''ortho''-Nitrophenyl-β-galactoside (ONPG) is a

colorimetric Colorimetry is "the science and technology used to quantify and describe physically the human color perception". It is similar to spectrophotometry, but is distinguished by its interest in reducing spectra to the physical correlates of color p ...

and

spectrophotometric Spectrophotometry is a branch of electromagnetic spectroscopy concerned with the quantitative measurement of the reflection or transmission properties of a material as a function of wavelength. Spectrophotometry uses photometers, known as spe ...

substrate Substrate may refer to: Physical layers *Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached ** Substrate (aquatic environment), the earthy material that exi ...

for detection of

β-galactosidase β-Galactosidase (EC 3.2.1.23, beta-gal or β-gal; systematic name β-D-galactoside galactohydrolase) is a glycoside hydrolase enzyme that catalyzes hydrolysis of terminal non-reducing β-D-galactose residues in β-D-galactosides. (This enzym ...

activity. This compound is normally colorless. However, if β-galactosidase is present, it

hydrolyzes Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

the ONPG molecule into

galactose Galactose (, ''wikt:galacto-, galacto-'' + ''wikt:-ose#Suffix 2, -ose'', ), sometimes abbreviated Gal, is a monosaccharide sugar that is about as sweetness, sweet as glucose, and about 65% as sweet as sucrose. It is an aldohexose and a C-4 epime ...

and ortho-nitrophenol. The latter compound has a yellow color that can be used to check for

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

activity by means of a

assay (at 420 nm wavelength). β-Galactosidase is required for

lactose Lactose is a disaccharide composed of galactose and glucose and has the molecular formula C12H22O11. Lactose makes up around 2–8% of milk (by mass). The name comes from (Genitive case, gen. ), the Latin word for milk, plus the suffix ''-o ...

utilization, so the intensity of the color produced can be used as a measure of the enzymatic rate. Though ONPG mimics lactose and is hydrolyzed by β-galactosidase, it is unable to act as an

inducer In molecular biology, an inducer is a molecule that regulates gene expression. An inducer functions in two ways; namely: *By disabling repressors. The gene is expressed because an inducer binds to the repressor. The binding of the inducer to the r ...

for the lac operon. Without another lactose analog that can act as an inducer, such as isopropyl β-D-1-thiogalactopyranoside (IPTG), β-galactosidase will not be transcribed and ONPG will not be hydrolyzed.

References

{{DEFAULTSORT:Nitrophenyl-beta-galactoside, ortho- Galactosides 2-Nitrophenyl compounds