Galactose

Galactose (Gal) (also called Brain Sugar) is a type of sugar which is less sweet than glucose and not very water-soluble. It is considered a nutritive sweetener because it has food energy.

Galactan is a polymer of the sugar galactose. It is found in hemicellulose and can be converted to galactose by hydrolysis.

Relationship to lactose
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 lactase, a β-galactosidase. In the human body, glucose is changed into galactose in order to enable the mammary glands to secrete lactose.

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).

Clinical significance
Two studies have suggested a possible link between galactose in milk and ovarian cancer. Other studies failed to show such a link.

There are some ongoing studies which suggest that Galactose may have a role in treatment of focal segmental glomerulosclerosis (a kidney disease resulting in kidney failure and proteinuria). This effect is likely to be a result of binding of Galactose to FSGS factor.

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.

Liver galactose metabolism
In the liver, galactose is converted to glucose 6-phosphate in the following reactions:

galacto-               uridyl                phosphogluco- kinase              transferase                mutase gal > gal 1 P --> glc 1 P ---> glc 6 P                            ^           \ /            v                         UDP-glc       UDP-gal ^            /                             \___________/                               epimerase

Metabolic disorders
There are 3 important disorders involving galactose: