InterPro: IPR004199 Glycoside hydrolase, family 42, domain 5

O-Glycosyl hydrolases EC:3.2.1. are a widespread group of enzymes that hydrolyse the glycosidic bond between two or more carbohydrates, or between a carbohydrate and a non-carbohydrate moiety. A classification system for glycosyl hydrolases, based on sequence similarity, has led to the definition of 85 different families [1, 2, 3]. This classification is available on the CAZy (CArbohydrate-Active EnZymes) web site [4]. Because the fold of proteins is better conserved than their sequences, some of the families can be grouped in clans.

Beta-galactosidase enzymes (EC:3.2.1.23) belong to the glycosyl hydrolase 42 family GH42. Beta-galactosidase is the product of the lac operon Z gene of Escherichia coli. This enzyme catalyses the hydrolysis of the disaccharide lactose to galactose and glucose, and can also convert lactose to allolactose, the inducer of the lac operon. This domain is found in single chain beta-galactosidases, which are comprised of five domains. The active site is located in a deep pocket built around the central alpha-beta barrel, with the other domains conferring specificity for a disaccharide substrate. This entry represents domain 5, which contains an N-terminal loop that swings towards the active site upon the deep binding of a ligand to produce a closed conformation [5]. This domain is also found in the amino-terminal portion of the small chain of dimeric beta-galactosidases.