Galactose
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 Haworth projection of D-galactose |
 Fischer projection of D-galactose |
Galactose (Gal) (also called brain sugar) is a type of sugar found in dairy products, in sugar beets and other gums and mucilages. It is also synthesized by the body, where it forms part of glycolipids and glycoproteins in several tissues. It is considered a nutritive sweetener because it has food energy. Galactose is less sweet than glucose and not very water-soluble.
Galactan is a polymer of the sugar galactose. It is found in hemicellulose and can be converted to galactose by hydrolysis.
Galactose is a monosaccharide constituent, together with glucose, of the disaccharide lactose. The hydrolysis of lactose to glucose and galactose is catalyzed by the enzyme beta-galactosidase, a lactase. In the human body, glucose is changed into galactose in order to enable the mammary glands to secrete lactose.
Two studies have suggested a possible link between galactose in milk and ovarian cancer.[link][link] Other studies failed to show such a link.
Structure and isomerism
The first and last -OH groups point the same way and the second and third -OH groups point the other way. D-Galactose has the same configuration at its penultimate carbon as D-glyceraldehyde. Galactose is an optical isomer of glucose.
Galactose and glucose are produced by hydrolysis of lactose by ß-galactosidase. This enzyme is produced by the lac operon in Escherichia coli (E. coli).
Metabolic disorders
There are 3 important disorders involving galactose:- Galactokinase deficiency causes cataracts and mental retardation. If a galactose-free diet starts sufficiently early, the cataracts will regress without complications however neurological damage is permanent.
- UDPgalactose-4-epimerase deficiency is extremely rare (only 2 reported cases). It causes nerve deafness.
- Galactose-1-phosphate uridyl transferase deficiency is the most problematic, as galactose-free diets do not have considerable long-term effects.
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