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

# Homocysteine

Homocysteine forms when methionine donates methyl groups. If it builds up, it damages vessels and favours thrombosis.

Updated 2026-11-01

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## Key facts

- It comes from S-adenosylhomocysteine after each methylation made by SAM.
- Two fates: back to methionine (methionine synthase, with B12 and 5-methyl-THF) or on to cysteine (cystathionine β-synthase, with B6).
- It rises with B12, folate or B6 deficiency, kidney failure and homocystinuria.
- It is not part of proteins.

**Formula**
C₄H₉NO₂S

**PubChem**
[CID 91552](https://pubchem.ncbi.nlm.nih.gov/compound/91552)

## Structure

- It has one more carbon than cysteine in the side chain, which ends in a thiol group (–SH).

## Clinical notes

- High homocysteine with high methylmalonic acid points to B12 deficiency; with normal methylmalonic acid, to folate deficiency.
- Hyperhomocysteinaemia is linked to atherosclerosis and to arterial and venous thrombosis.
- Classic homocystinuria (CBS) gives lens dislocation, a marfanoid habitus and thrombosis.

## Pathways

- [Methionine cycle and transsulfuration](https://durgical.com/en/biochemistry/pathways/methionine-cycle-and-transsulfuration/)

## Related conditions

- [Homocystinuria](https://durgical.com/en/conditions/homocystinuria/)
- [Deep vein thrombosis](https://durgical.com/en/conditions/deep-vein-thrombosis/)
- [Ischaemic stroke](https://durgical.com/en/conditions/ischaemic-stroke/)
- [Methylmalonic acidaemia](https://durgical.com/en/conditions/methylmalonic-acidaemia/)

## Related

- [S-adenosylmethionine (SAM)](https://durgical.com/en/biochemistry/molecules/s-adenosylmethionine/)
- [Methionine synthase](https://durgical.com/en/biochemistry/enzymes/methionine-synthase/)
- [Cystathionine β-synthase (CBS)](https://durgical.com/en/biochemistry/enzymes/cystathionine-synthase/)
- [One-carbon metabolism (folate, B12 and SAM)](https://durgical.com/en/biochemistry/topics/one-carbon-metabolism/)

## Test yourself

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

## Which two vitamins does remethylation of homocysteine to methionine need?

B12 (methylcobalamin) and folate as 5-methyltetrahydrofolate.

## How does methylmalonic acid separate B12 deficiency from folate deficiency?

Both raise homocysteine, but only B12 deficiency raises methylmalonic acid.

## Which B6 enzyme sends homocysteine towards cysteine?

Cystathionine β-synthase, which joins it to serine to make cystathionine.

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Molecules are shown in their neutral form, as the databases deposit them; in the cell the acids and phosphates are ionised.

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

Sources: [L-Homocysteine (CID 91552)](https://pubchem.ncbi.nlm.nih.gov/compound/91552) (PubChem, NCBI, 2026) · Public domain; [Homocysteine Test: MedlinePlus Medical Test](https://medlineplus.gov/lab-tests/homocysteine-test/) (MedlinePlus, US National Library of Medicine, 2026); [Sulfur amino acid metabolism](https://reactome.org/PathwayBrowser/#/R-HSA-1614635) (Reactome, 2026).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/biochemistry/molecules/homocysteine/
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