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

# Tryptophan

Tryptophan is an essential amino acid that gives serotonin, melatonin and part of our niacin (vitamin B3).

Updated 2026-11-01

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## Key facts

- Essential, with an indole ring; one of the least abundant amino acids in proteins.
- Tryptophan hydroxylase (with BH4) → 5-hydroxytryptophan → serotonin → melatonin in the pineal gland.
- Through the kynurenine pathway it forms NAD⁺: about 60 mg of tryptophan equal 1 mg of niacin; the route needs B6 and B2.
- It is glucogenic and ketogenic (gives alanine and acetoacetyl-CoA).

**Formula**
C₁₁H₁₂N₂O₂

**PubChem**
[CID 6305](https://pubchem.ncbi.nlm.nih.gov/compound/6305)

## Structure

- Its side chain is an indole (a benzene ring fused to a pyrrole), the bulkiest among protein amino acids.

## Clinical notes

- Carcinoid syndrome: the tumour spends tryptophan on serotonin and pellagra can appear from lack of niacin.
- Hartnup disease: tryptophan is poorly absorbed by gut and kidney; pellagra-like lesions appear.
- Antidepressants that block serotonin reuptake act on its product, serotonin.

## Pathways

- [Serotonin and melatonin synthesis](https://durgical.com/en/biochemistry/pathways/serotonin-and-melatonin-synthesis/)

## Related conditions

- [Carcinoid syndrome](https://durgical.com/en/conditions/carcinoid-syndrome/)
- [Hartnup disease](https://durgical.com/en/conditions/hartnup-disease/)

## Related

- [Molecules derived from amino acids](https://durgical.com/en/biochemistry/topics/molecules-derived-from-amino-acids/)
- [Glucogenic and ketogenic amino acids](https://durgical.com/en/biochemistry/topics/glucogenic-and-ketogenic-amino-acids/)
- [Tetrahydrobiopterin (BH4)](https://durgical.com/en/biochemistry/molecules/tetrahydrobiopterin/)
- [NAD⁺ (nicotinamide adenine dinucleotide)](https://durgical.com/en/biochemistry/molecules/nad/)

## Test yourself

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

## Which vitamin can be made from tryptophan?

Niacin (B3), through the kynurenine pathway, which ends in NAD⁺.

## Why can carcinoid syndrome cause pellagra?

The tumour diverts much of the tryptophan to serotonin, leaving little to make niacin.

## Where is melatonin made, and from what?

In the pineal gland, from serotonin, which in turn comes from tryptophan.

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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-Tryptophan (CID 6305)](https://pubchem.ncbi.nlm.nih.gov/compound/6305) (PubChem, NCBI, 2026) · Public domain; [Tryptophan catabolism](https://reactome.org/PathwayBrowser/#/R-HSA-71240) (Reactome, 2026); [Serotonin and melatonin biosynthesis](https://reactome.org/PathwayBrowser/#/R-HSA-209931) (Reactome, 2026).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/biochemistry/molecules/tryptophan/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/molecules/tryptophan/