This is the third domain of the five-domain beta-galactosidase enzyme that altogether catalyses the hydrolysis of beta(1-3) and beta(1-4) galactosyl bonds in oligosaccharides as well as the inverse reaction of enzymatic condensation and trans-glycosylation. This domain has an Ig-like fold [1].

1: Crystal structures of beta-galactosidase from Penicillium sp. and its complex with galactose. Rojas AL, Nagem RA, Neustroev KN, Arand M, Adamska M, Eneyskaya EV, Kulminskaya AA, Garratt RC, Golubev AM, Polikarpov I; J Mol Biol. 2004;343:1281-1292. PMID:15491613