The fate of fructose in the body

Fate of fructose in the liver
On the one hand, fructose is very quickly absorbed by the liver, where it is also metabolized using the enzyme "fructokinase" specific for fructose phosphorylation. Now let's look at the corresponding reaction:


 * Fructose + ATP → fructose-1-P + ADP
 * catalyzed by fructokinase

Fructose-1-P is not an intermediate product of glycolysis and its further transformation is catalyzed by the so-called aldolase B (different from aldolase A in glycolysis). The cleavage of Fructose-1-P produces two trioses – glyceraldehyde and dihydroxyacetone phosphate.


 * 'Dihydroxyacetone phosphate can be immediately involved as an intermediate of glycolysis.


 * 'Glyceraldehyde has a more complicated fate. It can be phosphorylated by a specific kinase to glyceraldehyde-3-phosphate, or it can be reduced to glycerol.
 * Phosphorylation is much more important, as it serves to connect glyceraldehyde to glycolysis.


 * Fructose-1-P' → dihydroxyacetone phosphate' + D-glyceraldehyde
 * catalyzed by specific aldolase B


 * Dihydroxyacetone phosphate' → glyceraldehyde-3-P' → glycolysis


 * D-glyceraldehyde' → glyceraldehyde-3-P' → glycolysis


 * or → glycerol → glycerol-3-phosphate' → triacylglycerols

Fructose metabolism is faster' than glucose metabolism, as the main regulatory (slowest) step of glycolysis catalyzed by phosphofructokinase is bypassed. As a result, this can lead to increased hepatic lipogenesis – from the excess pyruvate (and subsequently AcCoA) produced, an excessive amount of fatty acids and triacylglycerols are produced.

Alternative fate of fructose
To a lesser extent and also in other tissues (e.g. muscles) fructose is phosphorylated by hexokinase:


 * Fructose' + ATP' → Fructose-6-P' + ADP

The resulting Fructose-6-P is a direct intermediate product of glycolysis, and the route of connecting fructose therefore takes much less time. However, hexokinase has a higher Km for fructose and thus a low affinity.