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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

DOPAMINE

Made from L-DOPA by DOPA DECARBOXYLASE (aromatic L-amino acid decarboxylase, AADC)

DOPA DECARBOXYLASE (aromatic L-amino acid decarboxylase, AADC) — Catecholamines, serotonin & the tyrosine tree

Cofactors: Vitamin B₆ (PLP), CO₂ released

Inhibited by: CARBIDOPA — peripheral only, does not cross the blood–brain barrier

Deficiency: Parkinson disease — idiopathic loss of the dopaminergic neurons of the SUBSTANTIA NIGRA: bradykinesia, rigidity, resting tremor

Why L-DOPA rather than dopamine is the Parkinson's drug: dopamine cannot cross the blood–brain barrier but L-DOPA can, and DOPA decarboxylase then makes dopamine inside the CNS. Carbidopa inhibits the same enzyme in the PERIPHERY only, so more L-DOPA survives to reach the brain and the peripheral side effects drop.

Condition cards:Parkinson disease →

Becomes NOREPINEPHRINE via DOPAMINE β-MONOOXYGENASE (dopamine β-hydroxylase)

DOPAMINE β-MONOOXYGENASE (dopamine β-hydroxylase) — Catecholamines, serotonin & the tyrosine tree

Cofactors: Vitamin C (ascorbate), O₂

Happens inside the storage vesicle. Ascorbate is the reductant — one of the few places a vitamin C requirement is testable. Written β-monooxygenase or β-hydroxylase depending on the source; same enzyme.

Becomes Homovanillic acid (HVA) via MAO + COMT — dopamine's end product is HOMOVANILLIC ACID

MAO + COMT — dopamine's end product is HOMOVANILLIC ACID — Catecholamines, serotonin & the tyrosine tree

Monoamine oxidase does oxidative deamination, catechol-O-methyltransferase does O-methylation. Dopamine ends as HVA, NOT VMA — VMA comes from norepinephrine and epinephrine only. A very common mix-up, and HVA is the marker that goes with neuroblastoma.