Beta-galactosidase

protein
Name =
caption = β-galactosidase from "Penicillum" sp.


width = 300px
HGNCid = 4298
Symbol = GLB1
AltSymbols =
EntrezGene = 2720
OMIM = 230500
RefSeq = NM_000404
UniProt = P16278
PDB =
ECnumber = 3.2.1.23
Chromosome = 3
Arm =
Band =
LocusSupplementaryData = 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.cite book
title=Dorland's Illustrated Medical Dictionary
url=http://www.mercksource.com/pp/us/cns/cns_hl_dorlands.jspzQzpgzEzzSzppdocszSzuszSzcommonzSzdorlandszSzdorlandzSzdmd_g_01zPzhtm#12382991
accessdate = 2006-10-22] Alternate or nicknames are "beta-gal" or "β-gal". Lactase is often confused as an alternative name for β-galactosidase, but it is actually simply a sub-class of β-galactosidase.

tructure

The 1,021 amino acids of "E. coli" β-galactosidase were first sequenced in 1970.cite journal|author=Fowler et. al|title=The amino acid sequence of β-galactosidase|journal=J. Biol. Chem.| date=1970| url=http://www.jbc.org/cgi/reprint/245/19/5032] 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.cite journal |author=Matthews B |title=The structure of E. coli beta-galactosidase |journal=C R Biol |volume=328 |issue=6 |pages=549–56 |year=2005 |pmid=15950161] 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.cite journal|author=Gebler et al.|journal=J. Biol. Chem.|date=1991|title=Glu-537, not Glu-461, is the nucleophile in the active site of ("lacZ") β-galactosidase from "Escherichia coli"|url=http://www.jbc.org/cgi/reprint/267/16/11126] However, it is now known that Glu-461 is an acid catalyst. Instead, Glu-537 is the actual nucleophile,cite journal|author=Yuan et al.|title=Substitutions for Glu-537 of β-galactosidase from Escherichia coli cause large decreases in catalytic activity|journal=Biochem J|date=1994|url=http://www.pubmedcentral.gov/articlerender.fcgi?tool=pubmed&pubmedid=7909660|pmid=7909660] binding to a galactosyl intermediate.

In humans, the nucleophile of the hydrolysis reaction is Glu-268.cite journal |author=McCarter J, Burgoyne D, Miao S, Zhang S, Callahan J, Withers S |title=Identification of Glu-268 as the catalytic nucleophile of human lysosomal beta-galactosidase precursor by mass spectrometry |journal=J Biol Chem |volume=272 |issue=1 |pages=396–400 |year=1997 |pmid=8995274 |doi=10.1074/jbc.272.1.396]

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.

ee also

β-galactosidase assay is used frequently in genetics, molecular biology (see X-gal), and other life sciences: [ [http://openwetware.org/wiki/Beta-Galactosidase_Assay_(A_better_Miller) Beta-Galactosidase Assay (A better Miller) - OpenWetWare ] ]

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

References

External links

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* [http://www.reportergene.com/search/label/other%20reporters Updated research on beta-gal] and other reporters from Reportergene