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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)

Hydroxymethylbilane (linear tetrapyrrole)

Made from Porphobilinogen (PBG) by 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) →

Becomes Uroporphyrinogen III via Uroporphyrinogen III synthase (cosynthase)

Uroporphyrinogen III synthase (cosynthase) · irreversible — Heme synthesis (porphyrias)

Deficiency: Congenital erythropoietic porphyria (Günther, AR) — hydroxymethylbilane cyclises non-enzymatically into the dead-end type I isomer; severe photosensitivity and mutilation from infancy, red urine, erythrodontia

RING CLOSURE — the line that separates neurovisceral (before) from cutaneous (after) porphyrias.

Condition cards:Congenital erythropoietic porphyria (Gunther disease, CEP) →