Involved in siroheme-dependent heme b biosynthesis. Catalyzes the conversion of Fe-coproporphyrin III into heme by the oxidative decarboxylation of two propionate side chains. Fe-coproporphyrin III + 2 S-adenosyl-L-methionine = 2 5'-deoxyadenosine + 2 CO2 + heme b + 2 L-methionine Porphyrin-containing compound metabolism; protoheme biosynthesis. Belongs to the radical SAM superfamily.