Haworth projection of D-galactose

Fischer projection of D-galactose

Galactose (also called brain sugar) is a type of sugar found in dairy products, in sugar beets and other gums and mucilages. It is also synthetized 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.

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.

Individuals who lack this enzyme are 'lactose intolerant' and unable to realize the nutritive potential of milk sugars. The lactose then passes to the large intestine where it is digested by bacteria, producing gas and flatulence.

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.

Reactions

Hydrolysis

Galactose is hydrolysed by lactase.

Metabolic disorders

There are 3 important disorders involving galactose:

• Galactokinase deficiency causes cataracts. If a galactose-free diet starts sufficiently early, the cataracts will regress without complications.
• 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.