Involved in siroheme-dependent heme b biosynthesis. Catalyzes the conversion of didecarboxysiroheme into Fe-coproporphyrin III by oxidative loss of two acetic acid side chains. 12,18-didecarboxysiroheme + 2 AH2 + 2 S-adenosyl-L-methionine = 2 5'-deoxyadenosine + 2 A + 2 acetate + Fe-coproporphyrin III + 2 H(+) + 2 L-methionine Porphyrin-containing compound metabolism; protoheme biosynthesis. Belongs to the radical SAM superfamily.