Fructose Metabolism - Sweet & Swift
- Sources: Fruits, honey, high-fructose corn syrup, sucrose.
- Primary Site: Liver.
- Pathway:
- Fructose enters hepatocytes via GLUT5 (insulin-independent).
- Fructokinase (FK): Phosphorylates fructose.
- Fructose + ATP $\rightarrow$ Fructose-1-Phosphate (F-1-P) + ADP
- Aldolase B: Cleaves F-1-P.
- Fructose-1-Phosphate $\rightarrow$ Dihydroxyacetone Phosphate (DHAP) + Glyceraldehyde
- DHAP enters glycolysis.
- Glyceraldehyde is phosphorylated by Triokinase (Glyceraldehyde + ATP $\rightarrow$ Glyceraldehyde-3-P + ADP), then enters glycolysis.
- 📌 Mnemonic: 'FructoKinase Kicks it off, Aldolase B Breaks it down.'
⭐ Fructose metabolism bypasses Phosphofructokinase-1 (PFK-1), the major rate-limiting step of glycolysis. This allows rapid fructose processing, potentially leading to ↑ VLDL synthesis and lipogenesis.
Galactose Metabolism - Milky Way Fuel
- Sources: Primarily lactose (milk, dairy), hydrolyzed to galactose & glucose.
- Leloir Pathway: Converts galactose to glucose-1-phosphate. Key enzymes:
- Galactose + ATP $\xrightarrow{\text{Galactokinase (GALK)}}$ Galactose-1-P + ADP
- Galactose-1-P + UDP-Glucose $\xrightarrow{\text{GALT (Galactose-1-P Uridyltransferase)}}$ UDP-Galactose + Glucose-1-P
⭐ UDP-glucose is essential for the conversion of galactose-1-phosphate to glucose-1-phosphate via GALT.
- UDP-Galactose $\xrightarrow{\text{GALE (UDP-Galactose-4-Epimerase)}}$ UDP-Glucose (regenerated for GALT)
- Fate: Glucose-1-P $\rightarrow$ Glucose-6-P (enters glycolysis/gluconeogenesis) or glycogenesis.
- Clinical Note: Enzyme deficiencies (esp. GALT) cause galactosemia (cataracts, liver damage, intellectual disability).
- 📌 Mnemonic (Leloir enzymes): Kinase, Transferase, Epimerase (GalacKTE).
Disorders of Fructose Metabolism - Fruity Faults
- Two main disorders disrupt fructose metabolism, primarily due to enzyme deficiencies.
| Feature | Essential Fructosuria | Hereditary Fructose Intolerance (HFI) |
|---|---|---|
| Enzyme Defect | Fructokinase | Aldolase B |
| Clinical Features | Benign, asymptomatic, fructosuria | Severe: hypoglycemia, jaundice, vomiting, hepatomegaly, phosphate trapping |
| Inheritance | Autosomal Recessive | Autosomal Recessive |
| Treatment | None needed | Avoid fructose, sucrose, sorbitol |
⭐ In Hereditary Fructose Intolerance, accumulation of Fructose-1-Phosphate inhibits glycogenolysis and gluconeogenesis, leading to severe hypoglycemia on fructose ingestion.
Disorders of Galactose Metabolism - Galactic Glitches
- Galactose metabolism disorders result from enzyme deficiencies, leading to galactitol accumulation.
- Galactitol causes osmotic damage, notably cataracts.
- 📌 Galactosemia Affects Liver & Tiny tots (Classical GALT deficiency).
| Disorder | Enzyme Defect | Key Features | Treatment |
|---|---|---|---|
| Classical Galactosemia | GALT | Severe: FTT, liver damage, jaundice, cataracts, E. coli sepsis, intellectual disability | Galactose-free diet |
| Galactokinase Deficiency | GALK1 | Cataracts (infantile), otherwise mild | Galactose-free diet |
| GALE Deficiency | GALE (UDP-glucose-4-epimerase) | Variable: asymptomatic to severe (like classical); may involve only RBC/WBC | Galactose-free diet |
⭐ Newborn screening for classical galactosemia (GALT deficiency) is crucial to prevent severe complications like liver failure, sepsis, and intellectual disability.
High‑Yield Points - ⚡ Biggest Takeaways
- Essential fructosuria (fructokinase deficiency): benign; fructose in urine (reducing sugar).
- Hereditary Fructose Intolerance (aldolase B deficiency): Fructose-1-P accumulation inhibits glycogenolysis & gluconeogenesis, causing severe hypoglycemia, vomiting post fructose.
- Fructose metabolism bypasses PFK-1, a key rate-limiting step of glycolysis.
- Galactokinase deficiency: galactitol accumulation from unmetabolized galactose causes infantile cataracts.
- Classic galactosemia (GALT deficiency): Galactose-1-P accumulation causes cataracts, hepatomegaly, E. coli sepsis, intellectual disability.
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