Involved in heme degradation and iron utilization under anaerobic conditions. Catalyzes a radical-mediated mechanism facilitating iron liberation and the production of the tetrapyrrole product anaerobilin. Can use heme, mesoheme and deuteroheme as substrates. heme b + 2 reduced [flavodoxin] + 2 S-adenosyl-L-methionine = 5'-deoxyadenosine + anaerobilin + Fe(2+) + L-methionine + 2 oxidized [flavodoxin] + S-adenosyl-L-homocysteine Binds 1 [4Fe-4S] cluster. The cluster is coordinated with 3 cysteines and an exchangeable S-adenosyl-L-methionine. Inhibited by exposure to molecular oxygen. Belongs to the anaerobic coproporphyrinogen-III oxidase family. ChuW/HutW subfamily.