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β-galactosidase from Penicillum sp.
Identifiers
Symbol	GLB1
Entrez	2720
HUGO	4298
OMIM	230500
RefSeq	NM_000404
UniProt	P16278
Other data
EC number	3.2.1.23
Locus	Chr. 3 pter-p22

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.^[1] 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.

The 1,023 amino acids of E. coli β-galactosidase were first sequenced in 1970.^[2] 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.^[3]

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.^[4] However, it is now known that Glu-461 is an acid catalyst. Instead, Glu-537 is the actual nucleophile,^[5] binding to a galactosyl intermediate.

In humans, the nucleophile of the hydrolysis reaction is Glu-268.^[6]

β-galactosidase reaction

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.

β-galactosidase assay is used frequently in genetics, molecular biology (see X-gal), and other life sciences: [1]

IPTG induces activity of β-galactosidase by binding and inhibiting the lac repressor

1. ^ Dorland's Illustrated Medical Dictionary. Retrieved on October 22, 2006. 
2. ^ Fowler et. al (1970). "The amino acid sequence of β-galactosidase". J. Biol. Chem.. 
3. ^ Matthews B (2005). "The structure of E. coli beta-galactosidase". C R Biol 328 (6): 549-56. PMID 15950161. 
4. ^ Gebler et al. (1991). "Glu-537, not Glu-461, is the nucleophile in the active site of (lacZ) β-galactosidase from Escherichia coli". J. Biol. Chem.. 
5. ^ Yuan et al. (1994). "Substitutions for Glu-537 of β-galactosidase from Escherichia coli cause large decreases in catalytic activity". Biochem J. 
6. ^ McCarter J, Burgoyne D, Miao S, Zhang S, Callahan J, Withers S (1997). "Identification of Glu-268 as the catalytic nucleophile of human lysosomal beta-galactosidase precursor by mass spectrometry". J Biol Chem 272 (1): 396-400. PMID 8995274. 

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