A galactoside is a
glycoside
In chemistry, a glycoside is a molecule in which a sugar is bound to another functional group via a glycosidic bond. Glycosides play numerous important roles in living organisms. Many plants store chemicals in the form of inactive glycosides. ...
containing
galactose
Galactose (, '' galacto-'' + ''-ose'', "milk sugar"), sometimes abbreviated Gal, is a monosaccharide sugar that is about as sweet as glucose, and about 65% as sweet as sucrose. It is an aldohexose and a C-4 epimer of glucose. A galactose molecu ...
. The H of the OH group on carbon-1 of
galactose
Galactose (, '' galacto-'' + ''-ose'', "milk sugar"), sometimes abbreviated Gal, is a monosaccharide sugar that is about as sweet as glucose, and about 65% as sweet as sucrose. It is an aldohexose and a C-4 epimer of glucose. A galactose molecu ...
is replaced by an organic moiety.
Depending on whether the glycosidic bond lies "above" or "below" the plane of the galactose molecule, galactosides are classified as α-galactosides or β-galactosides.
A β-galactoside is a type of galactoside in which the
glycosidic bond
A glycosidic bond or glycosidic linkage is a type of covalent bond that joins a carbohydrate (sugar) molecule to another group, which may or may not be another carbohydrate.
A glycosidic bond is formed between the hemiacetal or hemiketal group ...
lies above the plane of the
galactose
Galactose (, '' galacto-'' + ''-ose'', "milk sugar"), sometimes abbreviated Gal, is a monosaccharide sugar that is about as sweet as glucose, and about 65% as sweet as sucrose. It is an aldohexose and a C-4 epimer of glucose. A galactose molecu ...
residue. The most commonly recognized and used β-galactoside in biochemistry is
lactose. However, other chemicals, such as
ONPG, are known, but these are typically synthesized for biochemical assays.
Galactosides play significant roles in metabolic processes of many organisms and are hydrolyzed by a class of enzymes called galactosidases and are classified according to what type of glycosidic linkage on the galactoside they will break.
For example, enzymes that hydrolyze the β-galactoside glycosidic bond are called
β-galactosidases, while those that hydrolyze the α-galactoside glycosidic bond are known as
α-galactosidases.
References
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