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Metagenomic approach for the isolation of a thermostable β-galactosidase with high tolerance of galactose and glucose from soil samples of Turpan Basin.

Zhang X, Li H, Li CJ, Ma T, Li G, Liu YH - BMC Microbiol. (2013)

Bottom Line: Furthermore, Gal308 displayed a very high tolerance of galactose and glucose, with the highest inhibition constant K(i,gal) (238 mM) and K(i,glu) (1725 mM) among β-galactosidases.This study will enrich the source of β-galactosidases, and attract some attentions to β-galactosidases from extreme habitats and metagenomic library.These properties make it a good candidate in the production of low-lactose milk and dairy products after further study.

View Article: PubMed Central - HTML - PubMed

Affiliation: School of life sciences, Sun Yat-sen University, Guangzhou 510275, People's Republic of China. lsslig@mail.sysu.edu.cn.

ABSTRACT

Background: β-Galactosidases can be used to produce low-lactose milk and dairy products for lactose intolerant people. Although commercial β-galactosidases have outstanding lactose hydrolysis ability, their thermostability is low, and reaction products have strong inhibition to these enzymes. In addition, the β-galactosidases possessing simultaneously high thermostability and tolerance of galactose and glucose are still seldom reported until now. Therefore, identification of novel β-galactosidases with high thermostability and tolerance to reaction products from unculturable microorganisms accounting for over 99% of microorganisms in the environment via metagenomic strategy is still urgently in demand.

Results: In the present study, a novel β-galactosidase (Gal308) consisting of 658 amino acids was identified from a metagenomic library from soil samples of Turpan Basin in China by functional screening. After being overexpressed in Escherichia coli and purified to homogeneity, the enzymatic properties of Gal308 with N-terminal fusion tag were investigated. The recombinant enzyme displayed a pH optimum of 6.8 and a temperature optimum of 78 °C, and was considerably stable in the temperature range of 40 °C - 70 °C with almost unchangeable activity after incubation for 60 min. Furthermore, Gal308 displayed a very high tolerance of galactose and glucose, with the highest inhibition constant K(i,gal) (238 mM) and K(i,glu) (1725 mM) among β-galactosidases. In addition, Gal308 also exhibited high enzymatic activity for its synthetic substrate o-nitrophenyl-β-D-galactopyranoside (ONPG, 185 U/mg) and natural substrate lactose (47.6 U/mg).

Conclusion: This study will enrich the source of β-galactosidases, and attract some attentions to β-galactosidases from extreme habitats and metagenomic library. Furthermore, the recombinant Gal308 fused with 156 amino acids exhibits many novel properties including high activity and thermostability at high temperatures, the pH optimum of 6.8, high enzyme activity for lactose, as well as high tolerance of galactose and glucose. These properties make it a good candidate in the production of low-lactose milk and dairy products after further study.

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Identification of the active site residues of Gal308 by alignment of the amino acid residues with other five homologous β-galactosidases from GH family 42. The GenBank accession numbers are as follows: Geobacillus thermocatenulatus, AAW56416; Truepera radiovictrix DSM17093, ADI14846; Thermus thermophilus, ABI35985; Alicyclobacillus acidocaldarius, AAZ81841; Bacillus circulans, AAA22260; This study (Gal308), AFD21844. The alignment was carried out using the Clustal W method. The number flanking the sequences represents amino acid positions of each sequence. Asterisks mean identity. The two putative catalytic residues (E195 and E368) of Gal308 were shown in box.
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Figure 1: Identification of the active site residues of Gal308 by alignment of the amino acid residues with other five homologous β-galactosidases from GH family 42. The GenBank accession numbers are as follows: Geobacillus thermocatenulatus, AAW56416; Truepera radiovictrix DSM17093, ADI14846; Thermus thermophilus, ABI35985; Alicyclobacillus acidocaldarius, AAZ81841; Bacillus circulans, AAA22260; This study (Gal308), AFD21844. The alignment was carried out using the Clustal W method. The number flanking the sequences represents amino acid positions of each sequence. Asterisks mean identity. The two putative catalytic residues (E195 and E368) of Gal308 were shown in box.

