Beta-galactosidase

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β-galactosidase, also called beta-gal or β-gal, 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] Lactase is often confused as an alternative name for β-galactosidase, but it is actually simply a sub-class of β-galactosidase.

Contents

Structure

The 1,024 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] 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.

In 1995, Dimri et al. proposed a new isoform for beta-galactosidase with optimum activity at pH 6.0 (Senescence Associated beta-gal or SA-beta-gal)[4] which would be specifically expressed in senescence (The irreversible growth arrest of cells). Even specific quantitative assays were developed for its detection.[5][6][7] Today it is known that this corresponds to an accumulation of the lysosomal endogenous beta-galactosidase.[8] and its expression is not required for senescence. However, it remains as the most widely used biomarker for senescent and aging cells, because it is reliable and easy to detect.

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