Catalyzes the oxidative degradation of the heme macrocyclic porphyrin ring in the presence of a suitable electron donor such as ascorbate or NADPH--cytochrome P450 reductase, with subsequent release of free iron. 3 AH2 + 2 H(+) + heme b + 3 O2 = 3 A + Fe(2+) + 3 H2O + mycobilin a 3 AH2 + 2 H(+) + heme b + 3 O2 = 3 A + Fe(2+) + 3 H2O + mycobilin b Homodimer. MhuD has the ability to bind 2 heme molecules per monomer but only the monoheme-protein complex is active. Belongs to the antibiotic biosynthesis monooxygenase family.