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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.

Pathway

Catecholamines, serotonin & the tyrosine tree · CNS and sympathetic ganglia (dopamine, NE); adrenal medulla only for epinephrine; melanocytes for melanin; liver for the degradation arm; serotonin in CNS raphe nuclei, gut enterochromaffin cells and platelets, with melatonin made from it in the pineal. Enzymes sit beside the arrows, ★ = rate-limiting, ⊕/⊖ = activators/inhibitors (toggle), −ATP +NADH = what the step spends and makes (toggle ⚡), dashed = allosteric signal, shaded bands = compartment. Drag to pan.

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Catecholamines, serotonin & the tyrosine tree

TYROSINE — the branch point

Made from BH₄ (tetrahydrobiopterin) by BH₄ is the cofactor for phenylalanine hydroxylase

BH₄ is the cofactor for phenylalanine hydroxylase — Catecholamines, serotonin & the tyrosine tree

Hydroxylase number one of three.

Made from Phenylalanine (essential) by PHENYLALANINE HYDROXYLASE (PAH)

PHENYLALANINE HYDROXYLASE (PAH) · irreversible — Catecholamines, serotonin & the tyrosine tree

Cofactors: BH₄ → BH₂, O₂

Deficiency: PHENYLKETONURIA — ~1 in 15,000 and the commonest inborn error of amino acid metabolism; ~98% are PAH itself. Screened 24–48 h AFTER protein feeding has started (test too early and you get a false negative, exactly like the urea cycle newborn). Treat within 7–10 days. Maternal PKU syndrome: an uncontrolled mother causes fetal microcephaly and cardiac defects even in a fetus WITHOUT PKU

Tyrosine is normally made here, which is why tyrosine becomes conditionally essential — and in a PKU patient, effectively essential — once this step is blocked. Treatment is restriction PLUS supplementation.

Condition cards:Phenylketonuria (classic PKU) →Maternal PKU syndrome →

Becomes L-DOPA via TYROSINE HYDROXYLASE ★

TYROSINE HYDROXYLASE ★ rate-limiting · irreversible — Catecholamines, serotonin & the tyrosine tree

Cofactors: BH₄ → BH₂, O₂

The committed, rate-limiting step of catecholamine synthesis — abundant in CNS, sympathetic ganglia and adrenal medulla. Same BH₄ dependency as PAH, which is the whole point of the DHPR story.

Becomes Tyramine (from aged food) via Bacterial decarboxylation during fermentation and ageing

Bacterial decarboxylation during fermentation and ageing — Catecholamines, serotonin & the tyrosine tree

Not a human enzyme — this happens in aged cheese, cured and fermented meat, tap beer and some wine.

Becomes 4-Hydroxyphenylpyruvate via Tyrosine aminotransferase

Tyrosine aminotransferase — Catecholamines, serotonin & the tyrosine tree

Cofactors: PLP (vitamin B₆)

Deficiency: Tyrosinemia type II (oculocutaneous) — corneal and skin lesions from tyrosine crystals

The catabolic route, in liver.