A novel β-galactosidase from Klebsiella oxytoca ZJUH1705 for efficient production of galacto-oligosaccharides from lactose.

Galacto-oligosaccharides (GOS), which can be produced by enzymatic transgalactosylation of lactose with β-galactosidases, have attracted much attention in recent years because of their prebiotic functions and wide uses in infant formula, infant foods, livestock feed, and pet food industries. In this study, a novel β-galactosidase-producing Klebsiella oxytoca ZJUH1705, identified by its 16S rRNA sequence (GenBank accession no. MH981243), was isolated. Two β-galactosidase genes, bga 1 encoding a 2058-bp fragment (GenBank accession no. MH986613) and bga 2 encoding a 3108-bp fragment (GenBank accession no. MN182756), were cloned from K. oxytoca ZJUH1705 and expressed in E. coli. The purified β-gal 1 and β-gal 2 had the specific activity of 217.56 U mg-1 and 57.9 U mg-1, respectively, at the optimal pH of 7.0. The reaction kinetic parameters Km, Vmax, and Kcat with oNPG as the substrate at 40 °C were 5.62 mM, 167.1 μmol mg-1 min-1, and 218.1 s-1, respectively, for β-gal 1 and 3.91 mM, 14.6 μmol mg-1 min-1, and 28.9 s-1, respectively, for β-gal 2. Although β-gal 1 had a higher enzyme activity for lactose hydrolysis, only β-gal 2 had a high transgalactosylation capacity. Using β-gal 2 with the addition ratio of ~ 2.5 U g-1 lactose, a high GOS yield of 45.5 ± 2.3% (w/w-1) was obtained from lactose (40% w/w-1 or 480 g L-1) in a phosphate buffer (100 mM, pH 7.0) at 40 °C in 48 h. Thus, the β-gal 2 from K. oxytoca ZJUH1705 would be a promising biocatalyst for GOS production from lactose.Key Points• A novel bacterial β-galactosidase producer was isolated and identified.• β-Galactosidases were cloned and expressed in heterologous strain and characterized.• Both enzymes have hydrolytic activity but only one have transglycosilation activity.• The developed process with β-gal 2 could provide an alternative for GOS production.