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Biochemistry · Enzyme

Galactose-1-phosphate uridylyltransferase (GALT) !

GALT is the central enzyme of the Leloir pathway: it turns galactose 1-phosphate into a sugar the body can use. Its absence causes classic galactosaemia.

Updated 2026-11-01

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Key facts

  • Galactose 1-phosphate + UDP-glucose ⇄ UDP-galactose + glucose 1-phosphate.
  • Ping-pong mechanism: the enzyme picks up UMP on a histidine and then hands it to galactose 1-phosphate.
  • The Q188R variant is the most common in classic galactosaemia in people of European descent; the Duarte variant is mild.
  • Structure 5IN3 (human) shows the enzyme with UMP on its histidine and glucose 1-phosphate in the active site.
EC number
2.7.7.12
Genes
GALT
Structure (PDB)
5IN3

Regulation

  • No major hormonal control; activity depends on the amounts of enzyme and substrate.

Inhibitors and toxins

  • No clinically used inhibitors

Clinical notes

  • Classic galactosaemia is picked up on newborn screening by measuring GALT activity or galactose in dried blood.
  • Even on a strict diet, speech and learning problems and ovarian failure can remain, because the body makes some galactose of its own.

Pathways

Test yourself

Think of the answer, then tap the question to see it.

Which molecule supplies the UDP in the GALT reaction?

UDP-glucose; glucose 1-phosphate leaves in exchange.

What is the Duarte variant of GALT?

A variant with partial activity; it usually causes a mild or symptom-free form.

Why can galactosaemia leave sequelae despite the diet?

The body makes its own galactose through the epimerase, and galactose 1-phosphate stays somewhat high.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: GALT: Galactose-1-phosphate uridylyltransferase (human) (UniProt, 2026); GALT gene: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); 5IN3: Crystal structure of glucose-1-phosphate bound nucleotidylated human galactose-1-phosphate uridylyltransferase (RCSB Protein Data Bank, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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