Porphobilinogen deaminase !

Porphobilinogen deaminase strings four pyrroles together; its deficiency causes acute intermittent porphyria.

Updated 2026-11-01

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Key facts

4 porphobilinogens + H₂O → hydroxymethylbilane + 4 NH₃, in the cytosol.
It uses its own cofactor, dipyrromethane (two pyrroles attached to a cysteine), as the primer on which the chain grows.
It has the lowest activity in the pathway: half (one mutated allele) can already lead to attacks when ALAS1 is induced.
The HMBS gene has an erythroid promoter and a housekeeping one: some mutations affect only the liver form.

It is not acutely regulated; its low activity turns it into a bottleneck when ALAS1 is induced.

Acute intermittent porphyria: autosomal dominant inheritance with low penetrance; most carriers never have an attack.
During an attack ALA and porphobilinogen build up: high urine porphobilinogen is the key test.
Enzyme activity in red cells and HMBS gene testing confirm the diagnosis and allow family screening.

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

Which disease does porphobilinogen deaminase deficiency cause?

Acute intermittent porphyria, autosomal dominant, with attacks of abdominal pain and neuropathy.

How many porphobilinogens does the enzyme link, and what does it release?

Four, in a linear chain (hydroxymethylbilane), releasing four ammonia molecules.

Why do most carriers of an HMBS mutation have no symptoms?

Half the activity is enough at rest; attacks come only when a trigger induces ALAS1.

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

Sources: HMBS: Porphobilinogen deaminase (human) (UniProt, 2026); HMBS gene: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); 3ECR: Structure of human porphobilinogen deaminase (RCSB Protein Data Bank, 2026); Porphyria: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026).

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Representative model, for educational purposes only.