Characterization of a novel GH36 α-galactosidase from Bacillus megaterium and its application in degradation of raffinose family oligosaccharides

A novel α-galactosidase gene (agaB) from Bacillus megaterium 3-7 was cloned and expressed in Escherichia coli. The gene coded for a protein with 741 amino acids and a calculated molecular mass of 85.4 kDa. The native structure of the recombined AgaB was determined to be a homotrimer. AgaB showed the highest identity of 57% with the characterized glycosyl hydrolase family 36 α-galactosidase from Clostridium stercorarium F-9. The enzyme exhibited a specific activity of 362.6 U/mg at 37 °C and pH 6.8. The enzyme showed strong resistance to proteases and great tolerance to galactose (Ki = 12.5 mM). AgaB displayed wide substrate specificity toward pNPGal, melibiose, raffinose and stachyose, with a Km of 0.42, 12.1, 17.0 and 25.4 mM, respectively. Furthermore, AgaB completely hydrolyzed raffinose and stachyose present in soybean milk at 37 °C within 4 h when combined with trypsin. These favorable properties make AgaB a potential candidate for applications in the food and feed industries.