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 comprises three steps, which may be:
Heme b³⁺ + O
₂ + NADPH + H⁺
→  α-meso-hydroxyheme³⁺ + NADP⁺
+ H
₂O
α-meso-hydroxyheme³⁺ + H⁺
+ O
₂ → verdoheme⁴⁺ + CO + H2O
verdoheme⁴⁺ + 7/2 NADPH + O
₂+ 3/2 H⁺
 → biliverdin + Fe²⁺ + 7/2 NADP⁺
+ H
₂O
The sum of these reactions is:
Heme b³⁺ + 3O
₂ + 9/2 NADPH + 7/2 H⁺
 → biliverdin + Fe²⁺ + CO + 9/2 NADP⁺
+ 3H
₂O
If the iron is initially in the +2 state, the reaction could be:
Heme b²⁺ + 3O₂ + 4 NADPH + 4 H⁺ →  biliverdin + Fe²⁺ + CO + 4 NADP⁺ + 3H₂O
This reaction can occur in virtually every cell; the classic example is the formation of a contusion, which forms different chromogens as it gradually heals: (red) heme to (green) biliverdin to (yellow) bilirubin.
 In terms of molecular mechanisms, the enzyme facilitates the intramolecular hydroxylation of one meso carbon centre in the heme.
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 comprises three steps, which may be:
Heme b³⁺ + O
₂ + NADPH + H⁺
→  α-meso-hydroxyheme³⁺ + NADP⁺
+ H
₂O
α-meso-hydroxyheme³⁺ + H⁺
+ O
₂ → verdoheme⁴⁺ + CO + H2O
verdoheme⁴⁺ + 7/2 NADPH + O
₂+ 3/2 H⁺
 → biliverdin + Fe²⁺ + 7/2 NADP⁺
+ H
₂O
The sum of these reactions is:
Heme b³⁺ + 3O
₂ + 9/2 NADPH + 7/2 H⁺
 → biliverdin + Fe²⁺ + CO + 9/2 NADP⁺
+ 3H
₂O
If the iron is initially in the +2 state, the reaction could be:
Heme b²⁺ + 3O₂ + 4 NADPH + 4 H⁺ →  biliverdin + Fe²⁺ + CO + 4 NADP⁺ + 3H₂O
This reaction can occur in virtually every cell; the classic example is the formation of a contusion, which forms different chromogens as it gradually heals: (red) heme to (green) biliverdin to (yellow) bilirubin.  In terms of molecular mechanisms, the enzyme facilitates the intramolecular hydroxylation of one meso carbon centre in the heme.
Heme oxygenase