Carnitine: the pathway in the body

This page shows the biochemical pathway behind the laboratory value Carnitine, lysine: which stations follow one another, which enzymes carry out each step and which cofactors they use. Every statement has a source. The page describes general textbook knowledge and says nothing about any individual person.

In brief

Carnitine is a small nitrogen-containing molecule that the body forms from lysine and methionine and also obtains from food. It carries long-chain fatty acids into the mitochondria, where they are broken down.

The pathway step by step

1. Lysine in protein → Trimethyllysine Methyltransferases · SAM Methyltransferases transfer three methyl groups from SAM to the lysine in the protein. When the protein is later broken down, trimethyllysine is released. Source 1
2. Trimethyllysine → Hydroxytrimethyllysine TMLH · Iron, Ascorbate The enzyme TMLH attaches an OH group. It contains iron, uses up α-ketoglutarate and is kept in its reactive form by ascorbate (vitamin C). Source 1, 2
3. Hydroxytrimethyllysine → Trimethylaminobutanal HTML aldolase · Vitamin B6 (PLP) An aldolase splits the molecule in two: an aldehyde and the amino acid glycine. The enzyme needs pyridoxal phosphate, the active form of vitamin B6. Source 1
4. Trimethylaminobutanal → γ-Butyrobetaine TMABA dehydrogenase · NAD⁺ A dehydrogenase converts the aldehyde into an acid; NAD⁺ accepts the electrons. Source 1
5. γ-Butyrobetaine → Carnitine BBOX · Iron, Ascorbate The enzyme BBOX adds the last OH group. This enzyme also contains iron, uses up α-ketoglutarate and works with ascorbate. Part of the carnitine also comes from food. Source 1, 2
6. Acyl-CoA → Acylcarnitine CPT1 The enzyme CPT1 on the outer membrane of the mitochondrion swaps coenzyme A for carnitine. This forms acylcarnitine, which can pass through the membrane. Source 3, 4
7. Acylcarnitine → Acylcarnitine inside CACT The translocase CACT carries acylcarnitine across the inner membrane and in exchange moves free carnitine out. Source 3
8. Acylcarnitine inside → Acyl-CoA in the matrix CPT2 · Coenzyme A The enzyme CPT2 swaps carnitine back for coenzyme A. The free carnitine is then available again for the next fatty acid. Source 3, 4
9. Acyl-CoA in the matrix → β-oxidation In the matrix, the fatty acid is shortened by two carbons in each round. This produces acetyl-CoA as well as NADH and FADH₂, which feed into the respiratory chain. Source 4

Cofactors in this pathway

• Vitamin C — As ascorbate, keeps the iron in TMLH and BBOX in its reactive form Source 1, 2In the ORY catalogue as a laboratory value: Vitamin C (Ascorbinsäure)
• Iron — Metal centre of the two hydroxylases TMLH and BBOX Source 1In the ORY catalogue as a laboratory value: Eisen
• Vitamin B6 — As pyridoxal phosphate, cofactor of the aldolase that cleaves hydroxy-trimethyllysine Source 1In the ORY catalogue as a laboratory value: Vitamin B6
• Niacin (NAD⁺) — NAD⁺ accepts the electrons when the aldehyde is oxidised to γ-butyrobetaine Source 1In the ORY catalogue as a laboratory value: NAD⁺ (Nicotinamidadenindinukleotid)
• α-Ketoglutarate — Consumed by TMLH and BBOX in each hydroxylation Source 1In the ORY catalogue as a laboratory value: Alpha-Ketoglutarat (2-Oxoglutarat)
• Methionine (SAM) — As S-adenosylmethionine, donates the three methyl groups for trimethyllysine Source 1In the ORY catalogue as a laboratory value: Methionin
• Lysine — Backbone of carnitine, built into proteins and methylated there Source 1In the ORY catalogue as a laboratory value: Lysin
• Pantothenic acid (coenzyme A) — Building block of coenzyme A, which CPT1 and CPT2 exchange for carnitine Source 3, 4

Sources

1. Strijbis K, Vaz FM, Distel B. Enzymology of the carnitine biosynthesis pathway. IUBMB Life 2010 · PubMed 20306513
2. Rebouche CJ. Ascorbic acid and carnitine biosynthesis. Am J Clin Nutr 1991 · PubMed 1962562
3. Longo N, Frigeni M, Pasquali M. Carnitine transport and fatty acid oxidation. Biochim Biophys Acta 2016 · PubMed 26828774
4. Houten SM, Wanders RJ. A general introduction to the biochemistry of mitochondrial fatty acid β-oxidation. J Inherit Metab Dis 2010 · PubMed 20195903

Related pathways

• Trans fatty acids — acyl-coa
• Dopamine, noradrenaline, adrenaline — Vitamin C (Ascorbinsäure), Eisen
• Spermidine — Eisen, Vitamin B6
• Alanine — Vitamin B6, NAD⁺ (Nicotinamidadenindinukleotid)
• Branched-chain amino acids — Vitamin B6, NAD⁺ (Nicotinamidadenindinukleotid)

As of 2026-09-16. Draft written by Claude to schema v2; sources checked in PubMed; expert review pending
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