Involved in heme d1 biosynthesis (PubMed:8982003, PubMed:25204657). Catalyzes the introduction of a double bond into the propionate side chain of pyrrole ring D of dihydro-heme d1, therefore converting dihydro-heme d1 to heme d1 (PubMed:25204657). A + dihydro-heme d1 = AH2 + heme d1 Porphyrin-containing compound metabolism. Monomer. Consists of an N-terminal domain that binds the heme c cofactor, followed by a long linker segment, and a C-terminal region that exhibits an eight-bladed heme d1-binding beta-propeller domain. The mutant exhibits decreased dissimilatory nitrite reductase (NIR) activity (PubMed:8982003). In the absence of this gene, the formation of the acrylic double bond of heme d1 does not take place and dihydro-heme d1 is inserted into the nitrite reductase NirS (PubMed:25204657). Belongs to the cytochrome c family.