Heme oxygenase
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Heme oxygenase (HO) is an enzyme that catalyzes the degradation of heme. This produces biliverdin, iron, and carbon monoxide.
Reaction
Heme oxygenase cleaves the heme ring at the alpha-methene bridge to form either biliverdin or, if the heme is still attached to a globin, verdoglobin. Biliverdin is subsequently converted to bilirubin by biliverdin reductase.
The reaction occurs as follows:
- Heme + 3 AH2 + 3 O2 → biliverdin + Fe2+ + CO + 3A + 3 H2O
Biliverdin.png
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This reaction can occur in virtually every cell; the classic example is the formation of a bruise, which goes through different colors as it gradually heals: red heme to green biliverdin to yellow bilirubin. Under normal physiological conditions, the activity of heme oxygenase is highest in the spleen, where old erythrocytes are sequestrated and destroyed.
Isoforms
There are three known isoforms of heme oxygenase.
Heme oxygenase 1 (HO-1) is an inducible isoform in response to stress such as oxidative stress, hypoxia, heavy metals, cytokines, etc. Heme oxygenase 2 (HO-2) is a constitutive isoform which is expressed under homeostatic conditions. Both HO-1 and HO-2 are ubiquitously expressed and catalytically active.
A third heme oxygenase (HO-3) is not catalytically active, but is thought to work in oxygen sensing.
See also
External links
Oxidoreductases: dioxygenases, including steroid hydroxylases (EC 1.14) | |
|---|---|
| 1.14.11 - 2-oxoglutarate | Prolyl hydroxylase - Lysyl hydroxylase |
| 1.14.13 - NADH or NADPH | Flavin-containing monooxygenase - Nitric oxide synthase - Cholesterol 7 alpha-hydroxylase - Methane monooxygenase - 3A4 -51A1 |
| 1.14.14 - reduced flavin or flavoprotein | 19A1 - 2D6 - 2E1 |
| 1.14.15 - reduced iron-sulfur protein | 11B1 - 11B2 - 11A1 |
| 1.14.16 - reduced pteridine | Phenylalanine hydroxylase - Tyrosine hydroxylase - Tryptophan hydroxylase |
| 1.14.17 - reduced ascorbate | Dopamine beta hydroxylase |
| 1.14.18-19 - other | Tyrosinase - Stearoyl-CoA desaturase-1 |
| 1.14.99 - miscellaneous | Cyclooxygenase - Heme oxygenase (HMOX1) - Squalene monooxygenase - 17A1 - 21A2 |
Metabolism: Porphyrin enzymes | |
|---|---|
| Porphyrin biosynthesis | initial mitochondrial: Aminolevulinic acid synthase - Porphobilinogen synthase
cytosolic: Porphobilinogen deaminase - Uroporphyrinogen III synthase - Uroporphyrinogen III decarboxylase - Coproporphyrinogen III oxidase terminal mitochondrial: Protoporphyrinogen oxidase - Ferrochelatase |
| Heme breakdown to bile | spleen: Heme oxygenase - Biliverdin reductase liver: UDP-glucuronosyltransferase |
| see also disorders, intermediates | |
Acknowledgement and Attribution Regarding Sources of Content
Some of the initial content on this page may be incorporated in part from copyleft sources in the public domain including wikis such as Wikipedia and AskDrWiki. Drug information for patients came from the The National Library of Medicine. Infectious disease information may have come from the Centers for Disease Control (CDC). Differential Diagnoses are drawn from clinicians as well as an amalgamation of 3 sources: 1.The Disease Database; 2. Kahan, Scott, Smith, Ellen G. In A Page: Signs and Symptoms. Malden, Massachusetts: Blackwell Publishing, 2004:3; 3. Sailer, Christian, Wasner, Susanne. Differential Diagnosis Pocket. Hermosa Beach, CA: Borm Bruckmeir Publishing LLC, 2002:7 .
