Biochemistry · MU Pleven

22. Metabolism of Galactose and Fructose. Enzyme Deficiencies. Synthesis of Hexosamines.

Covers the metabolism of galactose via the Leloir pathway, including the enzymes involved and the fate of its products, then the handling of fructose in the liver through fructokinase and aldolase B and its bypass of PFK-1. It sets out galactokinase, GALT, fructokinase and aldolase B deficiencies with their symptoms and dietary treatments, and closes with the hexosamine pathway, its key GFAT reaction and link to insulin resistance.

Inside: 4 sections · 5 labelled pictures · 10 flashcards · 2 min

Topic 22 of 76 in the MU Pleven syllabus for Biochemistry (Medicine, Year 2). The free AceMed note covers it in 4 sections, about 304 words (2 min of reading), with 10 flashcards.

In this note

  1. Metabolism of Galactose and Fructose.
  2. Metabolism of Galactose
  3. Metabolism of Fructose
  4. Synthesis of Hexosamines

Metabolism of Galactose

  • Dietary Source
  • Galactose mainly comes from lactose in milk.
  • Lactose → Glucose + Galactose via lactase in the small intestine.
  • Transport and Initial Conversion
  • Galactose is absorbed into enterocytes and transported to the liver.
  • Galactose is phosphorylated to Galactose-1-phosphate by:
  • Galactokinase
  • Galactose → Glucose Conversion (Leloir Pathway)
  • Galactose-1-phosphate reacts with UDP-glucose to form:
  • UDP-galactose and Glucose-1-phosphate
  • Enzyme: Galactose-1-phosphate uridyltransferase (GALT)
  • UDP-galactose is converted back to UDP-glucose by:
  • UDP-galactose-4-epimerase
  • Fate of Products
  • Glucose-1-phosphate → Glucose-6-phosphate → Energy production / glycogen synthesis.
  • UDP-galactose used for lactose synthesis in lactating mammary glands.
  • Clinical Importance — Enzyme Deficiencies
  • Galactokinase deficiency
  • Accumulation of galactose → converted to galactitol
  • Causes infantile cataracts; no severe systemic disease.
  • GALT deficiency (Classic Galactosemia)
  • Accumulation of galactose-1-phosphate (toxic)
  • Symptoms: hepatic failure, jaundice, vomiting, mental retardation, cataracts
  • Treatment: Exclude lactose and galactose from diet.

Metabolism of Fructose

  • Dietary Sources
  • Fruits, honey, and sucrose (sucrose = glucose + fructose).
  • Absorption and Transport
  • Fructose absorbed via GLUT5 in intestine; enters liver.
  • Fructose Pathway in Liver
  • Fructokinase converts fructose → Fructose-1-phosphate.
  • Aldolase B splits Fructose-1-phosphate → Dihydroxyacetone phosphate (DHAP) and Glyceraldehyde.
  • These enter glycolysis, gluconeogenesis, or lipogenesis.
  • Clinical Importance — Enzyme Deficiencies
  • Fructokinase deficiency (Essential Fructosuria)
  • Benign; fructose appears in urine; no toxicity.
  • Aldolase B deficiency (Hereditary Fructose Intolerance)
  • Fructose-1-phosphate accumulates → severe hypoglycemia, vomiting, liver failure.
  • Treatment: avoid fructose, sucrose, sorbitol.
  • Metabolic Significance
  • Fructose metabolism bypasses phosphofructokinase (PFK-1), a key regulatory step.
  • Therefore, fructose is rapidly converted to fat (lipogenesis) → contributes to fatty liver and metabolic syndrome.

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