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
- 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
Related conditions
- Carnitine palmitoyltransferase II (CPT-II) deficiency
- Primary carnitine deficiency
- Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency
Related
- Carnitine (L-carnitine)
- Carnitine shuttle
- Malonyl-CoA
- Acetyl-CoA carboxylase (ACC)
- Medium-chain acyl-CoA dehydrogenase (MCAD)
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.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: CPT1A: Carnitine O-palmitoyltransferase 1, liver isoform (human) (UniProt, 2026); CPT1A gene: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); Carnitine palmitoyltransferase I deficiency: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); Carnitine shuttle (Reactome, 2026).
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Representative model, for educational purposes only.