Catalyzes the sequential decarboxylation of the four acetate side chains of uroporphyrinogen to form coproporphyrinogen and participates in the fifth step in the heme biosynthetic pathway. Isomer I or isomer III of uroporphyrinogen may serve as substrate, but only coproporphyrinogen III can ultimately be converted to heme. In vitro also decarboxylates pentacarboxylate porphyrinogen I. 4 H(+) + uroporphyrinogen III = 4 CO2 + coproporphyrinogen III 4 H(+) + uroporphyrinogen I = 4 CO2 + coproporphyrinogen I Porphyrin-containing compound metabolism; protoporphyrin-IX biosynthesis; coproporphyrinogen-III from 5-aminolevulinate: step 4/4. Homodimer. Belongs to the uroporphyrinogen decarboxylase family.