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. heme b + 3 O2 + 3 reduced [NADPH--hemoprotein reductase] = biliverdin IXalpha + CO + Fe(2+) + H(+) + 3 H2O + 3 oxidized [NADPH--hemoprotein reductase] Homodimer. Constitutively expressed. Neither hmoB single mutant nor the hmoA hmoB double mutant display robust and reproducible phenotypes relative to the wild-type. Belongs to the antibiotic biosynthesis monooxygenase family.