[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Biochemistry](https://durgical.com/en/biochemistry/) › Porphobilinogen deaminase

[Biochemistry](https://durgical.com/en/biochemistry/) · Enzyme

# Porphobilinogen deaminase

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

Updated 2026-11-01

Loading the 3D model…

## 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.

**Also called**
Hydroxymethylbilane synthase (HMBS), PBG deaminase

**EC number**
[2.5.1.61](https://www.enzyme-database.org/query.php?ec=2.5.1.61)

**Genes**
HMBS

**Structure (PDB)**
[3ECR](https://www.rcsb.org/structure/3ECR)

## Regulation

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

## Inhibitors and toxins

- No inhibitors in clinical use

## Clinical notes

- 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.

## Pathways

- [Heme synthesis](https://durgical.com/en/biochemistry/pathways/heme-synthesis/)

## Related conditions

- [Acute intermittent porphyria](https://durgical.com/en/conditions/acute-intermittent-porphyria/)

## Related

- [Porphobilinogen](https://durgical.com/en/biochemistry/molecules/porphobilinogen/)
- [ALA dehydratase](https://durgical.com/en/biochemistry/enzymes/ala-dehydratase/)
- [Uroporphyrinogen decarboxylase](https://durgical.com/en/biochemistry/enzymes/uroporphyrinogen-decarboxylase/)
- [ALA synthase](https://durgical.com/en/biochemistry/enzymes/ala-synthase/)

## Test yourself

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.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/biochemistry/enzymes/porphobilinogen-deaminase/)

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

Sources: [HMBS: Porphobilinogen deaminase (human)](https://www.uniprot.org/uniprotkb/P08397/entry) (UniProt, 2026); [HMBS gene: MedlinePlus Genetics](https://medlineplus.gov/genetics/gene/hmbs/) (MedlinePlus, US National Library of Medicine, 2026); [3ECR: Structure of human porphobilinogen deaminase](https://www.rcsb.org/structure/3ECR) (RCSB Protein Data Bank, 2026); [Porphyria: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/porphyria/) (MedlinePlus, US National Library of Medicine, 2026).

[← All of biochemistry](https://durgical.com/en/biochemistry/)

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

---
Source: https://durgical.com/en/biochemistry/enzymes/porphobilinogen-deaminase/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/enzymes/porphobilinogen-deaminase/