Galactose Metabolism

Galactose Metabolism

Galactose Metabolism

  • 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

Location

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).

The Pathway

Galactose Metabolism Pathway

  • 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.

Results

Generates 1 intermediate molecule of glycolysis or gluconeogenesis for each molecule of galactose.

Energy Requirement

Requires 1 ATP

Significance

Allows galactose to be converted into intermediate molecules in the glycolysis or gluconeogenesis pathway.

Related Diseases

  • 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.

References

  1. David Hames and Nigel Hooper (2005). Biochemistry. Third ed. Taylor & Francis Group: New York.
  2. 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.
  3. John W. Pelley, Edward F. Goljan (2011). Biochemistry. Third edition. Philadelphia: USA.

Galactose Metabolism

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