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

Structure

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The 1,023 amino acids of E. coli Beta-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 beta-galactosidase consists of five domains, the third of which is an active site.

Reaction

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The active site of Beta-galactosidase catalyzes the hydrolysis of its disaccharide substrate via "shallow" and "deep" binding. Optimal activity of the enzyme requires monovalent sodium ions (Na⁺) as well as divalent magnesium ions (Mg²⁺). The beta-linkage of the substrate is cleaved by a terminal carboxyl group on the side chain of a glutamic acid.

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

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

Biology

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

In E. coli, beta-galcatosidase is produced by activation of the lac Z operon.

References

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EC 3.2.1