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.

Zoom

Catecholamines, serotonin & the tyrosine tree

NOREPINEPHRINE

Made from DOPAMINE by 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.

Made from Tyramine (from aged food) by THE CHEESE REACTION — tyramine displaces stored norepinephrine

THE CHEESE REACTION — tyramine displaces stored norepinephrine — Catecholamines, serotonin & the tyrosine tree

Normally gut and hepatic MAO destroy dietary tyramine on FIRST PASS and almost none reaches the circulation. On an MAOI that route is blocked: tyramine survives, acts as an indirect sympathomimetic, and displaces norepinephrine out of sympathetic nerve terminals → hypertensive crisis. SSRIs leave MAO intact, so they do not do this — one of the main reasons they replaced MAOIs.

Condition cards:MAOI–tyramine hypertensive crisis ('cheese reaction') →

Becomes EPINEPHRINE via PNMT — phenylethanolamine N-methyltransferase

PNMT — phenylethanolamine N-methyltransferase — Catecholamines, serotonin & the tyrosine tree

Cofactors: SAM → SAH

Activated by: CORTISOL, arriving from the adrenal cortex

ADRENAL MEDULLA ONLY, which is why epinephrine is essentially an adrenal hormone rather than a CNS neurotransmitter. Cortisol draining from the cortex through the portal supply induces PNMT — a neat anatomical reason the medulla sits where it does.

Becomes Normetanephrine via COMT — norepinephrine → NORmetanephrine

COMT — norepinephrine → NORmetanephrine — Catecholamines, serotonin & the tyrosine tree

O-methylation. The 'nor' rides along from norepinephrine, which is the easiest way to keep the two metanephrines straight.