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. Dorland's Illustrated Medical Dictionary, , , , , , 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.

Contents

Structure

Reaction

Biology

See also

References

Structure

The 1,023 amino acids of ''E. coli'' β-galactosidase were first sequenced in 1970. The amino acid sequence of β-galactosidase, Fowler et. al, , , J. Biol. Chem., 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. The structure of E. coli beta-galactosidase, Matthews B, , , C R Biol, 2005

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. Glu-537, not Glu-461, is the nucleophile in the active site of (''lacZ'') β-galactosidase from ''Escherichia coli'', Gebler et al., , , J. Biol. Chem., However, it is now known that Glu-461 is an acid catalyst. Instead, Glu-537 is the actual nucleophile, Substitutions for Glu-537 of β-galactosidase from Escherichia coli cause large decreases in catalytic activity, Yuan et al., , , Biochem J, binding to a galactosyl intermediate.
In humans, the nucleophile of the hydrolysis reaction is Glu-268. Identification of Glu-268 as the catalytic nucleophile of human lysosomal beta-galactosidase precursor by mass spectrometry, McCarter J, Burgoyne D, Miao S, Zhang S, Callahan J, Withers S, , , J Biol Chem, 1997

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.

See also

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

References