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[Biochemistry](https://durgical.com/en/biochemistry/) · Enzyme

# Branched-chain α-keto acid dehydrogenase (BCKDH)

BCKDH breaks down leucine, isoleucine and valine after their transamination. If it fails, urine smells like maple syrup.

Updated 2026-11-01

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## Key facts

- It oxidatively decarboxylates the keto acids of leucine, isoleucine and valine to their acyl-CoAs.
- A complex built like PDH and α-KGDH: E1 (BCKDHA and BCKDHB), E2 (DBT) and E3 (DLD).
- It needs thiamine pyrophosphate, lipoate, CoA, FAD and NAD⁺.
- Branched-chain amino acids are transaminated mainly in muscle, not in the liver.

**Also called**
BCKDH complex

**EC number**
[1.2.4.4](https://www.enzyme-database.org/query.php?ec=1.2.4.4)

**Genes**
BCKDHA

**Structure (PDB)**
[1X7Y](https://www.rcsb.org/structure/1X7Y)

## Structure

- 1X7Y: the human E1 component (α₂β₂ heterotetramer) with thiamine pyrophosphate.

## Regulation

- A specific kinase (BCKDK) switches it off by phosphorylation; a phosphatase (PPM1K) switches it back on.
- Branched-chain keto acids inhibit that kinase and so activate the complex.

## Inhibitors and toxins

- Arsenite (binds lipoate, as in PDH)

## Clinical notes

- Maple syrup urine disease: leucine, which is neurotoxic, builds up.
- Some mild forms respond to thiamine.
- Plasma alloisoleucine is the diagnostic marker.

## Pathways

- [Branched-chain amino acid breakdown (leucine route)](https://durgical.com/en/biochemistry/pathways/branched-chain-amino-acid-breakdown/)

## Related conditions

- [Maple syrup urine disease](https://durgical.com/en/conditions/maple-syrup-urine-disease/)
- [Propionic acidaemia](https://durgical.com/en/conditions/propionic-acidaemia/)
- [Methylmalonic acidaemia](https://durgical.com/en/conditions/methylmalonic-acidaemia/)

## Related

- [Pyruvate dehydrogenase complex (PDH)](https://durgical.com/en/biochemistry/enzymes/pyruvate-dehydrogenase-complex/)
- [α-Ketoglutarate dehydrogenase](https://durgical.com/en/biochemistry/enzymes/ketoglutarate-dehydrogenase/)
- [Glucogenic and ketogenic amino acids](https://durgical.com/en/biochemistry/topics/glucogenic-and-ketogenic-amino-acids/)
- [Transamination and deamination](https://durgical.com/en/biochemistry/topics/transamination-and-deamination/)

## Test yourself

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

## Which three amino acids does this complex break down?

Leucine, isoleucine and valine, after they are turned into their keto acids.

## Which two other complexes share its design and cofactors?

Pyruvate dehydrogenase and α-ketoglutarate dehydrogenase: all three use TPP, lipoate, CoA, FAD and NAD⁺ and share E3.

## Which plasma metabolite confirms deficiency of this complex?

Alloisoleucine, which only appears when branched-chain keto acids pile up.

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

Sources: [BCKDHA: 2-oxoisovalerate dehydrogenase subunit alpha, mitochondrial (human)](https://www.uniprot.org/uniprotkb/P12694/entry) (UniProt, 2026); [Maple syrup urine disease: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/maple-syrup-urine-disease/) (MedlinePlus, US National Library of Medicine, 2026); [1X7Y: Crystal structure of the human mitochondrial branched-chain alpha-ketoacid dehydrogenase](https://www.rcsb.org/structure/1X7Y) (RCSB Protein Data Bank, 2026).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/biochemistry/enzymes/branched-chain-keto-acid-dehydrogenase/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/enzymes/branched-chain-keto-acid-dehydrogenase/