[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Biochemistry](https://durgical.com/en/biochemistry/) › Carnitine palmitoyltransferase I (CPT-I)

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

# Carnitine palmitoyltransferase I (CPT-I)

CPT-I is the gate for fat into the mitochondrion: it moves the fatty acid from CoA to carnitine so it can cross.

Updated 2026-11-01

## Key facts

- On the outer mitochondrial membrane: acyl-CoA + carnitine → acylcarnitine + CoA.
- It is the rate-limiting step of long-chain β-oxidation.
- Malonyl-CoA inhibits it: when fat is being made it is not burned.
- Isoforms: CPT1A in liver and kidney, CPT1B in muscle and heart, CPT1C in neurons.

**Also called**
CPT1A (liver), CPT1B (muscle), CPT1C (brain)

**EC number**
[2.3.1.21](https://www.enzyme-database.org/query.php?ec=2.3.1.21)

**Genes**
CPT1A

## Regulation

- Inhibited by malonyl-CoA (the muscle isoform is far more sensitive).
- Fasting lowers malonyl-CoA and raises expression via PPARα: more fat enters.

## Inhibitors and toxins

- Malonyl-CoA (physiological)
- Etomoxir and perhexiline (experimental or of limited use)

## Clinical notes

- CPT1A deficiency causes hypoketotic hypoglycaemia and liver injury in infants during fasting; free blood carnitine is normal or high.
- A CPT1A variant common in Arctic populations is linked to hypoglycaemia in young children.

## Pathways

- [Fatty acid β-oxidation](https://durgical.com/en/biochemistry/pathways/fatty-acid-oxidation/)

## Related conditions

- [Carnitine palmitoyltransferase II (CPT-II) deficiency](https://durgical.com/en/conditions/carnitine-palmitoyltransferase-ii-deficiency/)
- [Primary carnitine deficiency](https://durgical.com/en/conditions/primary-carnitine-deficiency/)
- [Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency](https://durgical.com/en/conditions/medium-chain-acyl-coa-dehydrogenase-deficiency/)

## Related

- [Carnitine (L-carnitine)](https://durgical.com/en/biochemistry/molecules/carnitine/)
- [Carnitine shuttle](https://durgical.com/en/biochemistry/topics/carnitine-shuttle/)
- [Malonyl-CoA](https://durgical.com/en/biochemistry/molecules/malonyl-coa/)
- [Acetyl-CoA carboxylase (ACC)](https://durgical.com/en/biochemistry/enzymes/acetyl-coa-carboxylase/)
- [Medium-chain acyl-CoA dehydrogenase (MCAD)](https://durgical.com/en/biochemistry/enzymes/medium-chain-acyl-coa-dehydrogenase/)

## Test yourself

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

## Where is CPT-I and what reaction does it carry out?

On the outer mitochondrial membrane; it transfers the acyl group from CoA to carnitine, forming acylcarnitine.

## Which metabolite inhibits CPT-I and why does that make sense?

Malonyl-CoA, a synthesis intermediate: it prevents burning fatty acids that are being made.

## Do medium-chain fatty acids need CPT-I?

No: they cross the membrane and are activated inside the matrix, which is why MCT oils are used in long-chain defects.

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

Sources: [CPT1A: Carnitine O-palmitoyltransferase 1, liver isoform (human)](https://www.uniprot.org/uniprotkb/P50416/entry) (UniProt, 2026); [CPT1A gene: MedlinePlus Genetics](https://medlineplus.gov/genetics/gene/cpt1a/) (MedlinePlus, US National Library of Medicine, 2026); [Carnitine palmitoyltransferase I deficiency: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/carnitine-palmitoyltransferase-i-deficiency/) (MedlinePlus, US National Library of Medicine, 2026); [Carnitine shuttle](https://reactome.org/PathwayBrowser/#/R-HSA-200425) (Reactome, 2026).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/biochemistry/enzymes/carnitine-palmitoyltransferase-i/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/enzymes/carnitine-palmitoyltransferase-i/