Involved in coproporphyrin-dependent heme b biosynthesis. Catalyzes the decarboxylation of Fe-coproporphyrin III (coproheme) to heme b (protoheme IX), the last step of the pathway. The reaction occurs in a stepwise manner with a three-propionate harderoheme intermediate (By similarity). When reconstituted with heme, can generate oxygen using chlorite or hydrogen peroxide as substrate (in vitro), but has very low affinity for hydrogen peroxide and chlorite and extremely low enzyme activity (PubMed:15965735). Fe-coproporphyrin III + 2 H(+) + 2 H2O2 = 2 CO2 + 4 H2O + heme b Fe-coproporphyrin III + H(+) + H2O2 = CO2 + 2 H2O + harderoheme III H(+) + H2O2 + harderoheme III = CO2 + 2 H2O + heme b Fe-coproporphyrin III acts as both substrate and redox cofactor (By similarity). Was originally thought to use heme as a cofactor (PubMed:15965735). kcat is 0.77 sec(-1) with chlorite and 0.52 sec(-1) with hydrogen peroxide. The kcat values are at least 1000 times lower than those observed for well-characterized chlorite dismutases, and the specific activity is even lower. Porphyrin-containing compound metabolism; protoheme biosynthesis. Homopentamer. Belongs to the ChdC family. Type 1 subfamily.