E. coli β-galactosidase is a tetramer of four identical 1023-amino acid chains. Each chain consists of five domains, the third of which is an eight-stranded barrel that comprises much of the active site. β-galactosidase, also called lactase, beta-gal or β-gal, is a glycoside hydrolase enzyme that catalyzes the hydrolysis of β-galactosides into monosaccharides through the breaking of a glycosidic bond. β-galactosides include carbohydrates containing galactose where the glycosidic bond lies above the galactose molecule.