and increased catalytic efficiency of 2.1 times of that of the wild phytase, in the benefit of the development of economical feed enzyme industry.

Claims

1. Phytase YkAPPA variant with the following characteristics of having amino acid sequence substituting Leucine at the 162.sup.th site of the sequence set forth in SEQ ID NO.1 with glycine or proline, and having improved pepsin resistance and increased catalytic efficiency.

2. Phytase YkAPPA variant with the following characteristics of having amino acid sequence substituting glutamic acid at the 230.sup.th site of the sequence set forth in SEQ ID NO.1 with glycine, alanine, serine, threonine, aspartic acid, proline, or arginine, and having improved pepsin resistance and increased catalytic efficiency.

3. A polynucleotide encoding the phytase YeAPPA variant of claim 1.

4. Polynucleotide according to claim 3, is characterized of having the nucleotide sequence set in forth in SEQ ID NO.12, SEQ ID NO.13, SEQ ID NO.14, SEQ ID NO.15, SEQ ID NO.16, SEQ ID NO.17, SEQ ID NO.18, SEQ ID NO.19 or SEQ ID NO.20, respectively.

5. DNA constructor comprising the polynucleotide of claim 3.

6. Recombinant cell comprising the polynucleotide of claim 3.

7. A method of producing a phytase variant comprising the steps of transforming host with DNA constructor of claim 5 to obtain recombinant host cell; cultivating the recombinant host cell to produce the supernatant containing phytase variant; and recovering the said phytase variant.

8. (canceled)

Description

BRIEF DESCRIPTIONS OF THE DRAWINGS

[0032] FIG. 1 shows the comparison of effect of pepsin on activity of the modified phytase and the wild phytase.

[0033] FIG. 2 shows the comparison of the hydrolysis ability of the modified phytase and the wild phytase.

EMBODIMENT

[0034] The present invention is further illustrated with reference to the following Examples and the appended drawings, which should by no means be construed as limitations of the present invention.

Test Materials and Reagents

[0035] 1. Strains and vectors: Expression vetor pET-22b (+) and host strain BL21 (DE3) (Novagen)

[0036] 2. Enzymes and other biochemical reagents: restriction endonucleases (TaKaRa), ligase (Invitrogen), and pepsin p0685 (Sigma).

[0037] 3. Medium:

[0038] E. coli. LB medium: 1% of peptone, 0.5% of yeast extract, and 1% of NaCl, natural pH.

[0039] Suitable biology laboratory methods not particularly mentioned in the examples as below can be found in Sambrook, et al. (Molecular Cloning: A Laboratory Manual. 2nd, ed., Cold Spring Harbor Laboratory, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1989), and other kit laboratory manuals.

Example 1 Introduction of the Mutant Site to Wild Phytase

[0040] Gene encoding phytase YkAPPA having the nucleotide sequence as set in SEQ ID NO. 2 from Y. kristensenii was performed with site-directed mutagenesis by Overlap PCR to obtain genes encodng the phytase variants YeAPPA-L162G, YeAPPA-L162A, YeAPPA-E230G, YkAPPA-E230A, YkAPPA-E230S, YkAPPA-E230T, YkAPPA-E230D, YkAPPA-E230P, and YkAPPA-E230R, respectively. Overlap PCR was performed as being kept at 95 C. for 5 min, followed by 30 cycles of 94 C. for 30 sec, 55 C. for 30 sec, and 72 C. for 30-90 sec, and keep 72 C. for 10 min, using 20 mutation primers including the upper primer Ye-F and the reverse primer Ye-R for amplifying the full length of mutant gene, and the primers comprising the EcoRI and NotI sites marked in Italics or the mutant nucleotides marked in underlined for site-directed mutagenesis showed as below.

