- Fructose is an abundant sugar in the human diet.
- This dietary monosaccharide is present naturally in fruits and vegetables, either as free fructose or as part of the disaccharide sucrose, and as its polymer inulin.
- Sucrose (table sugar) is a disaccharide which when hydrolyzed yields fructose and glucose.
- The metabolism of fructose from dietary sources is referred to as fructolysis.
Location of Fructose Metabolism
- Fructose metabolism takes place primarily in the cytoplasm of cells of the liver.
- Substrate: Fructose (which is derived from breakdown of sucrose in small intestine).
The Pathway of Fructose Metabolism
There are two pathways for the metabolism of fructose; one occurs in muscle and adipose tissue, the other in liver.
- In muscle and adipose tissue, fructose can be phosphorylated by hexokinase (which is capable of phosphorylating both glucose and fructose) to form fructose 6-phosphate which then enters glycolysis.
- In liver, the cells contain mainly glucokinase instead of hexokinase and this enzyme phosphorylates only glucose. Thus in liver, fructose is metabolized instead by the fructose 1-phosphate pathway:
- Fructose is converted to fructose 1-phosphate by fructokinase with the use of an ATP.
- Fructose 1-phosphate is then split into glyceraldehyde and dihydroxyacetone phosphate by fructose 1-phosphate aldolase.
- The dihydroxyacetone feeds into glycolysis at the triose phosphate isomerase step.
- The glyceraldehyde is phosphorylated by triose kinase to glyceraldehyde 3-phosphate using another ATP and so also enters glycolysis.
Results: Generates 2 intermediate molecules of glycolysis for each molecule of fructose.
Energy Requirement: Requires 2 ATP.
- Allows fructose to be converted into intermediate molecules in the glycolysis pathway.
- Since this pathway bypasses the rate-limiting step in glycolysis, fructose is metabolized to pyruvate more rapidly than glucose.
- 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.