Function

D-Galactose is a monosaccharide sugar that serves as an energy source and glycosylation component. It is a C-4 epimer of glucose and often used as a source of carbon in culture media. Galactose is a component of the disaccharide lactose and released upon hydrolysis by β-galactosidase enzymes. It is converted to glucose via the Leloir pathway or metabolized via an alternative pathway, such as the DeLey-Doudoroff pathway.

D-Galactose is an energy-providing nutrient and also a necessary basic substrate for the biosynthesis of many macromolecules in the body. Metabolic pathways for D-Galactose are important not only for the provision of these pathways but also for the prevention of D-Galactose and D-Galactose metabolite accumulation. 

Metabolism

The main source of D-Galactose is lactose in the milk of mammals, but it can also be found in some fruits and vegetables. Utilization of D-Galactose in all living cells is initiated by the phosphorylation of the hexose by the enzyme galactokinase (E.C. 2.7.1.6) (GALK) to form D-Galactose-1-phosphate.

 In the presence of D-Galactose-1-phosphate uridyltransferase (E.C. 2.7.7.12) (GALT) D-Galactose-1-phosphate is exchanged with glucose-1-phosphate in UDP-glucose to form UDP-galactose. Glucose-1-phosphate will then enter the glycolytic pathway for energy production. 

Deficiency of the enzyme GALT in galactosemic patients leads to the accumulation of D-Galactose-1-phosphate. Classic galactosemia-a term that denotes the presence of D-Galactose in the blood is the rare inborn error of D-Galactose metabolism, diagnosed by the deficiency of the second enzyme of the D-Galactose assimilation pathway, GALT, which, in turn, is caused by mutations at the GALT gene. (PMID: 15256214, 11020650, 10408771); 

In organic chemistry, a hexose is a monosaccharide with six carbon atoms, having the chemical formula C6H12O6. Hexoses are classified by functional group, with aldohexoses having an aldehyde at position 1, and ketohexoses having a ketone at position 2. Galactose in the urine is a biomarker for the consumption of milk.