Mentions: To discover novel thermostable β-galactosidases, a metagenomic library consisting of approximately 8,000 clones was constructed using DNA extracted from soil samples of the Mountain of Flames of the Turpan Basin in China. Restriction analysis of 20 randomly selected clones from metagenomic library indicated that 95% of clones contained inserts of 2.5 to 7.5 kb in size, with an average size of 4.2 kb. Thus, the metagenomic library covered theoretically about 33.6 MB of soil microbial community DNA. One positive clone with bright blue color was finally identified from the metagenomic library. The activity of the positive clone was reconfirmed after retransformation, and then the plasmid of this clone was extracted and an insert of 5215 bp was sequenced. The ORF-finder and blastX analysis revealed the presence of an open reading frame of 1977 bp, which encoded a glycoside hydrolase family 42 (GH family 42) protein (Gal308) of 658 amino acids. A protein blast (blastp) search in the databases of NCBI indicated that the protein had the highest identity of 49% (291/599) with the β-galactosidase from one thermophilic microbe Geobacillus thermocatenulatus, as well as a low identity of only 38% (224/593) with the β-galactosidase from the other thermophilic microbe Thermoanaerobacterium thermosaccharolyticum DSM 571, suggesting that Gal308 is probably a novel thermostable β-galactosidase from unculturable microorganisms. In addition, multiple sequence alignment of Gal308 and other five homologous β-galactosidases from GH family 42 allowed the identification of the active site residues of Gal308 (Figure 1). Glu141 (E141) and Glu312 (E312) were regarded as the catalytic residues of a typical GH-42 β-galactosidase [19]. Thus, E195 and E368 (marked with two boxes), which located in two conserved regions, were thought to be the active site residues of Gal308 based on amino acid sequence alignment and the determined structure of β-galactosidase from T. Thermophilus (Figure 1).


Metagenomic approach for the isolation of a thermostable β-galactosidase with high tolerance of galactose and glucose from soil samples of Turpan Basin.

Zhang X, Li H, Li CJ, Ma T, Li G, Liu YH - BMC Microbiol. (2013)

Identification of the active site residues of Gal308 by alignment of the amino acid residues with other five homologous β-galactosidases from GH family 42. The GenBank accession numbers are as follows: Geobacillus thermocatenulatus, AAW56416; Truepera radiovictrix DSM17093, ADI14846; Thermus thermophilus, ABI35985; Alicyclobacillus acidocaldarius, AAZ81841; Bacillus circulans, AAA22260; This study (Gal308), AFD21844. The alignment was carried out using the Clustal W method. The number flanking the sequences represents amino acid positions of each sequence. Asterisks mean identity. The two putative catalytic residues (E195 and E368) of Gal308 were shown in box.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4016535&req=5

