Fructose Metabolism (Fructolysis)

1. 1

   Fructose + ATP → fructose-1-phosphate + ADP

   Hepatic fructokinase (ketohexokinase) 2.7.1.3 ST-0048 Irreversible ATP Mg2+
   In Fructose ATP

   Out Fructose-1-phosphate ADP
   › Notes

   Hepatic fructokinase (ketohexokinase) uses ATP and Mg2+ to phosphorylate fructose at carbon 1. This reaction is effectively irreversible and has high capacity but is not tightly feedback-regulated; rapid flux can transiently consume ATP and phosphate when fructose intake is very high.

2. 2

   Fructose-1-phosphate → dihydroxyacetone phosphate + glyceraldehyde

   Aldolase B (fructose-1-phosphate aldolase) 4.1.2.13 ST-0049 Irreversible/directional
   In Fructose-1-phosphate

   Out Dihydroxyacetone phosphate Glyceraldehyde
   › Notes

   Aldolase B (fructose-1-phosphate aldolase) cleaves fructose-1-phosphate without a redox cofactor. This is the defining hepatic fructolysis cleavage reaction and is functionally the pathway's critical/committed processing step because failure traps phosphate as fructose-1-phosphate.

3. 3

   Glyceraldehyde + ATP → glyceraldehyde-3-phosphate + ADP

   Triokinase (glyceraldehyde kinase) ST-0050 Irreversible/directional ATP Mg2+
   In Glyceraldehyde ATP

   Out Glyceraldehyde-3-phosphate ADP
   › Notes

   Triokinase (glyceraldehyde kinase) requires ATP and Mg2+, phosphorylating glyceraldehyde. Glyceraldehyde-3-phosphate and dihydroxyacetone phosphate can then enter glycolysis, gluconeogenesis, glycerol-3-phosphate synthesis, or lipogenesis.

4. 4

   Dihydroxyacetone phosphate ↔ glyceraldehyde-3-phosphate

   Triose phosphate isomerase 5.3.1.1 ST-0051 Reversible
   In Dihydroxyacetone phosphate

   Out Glyceraldehyde-3-phosphate
   › Notes

   Triose phosphate isomerase reversibly interconverts the products of fructolysis as needed for downstream glycolysis or gluconeogenesis. No additional ATP or redox cofactor is used.

5. 5

   Alternative extrahepatic entry: fructose + ATP → fructose-6-phosphate + ADP

   Hexokinase / Glucokinase 2.7.1.1 / 2.7.1.2 ST-0052 Irreversible/directional ATP Mg2+
   In Fructose ATP

   Out Fructose-6-phosphate ADP
   › Notes

   Hexokinase can phosphorylate fructose at carbon 6 using ATP and Mg2+, yielding fructose-6-phosphate for entry upstream of PFK-1. This route is generally minor because hexokinase preferentially phosphorylates glucose and because liver fructokinase efficiently clears portal fructose.

Showing all 5 steps.

Overview

Fructose is absorbed from the intestine and is metabolized chiefly in the liver by a pathway that converts it to triose phosphates. Hepatic fructolysis bypasses PFK-1, allowing rapid carbon entry downstream of glycolysis's principal regulatory step. Extrahepatic tissues can phosphorylate fructose through hexokinase, but this route is usually quantitatively minor at physiological fructose concentrations.

Cellular location

The principal fructolysis reactions occur in the cytosol of hepatocytes; kidney cortex and small-intestinal mucosa also express fructokinase and aldolase B. Fructose enters enterocytes mainly through GLUT5 and exits toward portal blood through GLUT2; hepatic uptake is mediated largely by GLUT2. The alternative hexokinase route is cytosolic in muscle, adipose tissue, and other cells.

Net energetics

Conversion of one fructose to two triose phosphates through the liver-specific pathway consumes 2 ATP (one each at fructokinase and triokinase) and produces no NADH, NADPH, FADH2, or GTP in the entry reactions. If both trioses proceed through glycolysis to pyruvate, the downstream payoff is the same as for two glyceraldehyde-3-phosphate molecules: 4 ATP and 2 NADH, giving net 2 ATP and 2 NADH per fructose from fructose to two pyruvate. The bypass of PFK-1 changes regulation, not the overall glycolytic ATP accounting.

Clinical significance

Because hepatic fructolysis supplies trioses below PFK-1, high fructose flux can favor hepatic glycerol-3-phosphate formation, de novo lipogenesis, and triacylglycerol synthesis when energy intake is excessive. Essential fructosuria results from fructokinase deficiency and is typically benign because fructose is excreted or metabolized by alternative pathways. Hereditary fructose intolerance, caused by aldolase B deficiency, produces fructose-1-phosphate accumulation, phosphate/ATP depletion, inhibition of glycogenolysis and gluconeogenesis, severe hypoglycemia, vomiting, jaundice, and potential liver failure after fructose, sucrose, or sorbitol ingestion.