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Metabolism of Fructose: Hereditary fructose intolerance, Fructokinase deficiency
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.
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 6phosphate 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 1phosphate pathway:
Fructose is converted to fructose 1phosphate by fructokinase with the use of an ATP.
Fructose 1phosphate is then split into glyceraldehyde and dihydroxyacetone phosphate by fructose 1phosphate aldolase.
The dihydroxyacetone feeds into glycolysis at the triose phosphate isomerase step.
The glyceraldehyde is phosphorylated by triose kinase to glyceraldehyde 3phosphate using another ATP and so also enters glycolysis.
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