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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

Heme synthesis (porphyrias) · Mitochondria (steps 1, 6–8) and cytosol (steps 2–5); ~85% in marrow erythroid precursors, the rest mostly liver (P450 turnover). Enzymes sit beside the arrows; a ★ marks the rate-limiting step. The map scrolls inside its panel.

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Heme synthesis (porphyrias)

Porphobilinogen (PBG)

Made from δ-Aminolevulinic acid (ALA) by ALA dehydratase (porphobilinogen synthase)

ALA dehydratase (porphobilinogen synthase) — Heme synthesis (porphyrias)

Cofactors: Zn²⁺

Inhibited by: LEAD (displaces zinc) — lead-sensitive step #1

Deficiency: ALA dehydratase-deficiency porphyria (rare, AR) — neurovisceral only; ALA ↑ with normal PBG, like lead poisoning

2 ALA → PBG. Cytosol.

Condition cards:Lead poisoning →ALA dehydratase deficiency porphyria →

Becomes Hydroxymethylbilane (linear tetrapyrrole) via PBG deaminase (hydroxymethylbilane synthase, HMBS)

PBG deaminase (hydroxymethylbilane synthase, HMBS) — Heme synthesis (porphyrias)

Deficiency: Acute intermittent porphyria (AD, ~50% activity) — colicky abdominal pain without peritoneal signs, vomiting, neuropsychiatric symptoms, motor neuropathy, tachycardia/hypertension; urine darkens to port-wine on standing; markedly ↑ urine PBG (and ALA); no photosensitivity; attacks after P450 inducers, OCPs/luteal phase, fasting; treat with IV hemin, IV glucose, givosiran

4 PBG → linear tetrapyrrole. Still BEFORE ring closure — accumulated ALA/PBG are not photoactive.

Condition cards:Acute intermittent porphyria (AIP) →