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Two β-galactosidases, β-gal I and β-gal II, from  Bifidobacterium breve  DSM 20213, which was isolated from the intestine of an infant, were overexpressed in  Escherichia coli  with co-expression of the chaperones GroEL/GroES, purified to electrophoretic homogeneity and biochemically characterized. Both β-gal I and β-gal II belong to glycoside hydrolase family 2 and are homodimers with native molecular masses of 220 and 211 kDa, respectively. The optimum pH and temperature for hydrolysis of the two substrates  o -nitrophenyl-β-D-galactopyranoside ( o NPG) and lactose were determined at pH 7.0 and 50°C for β-gal I, and at pH 6.5 and 55°C for β-gal II, respectively. The  k   cat / K   m  values for  o NPG and lactose hydrolysis are 722 and 7.4 mM −1 s −1  for β-gal I, and 543 and 25 mM −1 s −1  for β-gal II. Both β-gal I and β-gal II are only moderately inhibited by their reaction products D-galactose and D-glucose. Both enzymes were found to be very well suited for the production of galacto-oligosaccharides with total GOS yields of 33% and 44% of total sugars obtained with β-gal I and β-gal II, respectively. The predominant transgalactosylation products are β - D-Gal p -(1→6)-D-Glc (allolactose) and β - D-Gal p- (1→3)-D-Lac, accounting together for more than 75% and 65% of the GOS formed by transgalactosylation by β-gal I and β-gal II, respectively, indicating that both enzymes have a propensity to synthesize β - (1→6) and β - (1→3)-linked GOS. The resulting GOS mixtures contained relatively high fractions of allolactose, which results from the fact that glucose is a far better acceptor for galactosyl transfer than galactose and lactose, and intramolecular transgalactosylation contributes significantly to the formation of this disaccharide.

plos one

Two β-Galactosidases from the Human Isolate Bifidobacterium breve DSM 20213: Molecular Cloning and Expression, Biochemical Characterization and Synthesis of Galacto-Oligosaccharides