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[Biochemistry](https://durgical.com/en/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](https://www.enzyme-database.org/query.php?ec=2.7.7.12)

**Genes**
GALT

**Structure (PDB)**
[5IN3](https://www.rcsb.org/structure/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

- [Galactose metabolism](https://durgical.com/en/biochemistry/pathways/galactose-metabolism/)

## Related conditions

- [Classic galactosaemia](https://durgical.com/en/conditions/classic-galactosaemia/)
- [Cataract](https://durgical.com/en/conditions/cataract/)

## Related

- [Galactose](https://durgical.com/en/biochemistry/molecules/galactose/)
- [UDP-glucose](https://durgical.com/en/biochemistry/molecules/udp-glucose/)
- [Glucose](https://durgical.com/en/biochemistry/molecules/glucose/)
- [Glucose 6-phosphate](https://durgical.com/en/biochemistry/molecules/glucose-6-phosphate/)

## 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)](https://www.uniprot.org/uniprotkb/P07902/entry) (UniProt, 2026); [GALT gene: MedlinePlus Genetics](https://medlineplus.gov/genetics/gene/galt/) (MedlinePlus, US National Library of Medicine, 2026); [5IN3: Crystal structure of glucose-1-phosphate bound nucleotidylated human galactose-1-phosphate uridylyltransferase](https://www.rcsb.org/structure/5IN3) (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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Representative model, for educational purposes only.

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Source: https://durgical.com/en/biochemistry/enzymes/galactose-1-phosphate-uridylyltransferase/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/enzymes/galactose-1-phosphate-uridylyltransferase/