TABLE-US-00002 Yk-F:5-cgcgaattcgcaccgcttgcagcacaatctac-3 Yk-R:5-gatgcggccgcttaaatatggcaggctggctcg-3 L162G-F:5-cgggggtatgtaaaggcgacccagagaaaac-3 L162G-R:5-gttttctctgggtcgcctttacatacccccg-3 L162A-F:5-cgggggtatgtaaagcggacccagagaaaac-3 L162A-R:5-gttactctgggtccgcttacatacccccg-3 E230G-F:5-tcgaggtaaataaaggcgggacaaaagtctc-3 E230G-R:5-gagacttttgtcccgcctttatttacctcga-3 E230A-F:5-tcgaggtaaataaagcggggacaaaagtctc-3 E230A-R:5-gagacttttgtccccgctttatttacctcga-3 E230S-F:5-tcgaggtaaataaatctgggacaaaagtctc-3 E230S-R:5-gagacttttgtcccagattatttacctcga-3 E230T-F:5-tcgaggtaaataaaaccgggacaaaagtctc-3 E230T-R:5-gagacttttgtcccggttttatttacctcga-3 E230D-F:5-tcgaggtaaataaagatggggacaaaagtctc-3 E230D-R:5-gagacttttgtcccatctttatttacctcga-3 E230P-F:5-tcgaggtaaataaaccggggacaaaagtctc-3 E230P-R:5-gagacttttgtccccggtttatttacctcga-3 E230R-F:5-tcgaggtaaataaacgtgggacaaaagtctc-3 E230R-R:5-gagacttttgtcccacgtttatttacctcga-3

[0041] The modified gene was recovered, connected with the vector pEASY-T3, and sequenced.

Example 2 Expressing the Phytase in E coli

[0042] The modified genes encoding the phytase variants were inserted into expression vector pET-22b (+), and transformed into E coli. Strain BL21 (DE3), which were induced by IPTG in 1 mM, cultivated for 5 h at 24 C. to express the phytase, followed by being purified by columns Ni-NTA and DEAE to obtain the mutant protein with the same molecular weight as that of the wild.

Example 3 Measuring the Activity of the Phytase Variants

Measuring Effect of Pepsin on the Enzyme Activity of the Phytase Variants

[0043] 1 unit of phytase activity is determined to be the enzyme amount releasing 1 mol of phosphate for 1 minute. The absolute value of the measured phytase activity may be calculated based on the standard curve of inorganic phosphate in dilution.

[0044] The effect of pepsin on the activity of the purified mutant phytase was determined by detecting the remaining activity after being treated in pH 2 for 2 hours with the different concentrations of pepsin in a mass ratio to phytase ranging from 1/1000 to 1/1. The activity of phytase was detected by ferric molybdenum sulfate blue method by adding 50 ul of phytase solution to 950 ul of sodium phytate substrate in 1.5 mmol/L to react for 30 min at 37 C., followed by adding 1 mL of 10% (m/v) TCA to stop the reaction, and 2 mL of developing color reagent. After developing, OD is measured at 700 nm to calculate the phytase activity. As showed in A and B of FIG. 1, in the case of the ratio pepsin to phytase ranging from 1/1000 to 1/20, the phytase variants remain far more enzyme activity after being treated for 2 h in different concentration of pepsin, than that of the wild phytase, wherein the retained activity of the phytase variants YkAPPA-E230G, YkAPPA-E230A, YkAPPA-L162G, YkAPPA-L162A, YkAPPA-E230S, YkAPPA-E230D and YkAPPA-E230T were 83%, 76%, 50%, 42%, 34%, 12% and more than 12% in order, and the retained activity of the phytase variants YkAPPA-E230P and YkAPPA-E230R with the strong rigid side chains were more than 64% and more than 49% in order, but the wild phytase almost lost activity, demonstrating that pepsin resistance of phytase variants were improved.

Measuring the Optimal pH and Optimal Temperature

[0045] The purified phytase variants were performed the enzymatic reactions in the substrate solutions with the different pHs using 0.1 mol/L of Glycine-HCl buffer (pH1.03.0), 0.1 mol/L of acetic acid-sodium acetate buffer (pH36), 0.1 mol/L of Tris-Hcl buffer (pH68) and 0.1 mol/L of glycine-sodium hydroxide buffer (pH812.0) at 37 C. to deterimine the optimal pH. As showed in Table 1, the optimal pH values of the eights phytase variants were pH 4.5 similar f to that of the wild enzyme, other than the optimal pH of the phytase variant YkAPPA-E230R decreased 0.5 pH units. And, the phytase variants YkAPPA-E230G, YkAPPA-E230A, YkAPPA-E230R, YkAPPA-L162G, and YkAPPA-L162A were more acid stable than the wild phytase, wherein the phytase variants YkAPPA-E230G, YkAPPA-E230A, and YkAPPA-E230R can retain more than 85% of enzyme activity, but the wild phytase only retained 64% of enzyme activity after being treated in pH 1.0 to 1.5 for 1 hour. And, phytase variants YkAPPA-E230P, YkAPPA-E230S, YkAPPA-E230T, and YkAPPA-E230D had the similar acid stability as the wild phytase.

