Serotonin Tryptophan — FRCA Primary MCQ

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ModeratePhysiology - Endocrine & MetabolicSerotonin TryptophanFRCA Primary

Serotonin is synthesised through hydroxylation of an essential aromatic amino acid, followed by pyridoxal phosphate-dependent decarboxylation. Which amino acid is its precursor?

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Correct answer: C — Tryptophan

Tryptophan is the amino acid precursor of serotonin (5-hydroxytryptamine). L-tryptophan is first hydroxylated to 5-hydroxytryptophan by tryptophan hydroxylase; this is the rate-limiting step and requires tetrahydrobiopterin, oxygen and iron. Aromatic L-amino acid decarboxylase then converts 5-hydroxytryptophan to serotonin using pyridoxal phosphate. Histidine is the precursor of histamine. Phenylalanine can be hydroxylated to tyrosine, while tyrosine is the precursor of the catecholamines dopamine, noradrenaline and adrenaline. Glutamine contributes to glutamate and GABA metabolism rather than serotonin synthesis. Platelets take up and store serotonin but do not provide the principal biosynthetic pathway.

Reference: A Comprehensive Review of Nutritional Influences on the Serotonergic System, serotonin biosynthesis figure, 2025. https://pubmed.ncbi.nlm.nih.gov/40998119/