galactoses

Galactose is a six-carbon aldose sugar that is an isomer of glucose. It exists primarily in cyclic forms in solution and occurs in two anomeric forms, alpha and beta, that can interconvert (mutarotation). In nature it is most familiar as part of lactose, the disaccharide of milk, in which galactose is joined to glucose by a beta-1,4-glycosidic bond. Free galactose also occurs in small amounts in foods and can be produced endogenously during the breakdown of complex carbohydrates.

Biological role: In animals galactose is not a major dietary energy source, but it is a key precursor for the biosynthesis of galactose-containing glycoconjugates. It is converted to UDP-galactose, which together with glucose-1-phosphate can contribute to the formation of lactose in milk and to glycoprotein and glycolipid biosynthesis.

Metabolism: Galactose is processed mainly via the Leloir pathway. Galactokinase (GALK) converts galactose to galactose-1-phosphate. Galactose-1-phosphate uridylyltransferase (GALT) transfers a uridylyl group from UDP-glucose to form UDP-galactose and glucose-1-phosphate. UDP-galactose epimerase (GALE) interconverts UDP-galactose and UDP-glucose, linking galactose metabolism to glycolysis and biosynthesis. Defects in these steps cause galactosemia, a group of inherited disorders that can affect the liver, brain, eyes, and growth.

Dietary sources and clinical notes: The main dietary source of galactose is lactose in dairy products. Galactose is also present in certain plant polysaccharides and oligosaccharides. In humans, galactosemia requires dietary restriction of lactose-containing foods in affected individuals. In research settings, galactose is used as a carbon source in certain microbial and cell culture systems and as a tool for studying glycosylation pathways.