Probably acts as a heme chaperone, transferring heme to the NarI subunit of the respiratory enzyme nitrate reductase; transfer may be stimulated by NADH. Binds one molecule of heme per monomer, possibly covalently. Heme binding is not affected by either [4Fe-4S] or S-adenosyl-L-methionine (SAM)-binding. Does not have coproporphyrinogen III dehydrogenase activity in vitro (PubMed:29282292). Binds 1 [4Fe-4S] cluster. The cluster is coordinated with 3 cysteines and an exchangeable S-adenosyl-L-methionine (Probable). Binding of the [4Fe-4S] cofactor promotes dimerization. Very slightly decreased anaerobic growth in minimal medium with KNO(3) and glycerol. Might carry two S-adenosyl-L-methionine binding sites with only one binding to the iron-sulfur cluster. Belongs to the anaerobic coproporphyrinogen-III oxidase family. HemW subfamily.