Structure, Function and Applications of Microbial β-galactosidase (Lactase)

Abstract

A β-galactosidase (EC 3.2.1.23, β-D-galactoside galactohydrolase), commonly known as lactase is known to catalyze not only hydrolyze β-D-galactoside linkage of lactose or other β-galactosides into monosaccharides, glucose and galactose but also has transgalactosylation activity to synthesize galacto-oligosaccharides. Both reaction activities are well characterized and applied in many food industries. Although β-galactosidases are widely distributed in nature, the most thoroughly studied β-galactosidases are obtained from E. coli, and commercially used β-galactosidases are from mainly fungi and yeasts. Galacto-oligosaccharides, so-called prebiotics, have been shown to employ this growth-stimulating effect on probiotic bacteria, including bifidobacteria. Biochemical and molecular aspects of the β-galactosidase genes from different microorganisms have been studied, but the known structure and function of different β-galactosidases are limited. β-Galactosidases which belongs to the 4/7 superfamily of the glycoside hydrolase families (GHs) are currently divided into GH-1, GH-2, GH-35 and GH-42, and yet the four families are so distantly related to each other, and the hydrolytic and transgalactosylation activities of the isoenzymes appears to be different. The known 3D structures and functions of β-galactosidases from prokaryotic E. coli (mesophilic), three extremophiles like Thermus thermophilus (thermophilic), Sulfolobus solfataricus (thermophilic), Arthrobacter (psychrotrophic), and eukaryote Penicillium were compared. The sequence, homology and multiple isozymes of lactic bacterial β-galactosidases were supplemented for their phylogenetic analysis. The applications of this enzyme on the oligosaccharides and prebiotics were discussed in details.