- Galactose is a major dietary sugar for humans.
- The hydrolysis of the disaccharide lactose (in milk) yields galactose and glucose.
- Galactose and glucose are epimers that differ in their configuration at C-4.
- Thus the entry of galactose into glycolysis requires an epimerization reaction.
- This occurs via a four-step pathway called the galactose–glucose interconversion pathway.
Galactose metabolism takes place primarily in the cytoplasm of cells of the liver.
Substrate: Galactose (which is derived from breakdown of lactose in small intestine).
- Galactose is phosphorylated by galactokinase to give galactose 1-phosphate.
- Galactose 1-phosphate uridylyl transferase catalyzes the transfer of a uridyl group from UDP-glucose to galactose 1-phosphate to form UDP-galactose and glucose 1-phosphate.
- The UDP-galactose is converted back to UDP-glucose by UDP-galactose 4-epimerase. Thus, overall, UDP-glucose is not consumed in the reaction pathway.
- Finally the glucose 1-phosphate is converted to glucose 6-phosphate by phosphoglucomutase.
- The glucose 6-phosphate then enters glycolysis.
Generates 1 intermediate molecule of glycolysis or gluconeogenesis for each molecule of galactose.
Requires 1 ATP
Allows galactose to be converted into intermediate molecules in the glycolysis or gluconeogenesis pathway.
- Galactosemia is a genetic disease caused by an inability to convert galactose to glucose.
- Lack of the second enzyme in this pathway, galactose 1-phosphate uridylyl transferase, leads to the disease through the accumulation of toxic products, including galactitol formed by the reduction of galactose.
- David Hames and Nigel Hooper (2005). Biochemistry. Third ed. Taylor & Francis Group: New York.
- Smith, C. M., Marks, A. D., Lieberman, M. A., Marks, D. B., & Marks, D. B. (2005). Marks’ basic medical biochemistry: A clinical approach. Philadelphia: Lippincott Williams & Wilkins.
- John W. Pelley, Edward F. Goljan (2011). Biochemistry. Third edition. Philadelphia: USA.