Involved in heme d1 biosynthesis. Catalyzes the decarboxylation of siroheme into didecarboxysiroheme. 2 H(+) + siroheme = 12,18-didecarboxysiroheme + 2 CO2 Porphyrin-containing compound metabolism. Probably forms a complex composed of NirD, NirL, NirG and NirH. All proteins are required for the total conversion of siroheme to didecarboxysiroheme. Insertional mutagenesis results in the loss of respiratory nitrite reductase activity in vivo and in vitro. Mutant strains synthesize a periplasmic cytochrome cd1 that lacks heme d1. Belongs to the Ahb/Nir family.