Beta-galactosidase
From Wikipedia, the free encyclopedia
| galactosidase, beta 1
| |
| Identifiers | |
| Symbol(s) | GLB1 |
| Entrez | 2720 |
| OMIM | 230500 |
| RefSeq | NM_000404 |
| UniProt | P16278 |
| Other data | |
| EC number | 3.2.1.23 |
| Locus | Chr. 3 pter-p22 |
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.<ref name=dorland> Dorland's Illustrated Medical Dictionary. Retrieved on October 22, 2006.</ref> 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.
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[edit] Structure
The 1,023 amino acids of E. coli Beta-galactosidase were first sequenced in 1970.<ref name=fowler>Fowler et. al (1970). "The amino acid sequence of beta-galactosidase". J. Biol. Chem..</ref> 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.<ref name=matthews>Matthews et. al. (2004). "The structure of E. coli Beta-galactosidase". C. R. Biologies.</ref>
[edit] Reaction
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 kalium ions (K+) 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;<ref name=gebler>Gebler et. al. (1991). "Glu-537, not Glu-461, is the nucleophile in the active site of (lac Z) beta-galactosidase from Escherichia coli". J. Biol. Chem..</ref> it is now known that Glu-461 is an acid catalyst. Glu-537 is the actual nucleophile,<ref name=yuan>Yuan et. al. (1994). "Substitutions for Glu-537 of Beta-galactosidase from Escherichia coli cause large decreases in catalytic activity". Biochem J.</ref> binding to a galactosyl intermdiate.
In humans, the nucleophile of the hydrolysis reaction is Glu-268.<ref name=mccarter>McCarter et. al. (1997). "Identification of Glu-268 as the Catalytic Nucleophile of Human Lysosomal Beta-galactosidase Precursor by Mass Spectrometry".</ref>
[edit] Biology
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.
[edit] References
<references/>fr:Bêta-galactosidase
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