TABLE-US-00003 TABLE 1 Comparison of the effect temperature and pH on the activity and stability of the modified phytase and the wild phytase pH stability of Thermo- the phytase stability of Optimal after being treated phytase kept Optimal temper- in different for 30 min Variants pH ature pHs for 1 h at 60 C. YkAPPA 4.5 55 C. pH 1-1.5, 64-77%; 16% pH 2-10, >91% YkAPPA- 4.5 55 C. pH 1-1.5, >92%; 35% E230G pH 2-10, >99% YkAPPA- 4.5 55 C. pH 1-1.5, >87%; 16% E230A pH 2-10, >99% YkAPPA- 4.5 60 C. pH 1-1.5, <78%; 42% E230P pH 2-10, >88% YkAPPA- 4.0 55 C. pH 1-1.5, >87%; 23% E230R pH 2-10, >95% YkAPPA- 4.5 55 C. pH 1-1.5, <79%; 34% E230S pH 2-10, >89% YkAPPA- 4.5 55 C. pH 1-1.5, <78%; 33% E230T pH 2-10, >91% YkAPPA- 4.5 55 C. pH 1-1.5, <77%; 17% E230D pH 2-10, >91% YkAPPA- 4.5 55 C. pH 1-1.5, >90%; 17% L162G pH 2-10 > 100% YkAPPA- 4.5 55 C. pH 1-1.5, >85%; 17% L162A pH 2-10 > 93%

Measuring Kinetic Parameter of the Phytase Variants

[0046] The activity of phytase was measured with sodium phytate as substrate in different concentrations of 0.0625 mmol/L, 0.1 mmol/L, 0.125 mmol/L, 0.2 mmol/L, 0.25 mmol/L, 0.5 mmol/L, 1.0 mmol/L and 1.5 mmol/L at the optimal temperature and pH, followed by calculating the values of k.sub.m and V.sub.max by double reciprocal method for Michaelis equation, and K.sub.cat according to the theoretical molecular weight. As showed in Table 2, the affinity (k.sub.m) for each of phytase variants to substrates was almost similar to that for the wild phytase. Reaction rate V.sub.max and conversion rate K.sub.cat of the phytase variant YkAPPA-E230G are greatly increased to 1.9 times of that of the wild phytase, and catalytic efficiency K.sub.cat/k.sub.m was 2.1 times of that of the wild phytase, and reaction rate V.sub.max and conversion rate K.sub.cat of the phytase variant YkAPPA-L162G was increased to 1.6 to 1.8 times of that of the wild phytase. Reaction rate V.sub.max, conversion rate K.sub.cat of the phytase variant YkAPPA-E230A was 1.3 times of those of the wild phytase. And, the catalytic properties including reaction rate, turnover rate and catalytic efficiency of the other phytase variants were similar to those of the wild phytase.

TABLE-US-00004 TABLE 2 Comparison of the enzymatic properties of the modified phytase and the wild phytase Km Vmax Kcat Kcat/Km Variants (mM) (U mg.sup.1) (S.sup.1) (S.sup.1 mM.sup.1) YkAPPA 0.09 3554 2719 29423 YkAPPA-E230G 0.10 7097 5429 61690 YkAPPA-E230A 0.09 4533 3468 37685 YkAPPA-E230P 0.08 3177 2430 29833 YkAPPA-E230R 0.11 4329 3312 29883 YkAPPA-E230S 0.09 3795 2903 29298 YkAPPA-E230T 0.08 3247 2484 29767 YkAPPA-E230D 0.09 3587 2744 29088 YkAPPA-L162G 0.09 6321 4836 46084 YkAPPA-L162A 0.10 3917 2996 29537

Example 3 Measuring Activity of the Phytase Variants

[0047] The gastrointestinal environment of animals was simulated with different pH ranging from 1.0 to 5.5 and in the different ratio of pepsin to phytase ranging from 1/100 to 1/1, so as to determine hydrolysis ability of the variant YkAPPA-E230G taking corn starch as a substrate. As showed in FIG. 2, for the variant YkAPPA-E230G, the amount of inorganic phosphorus released by hydrolyzing the corn starch was the most which was 2 times of that of the wild phytase without adding pepsin, and increased to 11 times and 24 times when adding pepsin in a ratio of 1/10 and 1/1 respectively, in case of pH 4.5.