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Open Access Research article

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

Xia Zhang1, He Li2, Chang-Jie Li1, Teng Ma1, Gang Li1* and Yu-Huan Liu1*

Author Affiliations

1 School of life sciences, Sun Yat-sen University, Guangzhou 510275, People’s Republic of China

2 Guangdong Provincial Key Laboratory of Pharmaceutical Bioactive Substances, School of Basic Courses, Guangdong Pharmaceutical University, Guangzhou 510006, People’s Republic of China

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BMC Microbiology 2013, 13:237  doi:10.1186/1471-2180-13-237

Published: 24 October 2013

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 Ki,gal (238 mM) and Ki,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.

Keywords:
β-Galactosidases; Metagenomic library; Enzyme characterization; Thermostability; Tolerance of galactose and glucose