Beta-galactosidase

β-galactosidase is a hydrolase enzyme that catalyzes the hydrolysis of β-galactosides into monosaccharides. Substrates of different β-galactosidases include ganglioside GM1, lactosylceramides, lactose, and various glycoproteins. Alternate or nicknames are "beta-gal" or "β-gal". Lactase is often confused as an alternate name for β-galactosidase, but it is actually simply a sub-class of β-galactosidase.

Structure
The 1,023 amino acids of E. coli β-galactosidase were first sequenced in 1970. Four such chains comprise the protein, which was discovered to be a 464-kDa tetramer with 222-point symmetry twenty-four years later. Each unit of β-galactosidase consists of five domains, the third of which is an active site. This enzyme can be split in two peptides, LacZα and LacZΩ, none of which is active by itself but both spontaneously reassemble into a functional enzyme. This characteristic is used in many cloning vectors to achieve α-complementation in specific laboratory strains of E. coli, where the small LacZα peptide is encoded by the plasmid while the large LacZΩ is encoded in trans by the bacterial chromosome. When DNA fragments are inserted in the vector and production of LacZα is disrupted, the cells exhibit no β-galactosidase activity: this allows the blue/white screening of recombinant clones.

Reaction
The active site of β-galactosidase catalyzes the hydrolysis of its disaccharide substrate via "shallow" and "deep" binding. Monovalent potassium ions (K+) as well as divalent magnesium ions (Mg2+) are required for the enzyme's optimal activity. The beta-linkage of the substrate is cleaved by a terminal carboxyl group on the side chain of a glutamic acid.

In E. coli, Glu-461 was thought to be the nucleophile in the substitution reaction. However, it is now known that Glu-461 is an acid catalyst. Instead, Glu-537 is the actual nucleophile, binding to a galactosyl intermediate.

In humans, the nucleophile of the hydrolysis reaction is Glu-268.

Biology
β-galactosidase is an essential enzyme in the human body. Deficiencies in the protein can result in galactosialidosis or Morquio B syndrome.

In E. coli, β-galactosidase is produced by activation of the lac operon, as the lacZ gene.