Galacto-oligosaccharide production by a thermostable recombinant β-galactosidase from Thermotoga maritima

A beta-galactosidase from Thermotoga maritima produced galacto-oligosaccharides (GOS) from lactose by transgalactosylation when expressed in Escherichia coli. The enzyme activity for GOS production was maximal at pH 6.0 and 90 degrees C. In thermal stability experiments, the enzyme followed first-order kinetics of pH and thermal inactivation, and half-lives at pH 5.0, pH 8.0, 80 degrees C, and 95 degrees C were 27 h, 82 h, 41 h, and 14 min, respectively, suggesting that the enzyme was stable below 80 degrees C and in the pH range of 5.0-8.0. Mn2+ was the most effective divalent cation for GOS production. Cu2+ and EDTA inhibited more than 84% of enzyme activity. GOS production increased with increasing lactose concentrations and peaked at 500 g lactose/l. Among tested enzyme concentrations, the highest production of GOS was obtained at 1.5 units enzyme/ml. Under the optimal conditions of pH 6.0, 80 degrees C, 500 g lactose/l, and 1.5 units enzyme/ml, GOS production was 91 g/l for 300 min, with a GOS productivity of 18.2 g/l center dot h and a conversion yield of GOS to lactose of 18%.