Allows bacterial pathogens to use the host heme as an iron source. Catalyzes the oxidative degradation of the heme macrocyclic porphyrin ring to the oxo-bilirubin chromophore staphylobilin (a mixture of the linear tetrapyrroles 5-oxo-delta-bilirubin and 15-oxo-beta-bilirubin) in the presence of a suitable electron donor such as ascorbate or NADPH--cytochrome P450 reductase, with subsequent release of free iron. 5 AH2 + 2 H(+) + heme b + 4 O2 = 5 A + delta-staphylobilin + Fe(2+) + formaldehyde + 4 H2O 5 AH2 + 2 H(+) + heme b + 4 O2 = 5 A + beta-staphylobilin + Fe(2+) + formaldehyde + 4 H2O Homodimer. IsdG seems to carry out oxygenation of the heme without the assistance of any of the prosthetic groups or cofactors normally associated with activation of molecular oxygen. Belongs to the antibiotic biosynthesis monooxygenase family. Heme-degrading monooxygenase IsdG subfamily.