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.

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

BH₄ (tetrahydrobiopterin)

Made from BH₂ (dihydrobiopterin) by DIHYDROPTERIDINE REDUCTASE (DHPR) regenerates BH₄

DIHYDROPTERIDINE REDUCTASE (DHPR) regenerates BH₄ — Catecholamines, serotonin & the tyrosine tree

Cofactors: NADH → NAD⁺

Deficiency: DHPR deficiency — about 2% of hyperphenylalaninemia, and the reason a perfectly diet-controlled PKU patient can still decline. Diet addresses phenylalanine hydroxylase only; losing BH₄ stalls tyrosine hydroxylase and tryptophan hydroxylase too, so DOPAMINE, NOREPINEPHRINE AND SEROTONIN all fail — movement disorder plus seizures. The giveaway is elevated urinary biopterin after a phenylalanine load

Every time BH₄ is used by a hydroxylase it is oxidised to BH₂, and DHPR is what puts it back. Without DHPR, BH₄ is progressively used up and never replenished.

Condition cards:BH₄-deficient hyperphenylalaninemia (dihydropteridine reductase / DHPR deficiency, variant PKU) →

Becomes TYROSINE — the branch point via BH₄ is the cofactor for phenylalanine hydroxylase

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

Hydroxylase number one of three.

Becomes L-DOPA via BH₄ is ALSO the cofactor for tyrosine hydroxylase

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

Hydroxylase number two.

Becomes 5-Hydroxytryptophan (5-HTP) via BH₄ is the cofactor for TRYPTOPHAN HYDROXYLASE too — 3 of 3

BH₄ is the cofactor for TRYPTOPHAN HYDROXYLASE too — 3 of 3 — Catecholamines, serotonin & the tyrosine tree

Hydroxylase number three, and the one people forget. All three aromatic amino acid hydroxylases — phenylalanine, tyrosine and tryptophan — run on the same BH₄ pool and are regenerated by the same DHPR. That is why a DHPR defect silences dopamine, norepinephrine AND serotonin at once while a phenylalanine-restricted diet only addresses PAH.