An acid-tolerant and cold-active β-galactosidase potentially suitable to process milk and whey samples.

A novel β-galactosidase gene (gal M ) was cloned from an aquatic habitat metagenome. The analysis of its translated sequence (Gal M ) revealed its phylogenetic closeness towards Verrucomicrobia sp. The sequence comparison and homology structure analysis designated it a member of GH42 family. The three-dimensional homology model of Gal M depicted a typical (β/α)8 TIM-barrel containing the catalytic core. The gene (gal M ) was expressed in a heterologous host, Escherichia coli, and the purified protein (Gal M ) was subjected to biochemical characterization. It displayed β-galactosidase activity in a wide range of pH (2.0 to 9.0) and temperature (4 to 60 °C). The heat exposed protein showed considerable stability at 40 and 50 °C, with the half-life of about 100 h and 35 h, respectively. The presence of Na, Mg, K, Ca, and Mn metals was favorable to the catalytic efficiency of Gal M , which is a desirable catalytic feature, as these metals exist in milk. It showed remarkable tolerance of glucose and galactose in the reaction. Furthermore, Gal M discerned transglycosylation activity that is useful in galacto-oligosaccharides' production. These biochemical properties specify the suitability of this biocatalyst for milk and whey processing applications. KEY POINTS: • A novel β-galactosidase gene was identified and characterized from an aquatic habitat. • It was active in extreme acidic to mild alkaline pH and at cold to moderate temperatures. • The β-galactosidase was capable to hydrolyze lactose in milk and whey.