Metagenomic identification, purification and characterisation of the Bifidobacterium adolescentis BgaC β-galactosidase

Members of the human gut microbiota use glycoside hydrolase (GH) enzymes, such as beta-galactosidases, to forage on host mucin glycans and dietary fibres. A human faecal metagenomic fosmid library was constructed and functionally screened to identify novel beta-galactosidases. Out of the 16,000 clones screened, 30 beta-galactosidase-positive clones were identified. The beta-galactosidase gene found in the majority of the clones was BAD_1582 from Bifidobacterium adolescentis, subsequently named bgaC. This gene was cloned with a hexahistidine tag, expressed in Escherichia coli and His-tagged-BgaC was purified using Ni2+-NTA affinity chromatography and size filtration. The enzyme had optimal activity at pH 7.0 and 37 degrees C, with a wide range of pH (4-10) and temperature (0-40 degrees C) stability. It required a divalent metal ion co-factor; maximum activity was detected with Mg2+, while Cu2+ and Mn2+ were inhibitory. Kinetic parameters were determined using ortho-nitrophenyl-beta-d-galactopyranoside (ONPG) and lactose substrates. BgaC had a V-max of 107 mu mol/min/mg and a K-m of 2.5 mM for ONPG and a V-max of 22 mu mol/min/mg and a K-m of 3.7 mM for lactose. It exhibited low product inhibition by galactose with a K-i of 116 mM and high tolerance for glucose (66% activity retained in presence of 700 mM glucose). In addition, BgaC possessed transglycosylation activity to produce galactooligosaccharides (GOS) from lactose, as determined by TLC and HPLC analysis. The enzymatic characteristics of B. adolescentis BgaC make it an ideal candidate for dairy industry applications and prebiotic manufacture.

Automated summary

The study identified a novel beta-galactosidase gene, bgaC, from Bifidobacterium adolescentis, which showed optimal activity at pH 7.0 and 37 degrees C, with a wide range of pH and temperature stability. BgaC required a divalent metal ion co-factor and exhibited low product inhibition by galactose and high tolerance for glucose. The enzyme also had transglycosylation activity to produce galactooligosaccharides (GOS) from lactose, making it an ideal candidate for dairy industry applications and prebiotic manufacture.