Learn / Metabolism
Metabolic pathways
Every FMK pathway as a clickable map. Pick one pathway to hammer it — next step, enzyme, regulator, cofactor, and the facts that get tested (NADPH and who needs it, rate-limiting steps, deficiencies) — or set it to all pathways for a mixed drill. Every answer says which enzyme and which pathway.
Fructose metabolism
Fructose (GLUT5 in gut, GLUT2 into liver) is phosphorylated by fructokinase to F1P and split by aldolase B into DHAP + glyceraldehyde — entering glycolysis BELOW PFK-1, so its flux is unregulated → lipogenesis, VLDL, uric acid. Aldolase B deficiency traps phosphate as F1P.
Click any metabolite on the map for its reactions. Source: FMK-03.4 Fructose, galactose & ethanol.
Reactions
| Step | Enzyme | Cofactors | Regulation |
|---|---|---|---|
| Fructose → Fructose-1-phosphate | Fructokinase Essential fructosuria — benign; fructose in blood/urine (positive copper-reduction test, negative glucose oxidase); no treatment Essential fructosuria → |
ATP → ADP | |
| Fructose-1-phosphate → DHAP | Aldolase B ★ Hereditary fructose intolerance — F1P accumulates, traps Pi → ↓ATP → gluconeogenesis and glycogenolysis fail: vomiting, hypoglycemia, jaundice, hepatomegaly after fruit/sucrose; avoid fructose, sucrose AND sorbitol Hereditary fructose intolerance (secondary hyperuricemia) → |
— | |
| Fructose-1-phosphate → Glyceraldehyde | Aldolase BHereditary fructose intolerance (secondary hyperuricemia) → | — | |
| Glyceraldehyde → Glyceraldehyde-3-phosphate | Triokinase | ATP → ADP | |
| DHAP → Glycolysis / gluconeogenesis | Triose phosphate isomerase → glycolysis | — | |
| Glyceraldehyde-3-phosphate → Glycolysis / gluconeogenesis | Enters glycolysis below PFK-1 | — | |
| Fructose → Fructose-6-phosphate (extrahepatic) | Hexokinase (muscle, adipose, kidney) | ATP → ADP | |
| Sorbitol → Fructose | Sorbitol dehydrogenase | NAD⁺ → NADH |
Conditions that live on this map
Essential fructosuriaHereditary fructose intolerance (secondary hyperuricemia)High-fructose diet: NAFLD, hyperuricemia and metabolic syndrome
Blue pills sit on one specific arrow; grey ones are whole-pathway problems. Each opens the full condition card, which links back here with the arrow lit.
Facts worth knowing
Transporter
- Intestinal absorption of fructose uses — GLUT-5 (then GLUT-2 into the liver via the portal vein)
Regulation
- Why fructose is 'unregulated' compared with glucose — It enters as F1P → trioses, downstream of PFK-1 — the main glycolytic checkpoint is skipped
- Metabolic consequences of high fructose intake — ↑ de novo lipogenesis & VLDL (hypertriglyceridemia, NAFLD), ↑ uric acid (gout), insulin resistance
Enzyme deficiency
- Benign vs dangerous fructose disorder — Fructokinase deficiency = essential fructosuria (benign). Aldolase B deficiency = hereditary fructose intolerance (hypoglycemia, liver failure)
- Why HFI causes hypoglycemia — F1P sequesters inorganic phosphate → ATP can't be regenerated → glycogen phosphorylase and gluconeogenesis stall
- Deficiency of Fructokinase causes… — Essential fructosuria — benign; fructose in blood/urine (positive copper-reduction test, negative glucose oxidase); no treatment (Liver, kidney, intestine. High Vmax — fructose is grabbed fast, consuming ATP.)
- Deficiency of Aldolase B causes… — Hereditary fructose intolerance — F1P accumulates, traps Pi → ↓ATP → gluconeogenesis and glycogenolysis fail: vomiting, hypoglycemia, jaundice, hepatomegaly after fruit/sucrose; avoid fructose, sucrose AND sorbitol (The regulated step of hepatic fructose handling.)
Rate-limiting step
- Rate-limiting enzyme of fructose metabolism — Aldolase B (The regulated step of hepatic fructose handling.)