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

# Heme oxygenase

Heme oxygenase breaks the heme ring, releasing biliverdin, iron and carbon monoxide.

Updated 2026-11-01

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## Key facts

- Heme + 3 O₂ + electrons from NADPH → biliverdin IXα + Fe²⁺ + CO.
- It is anchored to the endoplasmic reticulum and gets its electrons from NADPH–cytochrome P450 reductase.
- It cuts the α bridge of the ring; it is the main source of endogenous CO (that is why exhaled CO rises in hemolysis).
- HO-1 is abundant in spleen and liver macrophages; HO-2 is constitutive in brain and testis.

**Also called**
HO-1 (inducible), HO-2 (constitutive)

**EC number**
[1.14.14.18](https://www.enzyme-database.org/query.php?ec=1.14.14.18)

**Genes**
HMOX1, HMOX2

**Structure (PDB)**
[1N45](https://www.rcsb.org/structure/1N45)

## Regulation

- HO-1 is induced by its substrate (heme), oxidative stress, hypoxia, heavy metals and inflammation (via Nrf2).

## Inhibitors and toxins

- Tin and zinc protoporphyrins (competitive)

## Clinical notes

- Its products are protective: biliverdin and bilirubin are antioxidants, and CO has vasodilating and anti-inflammatory effects.
- HO-1 deficiency (very rare) causes severe hemolysis, inflammation and endothelial damage.
- In hemolysis it works harder: more unconjugated bilirubin, more exhaled CO and more recycled iron.

## Pathways

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

## Related conditions

- [Sickle cell disease](https://durgical.com/en/conditions/sickle-cell-disease/)
- [Neonatal jaundice](https://durgical.com/en/conditions/neonatal-jaundice/)

## Related

- [Heme](https://durgical.com/en/biochemistry/molecules/heme/)
- [Biliverdin reductase](https://durgical.com/en/biochemistry/enzymes/biliverdin-reductase/)
- [Bilirubin](https://durgical.com/en/biochemistry/molecules/bilirubin/)
- [Iron metabolism](https://durgical.com/en/biochemistry/topics/iron-metabolism/)
- [Hemoglobin and 2,3-BPG](https://durgical.com/en/biochemistry/enzymes/hemoglobin-and-2-3-bpg/)

## Test yourself

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

## What are the three products of heme oxygenase?

Biliverdin, Fe²⁺ and carbon monoxide.

## Why does exhaled CO rise in hemolysis?

Because more heme is broken down, and heme oxygenase is the body's main source of CO.

## Which heme oxygenase isoform is inducible?

HO-1, induced by heme, oxidative stress and inflammation; HO-2 is constitutive.

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

Sources: [HMOX1: Heme oxygenase 1 (human)](https://www.uniprot.org/uniprotkb/P09601/entry) (UniProt, 2026); [1N45: X-RAY CRYSTAL STRUCTURE OF HUMAN HEME OXYGENASE-1 (HO-1) IN COMPLEX WITH ITS SUBSTRATE HEME](https://www.rcsb.org/structure/1N45) (RCSB Protein Data Bank, 2026); [Reactome | Heme degradation](https://reactome.org/PathwayBrowser/#/R-HSA-189483) (Reactome, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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Representative model, for educational purposes only.

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