Characterization of recombinant β- galactosidase and its use in enzymatic synthesis of lactulose from lactose and fructose

The gene encoding a β- galactosidase was cloned from Lactobacillus plantarum FMNP01 and expressed in Escherichia coli BL21(DE3). The characteristics of this purified recombinant enzyme, L.pFMNP01Gal, were determined, and its transgalactosylation reaction conditions for the production of lactulose were optimized. Using ONPG as substrate, the L.pFMNP01Gal showed specific activity of 980 U/g with a Km of 6.86 mM and a Kcat of 22.47/s. This enzyme was most stable at 40–50 °C, and exhibited optimum catalytic activity at 40 °C and pH 7.0. The activity of L.pFMNP01Gal was greatly inhibited by Cu²⁺, while other tested metal ions had little influence on it. For the optimization of transgalactosylation reaction, high lactulose production was achieved when 60% (W/V) sugars were used as substrates with a lactose/fructose mass ratio of 2:1, and 2 U/mL of L.pFMNP01Gal as catalyst. Under these optimum conditions, 18.38 ± 2.17 g/L of lactulose was synthesized in 6 h at 50 °C. This study provides an alternative method for enzymatic synthesis of lactulose.