Involved in coproporphyrin-dependent heme b biosynthesis (PubMed:25908396). Catalyzes the decarboxylation of Fe-coproporphyrin III (coproheme) to heme b (protoheme IX), the last step of the pathway (By similarity). The reaction occurs in a stepwise manner with a three-propionate harderoheme intermediate (By similarity). Can stimulate the generation of protoporphyrin IX, but not coproporphyrin III, by HemY (PubMed:27597779). 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:25908396). Porphyrin-containing compound metabolism; protoheme biosynthesis. Belongs to the ChdC family. Type 1 subfamily.