Heme oxygenase
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Heme oxygenase (HO) is an enzyme that catalyzes the degradation of heme. This results in the degradation of heme and the production of iron, carbon monoxide and biliverdin. Heme oxygenase cleaves the heme ring at the alpha methene bridge to form biliverdin. Biliverdin is subsequently converted to bilirubin by biliverdin reductase.
It catalyzes the following chemical reaction:
Heme + 3 AH2 + 3 O2 = biliverdin + Fe2+ + CO + 3 A + 3 H2O
This reaction can occur in virtually every cell, and the classic example is the formation of bruise and the changing colours which develop during its sequential disappearance (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.
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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 thought to perhaps work in oxygen sensing.
