Abstract
A novel thermostable β-galactosidase gene, designated as GkGal1A, from the thermophilic bacterium Geobacillus kaustophilus HTA426 was cloned and heterologously overexpressed in Escherichia coli(E. coli). Based on the sequence analysis, GkGal1A belongs to the glycosyl hydrolase family 1 that was the first β-galactosidase of bacterial origins expressed by us in this family. The apparent molecular weight of GkGal1A determined by sodium deodecyl sulfate-polyacrylamide gel electrophoresis is 52000. It exhibited the highest activity toward p-nitrophenyl-β-D-galactopyranoside at pH 7.8 and 70 °C and displayed high thermal stability. Divalent cations are prerequisite for the activity of GKGal1A, with the highest activity in the presence of Mn2+. Moreover, the three-dimensional structure of GkGal1A was modeled to speculate the structure of the catalytic residues and the reaction mechanism. The catalytic residues consisting of Glu166 and Glu355 were verified by site-directed mutagenesis.
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