Uroporphyrinogen decarboxylase !

Uroporphyrinogen decarboxylase removes four carboxyls from the ring; when it fails in the liver, porphyria cutanea tarda appears.

Updated 2026-11-01

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Key facts

Uroporphyrinogen III → coproporphyrinogen III + 4 CO₂, in the cytosol.
It decarboxylates the four acetate side chains (turning them into methyls) in order, with no cofactors or metals.
It is a dimer; structure 1R3Y shows the product, coproporphyrinogen III, in the active-site cleft.
In an iron-loaded liver an inhibitor (uroporphomethene) forms from the substrate itself.

Liver iron, alcohol, estrogens, smoking and hepatitis C favour the oxidative stress that inhibits it.

Porphyria cutanea tarda: most cases are acquired (type 1); about a quarter carry a heterozygous UROD mutation (type 2).
Two mutated copies cause hepatoerythropoietic porphyria, severe and starting in childhood.
Uroporphyrin and seven-carboxyl porphyrins build up in urine, which looks pinkish and fluoresces under Wood light.

Think of the answer, then tap the question to see it.

Which porphyria results from low liver uroporphyrinogen decarboxylase activity?

Porphyria cutanea tarda, the most common, with blisters on sun-exposed skin.

Why does iron overload worsen porphyria cutanea tarda?

Iron promotes oxidation of the substrate to uroporphomethene, which inhibits the enzyme; that is why phlebotomy helps.

Which chemical groups does this enzyme remove?

Four carboxyls from the acetate side chains, which are left as methyls.

Educational information, not medical advice. For any symptom, see a health professional.

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