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 intermediate. 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. Porphyrin-containing compound metabolism; protoheme biosynthesis. Homopentamer. Belongs to the ChdC family. Type 1 subfamily.