Characterisation of a thermostable family 42 β-galactosidase (BgalC) family from Thermotoga maritima showing efficient lactose hydrolysis

A β-galactosidase gene (TM_1195) of Thermotoga maritima was cloned and expressed in Escherichia coli. The recombinant β-galactosidase (BgalC), belonging to glycosyl hydrolase (GH) family 42, was purified to homogeneity with 23.4-fold purification and a recovery of 36.6%. Its molecular mass was estimated to be 78 kDa by SDS–PAGE. BgalC exhibited maximum activity at an optimal pH of 5.5 and an optimum temperature of 80 °C. The enzyme displayed important properties, such as stability over a broad pH range of 5.0–9.0 and thermostability up to 75 °C. Km values of BgalC for p-nitrophenyl-β-galactopyranoside (pNPGal), o-nitrophenyl-β-galactopyranoside (oNPGal) and lactose were 1.21, 7.31 and 6.5 mM, respectively. BgalC was efficient in complete removal of lactose from milk. BgalC is significantly one of the few β-galactosidases from family 42 displaying significant hydrolysis of lactose. These properties make BgalC an ideal candidate for commercial use, in the production of lactose-free milk.