Key enzyme in heme biosynthesis. Catalyzes the oxidative decarboxylation of propionic acid side chains of rings A and B of coproporphyrinogen III (By similarity). coproporphyrinogen III + 2 H(+) + O2 = 2 CO2 + 2 H2O + protoporphyrinogen IX Porphyrin-containing compound metabolism; protoporphyrin-IX biosynthesis; protoporphyrinogen-IX from coproporphyrinogen-III (O2 route): step 1/1. Porphyrin-containing compound metabolism; chlorophyll biosynthesis. Homodimer. Belongs to the aerobic coproporphyrinogen-III oxidase family.