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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)

Uroporphyrinogen III

Made from Hydroxymethylbilane (linear tetrapyrrole) by 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) →

Becomes Coproporphyrinogen III via Uroporphyrinogen decarboxylase (UROD)

Uroporphyrinogen decarboxylase (UROD) — Heme synthesis (porphyrias)

Deficiency: Porphyria cutanea tarda (the MOST common porphyria; often acquired with hepatitis C, alcohol, estrogen, iron overload) — blisters and skin fragility on sun-exposed skin, hypertrichosis, onset 4th–5th decade; tea-coloured urine with ↑ uroporphyrin that fluoresces coral-pink under Wood's lamp; no acute attacks; treat with phlebotomy, low-dose hydroxychloroquine, treat HCV

4 acetate → methyl. Cytosol.

Condition cards:Porphyria cutanea tarda (PCT) →