Fructose metabolism

Fruits and berries accumulate fructose, which honey abundantly contains. During carbohydrate digestion, the small intestine forms fructose from sucrose. In the human body, the epithelial cells of the intestine convert part of the absorbed fructose into glucose before it enters the portal vein. While somatic cells of the human body use glucose as an energy source, certain cells like spermatozoa utilize fructose. The concentration of fructose in human sperm reaches up to 12 mmol/l. Spermatozoa synthesize fructose from glucose. A small amount of fructose also accumulates in the lens of the eye. Fructose metabolism begins with activation catalyzed by hexokinase and fructokinase, similar to glucose. Hexokinase activates fructose in many cells of the human body (except the liver). This enzyme activates various hexoses, mainly epimers of glucose, forming hexose-6-phosphates. Due to hexokinase’s low affinity for fructose, it only functions in the presence of high levels of fructose. The activation of fructose by hexokinase stops when glucose-6-phosphate accumulates in the cell, inhibiting this enzyme. Glucose also inhibits the activation of fructose by hexokinase.

D-fructose+ATP-hexokinase(Mg²⁺)->D-fructose-1-phosphate+ADP.

In the liver, fructose metabolism begins slightly differently. Only fructokinase present in liver cells activates fructose, converting it into fructose-1-phosphate.

D-fructose+ATP-fructokinase(Mg²⁺)->D-fructose-1-phosphate+ADP.

Source | Glossary of Most Commonly Used Biomedical Terms and Concepts | Lithuanian University of Health Sciences | Academician Professor Antanas Praškevičius, Professor Laima Ivanovienė