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).

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

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.

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.

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) (PubChem, NCBI, 2026) · Public domain; Tryptophan catabolism (Reactome, 2026); Serotonin and melatonin biosynthesis (Reactome, 2026).

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