Characterization of a thermostable recombinant beta-galactosidase from Thermotoga maritima

Aims: Characterization of a thermostable recombinant beta-galactosidase from Thermotoga maritima for the hydrolysis of lactose and the production of galacto-oligosaccharides. Methods and Results: A putative beta-galactosidase gene of Thermotoga maritima was expressed in Escherichia coli as a carboxyl terminal His-tagged recombinant enzyme. The gene encoded a 1100-amino acid protein with a calculated molecular weight of 129 501. The expressed enzyme was purified by heat treatment, His-tag affinity chromatography, and gel filtration. The optimum temperatures for beta-galactosidase activity were 85 and 80degreesC with oNPG and lactose, respectively. The optimum pH value was 6.5 for both oNPG and lactose. In thermostability experiments, the enzyme followed first-order kinetics of thermal inactivation and its half-life times at 80 and 90degreesC were 16 h and 16 min, respectively. Mn2+ was the most effective divalent cation for beta-galactosidase activity on both oNPG and lactose. The K-m and V-max values of the thermostable enzyme for oNPG at 80degreesC were 0.33 mm and 79.6 mumol oNP min(-1) mg(-1). For lactose, the K-m and V-max values were dependent on substrate concentrations; 1.6 and 63.3 at lower concentrations up to 10 mm of lactose and 27.8 mm and 139 mumol glucose min(-1) mg(-1) at higher concentrations, respectively. The enzyme displayed non-Michaelis-Menten reaction kinetics with substrate activation, which was explained by simultaneous reactions of hydrolysis and transgalactosylation. Conclusions: The results suggest that the thermostable enzyme may be suitable for both the hydrolysis of lactose and the production of galacto-oligosaccharides. Significance and Impact of the Study: The findings of this work contribute to the knowledge of hydrolysis and transgalactosylation performed by beta-galactosidase of hyperthermophilic bacteria.