Learn / Metabolism
Metabolic pathways
Every FMK pathway as a clickable map. Pick one pathway to hammer it — next step, enzyme, regulator, cofactor, and the facts that get tested (NADPH and who needs it, rate-limiting steps, deficiencies) — or set it to all pathways for a mixed drill. Every answer says which enzyme and which pathway.
Catecholamines, serotonin & the tyrosine tree
SEROTONIN (5-HT)
Made from 5-Hydroxytryptophan (5-HTP) by AADC again — the same decarboxylase that makes dopamine
AADC again — the same decarboxylase that makes dopamine — Catecholamines, serotonin & the tyrosine tree
Cofactors: PLP (vitamin B₆), CO₂ released
Deficiency: AADC deficiency — a rare combined deficiency of BOTH dopamine and serotonin, exactly as the shared enzyme predicts
Aromatic L-amino acid decarboxylase is not dopamine-specific: it decarboxylates L-DOPA to dopamine on the tyrosine branch and 5-HTP to serotonin here. One enzyme, both monoamine pathways — which is also why carbidopa, an AADC inhibitor, is peripherally active on more than just L-DOPA.
Becomes 5-HIAA → urine via MAO → 5-HIAA (5-hydroxyindoleacetic acid)
MAO → 5-HIAA (5-hydroxyindoleacetic acid) · irreversible — Catecholamines, serotonin & the tyrosine tree
Deficiency: CARCINOID SYNDROME — a serotonin-secreting neuroendocrine tumour: flushing, diarrhoea, wheezing and right-sided valvular disease, diagnosed by elevated urinary 5-HIAA. The serotonin equivalent of using VMA and metanephrines to catch a pheochromocytoma
Monoamine oxidase finishes serotonin just as it finishes the catecholamines; the urinary end product is 5-HIAA. Serotonin is also N-acetylated and O-methylated in the PINEAL gland to MELATONIN.