Figure 1: Identification of the active site residues of Gal308 by alignment of the amino acid residues with other five homologous β-galactosidases from GH family 42. The GenBank accession numbers are as follows: Geobacillus thermocatenulatus, AAW56416; Truepera radiovictrix DSM17093, ADI14846; Thermus thermophilus, ABI35985; Alicyclobacillus acidocaldarius, AAZ81841; Bacillus circulans, AAA22260; This study (Gal308), AFD21844. The alignment was carried out using the Clustal W method. The number flanking the sequences represents amino acid positions of each sequence. Asterisks mean identity. The two putative catalytic residues (E195 and E368) of Gal308 were shown in box.
Mentions: To discover novel thermostable β-galactosidases, a metagenomic library consisting of approximately 8,000 clones was constructed using DNA extracted from soil samples of the Mountain of Flames of the Turpan Basin in China. Restriction analysis of 20 randomly selected clones from metagenomic library indicated that 95% of clones contained inserts of 2.5 to 7.5 kb in size, with an average size of 4.2 kb. Thus, the metagenomic library covered theoretically about 33.6 MB of soil microbial community DNA. One positive clone with bright blue color was finally identified from the metagenomic library. The activity of the positive clone was reconfirmed after retransformation, and then the plasmid of this clone was extracted and an insert of 5215 bp was sequenced. The ORF-finder and blastX analysis revealed the presence of an open reading frame of 1977 bp, which encoded a glycoside hydrolase family 42 (GH family 42) protein (Gal308) of 658 amino acids. A protein blast (blastp) search in the databases of NCBI indicated that the protein had the highest identity of 49% (291/599) with the β-galactosidase from one thermophilic microbe Geobacillus thermocatenulatus, as well as a low identity of only 38% (224/593) with the β-galactosidase from the other thermophilic microbe Thermoanaerobacterium thermosaccharolyticum DSM 571, suggesting that Gal308 is probably a novel thermostable β-galactosidase from unculturable microorganisms. In addition, multiple sequence alignment of Gal308 and other five homologous β-galactosidases from GH family 42 allowed the identification of the active site residues of Gal308 (Figure 1). Glu141 (E141) and Glu312 (E312) were regarded as the catalytic residues of a typical GH-42 β-galactosidase [19]. Thus, E195 and E368 (marked with two boxes), which located in two conserved regions, were thought to be the active site residues of Gal308 based on amino acid sequence alignment and the determined structure of β-galactosidase from T. Thermophilus (Figure 1).

Bottom Line: Furthermore, Gal308 displayed a very high tolerance of galactose and glucose, with the highest inhibition constant K(i,gal) (238 mM) and K(i,glu) (1725 mM) among β-galactosidases.This study will enrich the source of β-galactosidases, and attract some attentions to β-galactosidases from extreme habitats and metagenomic library.These properties make it a good candidate in the production of low-lactose milk and dairy products after further study.

View Article: PubMed Central - HTML - PubMed

Affiliation: School of life sciences, Sun Yat-sen University, Guangzhou 510275, People's Republic of China. lsslig@mail.sysu.edu.cn.

ABSTRACT

Background: β-Galactosidases can be used to produce low-lactose milk and dairy products for lactose intolerant people. Although commercial β-galactosidases have outstanding lactose hydrolysis ability, their thermostability is low, and reaction products have strong inhibition to these enzymes. In addition, the β-galactosidases possessing simultaneously high thermostability and tolerance of galactose and glucose are still seldom reported until now. Therefore, identification of novel β-galactosidases with high thermostability and tolerance to reaction products from unculturable microorganisms accounting for over 99% of microorganisms in the environment via metagenomic strategy is still urgently in demand.

Results: In the present study, a novel β-galactosidase (Gal308) consisting of 658 amino acids was identified from a metagenomic library from soil samples of Turpan Basin in China by functional screening. After being overexpressed in Escherichia coli and purified to homogeneity, the enzymatic properties of Gal308 with N-terminal fusion tag were investigated. The recombinant enzyme displayed a pH optimum of 6.8 and a temperature optimum of 78 °C, and was considerably stable in the temperature range of 40 °C - 70 °C with almost unchangeable activity after incubation for 60 min. Furthermore, Gal308 displayed a very high tolerance of galactose and glucose, with the highest inhibition constant K(i,gal) (238 mM) and K(i,glu) (1725 mM) among β-galactosidases. In addition, Gal308 also exhibited high enzymatic activity for its synthetic substrate o-nitrophenyl-β-D-galactopyranoside (ONPG, 185 U/mg) and natural substrate lactose (47.6 U/mg).

Conclusion: This study will enrich the source of β-galactosidases, and attract some attentions to β-galactosidases from extreme habitats and metagenomic library. Furthermore, the recombinant Gal308 fused with 156 amino acids exhibits many novel properties including high activity and thermostability at high temperatures, the pH optimum of 6.8, high enzyme activity for lactose, as well as high tolerance of galactose and glucose. These properties make it a good candidate in the production of low-lactose milk and dairy products after further study.

Show MeSH
Related in: MedlinePlus