Involved in heme d1 biosynthesis (PubMed:29076625). Radical SAM enzyme that catalyzes the removal of two propionate side chains from the intermediate 12,18-didecarboxysiroheme (DDSH) and may introduce the keto functions on rings A and B, yielding the heme d1 precursor dihydro-heme d1 (PubMed:29076625). Binds 2 [4Fe-4S] clusters per subunit. Porphyrin-containing compound metabolism. Belongs to the radical SAM superfamily.