An enzyme that catalyzes the decarboxylation of UROPORPHYRINOGEN III to coproporphyrinogen III by the conversion of four acetate groups to four methyl groups. It is the fifth enzyme in the 8-enzyme biosynthetic pathway of heme. Several forms of cutaneous porphyrias are results of this enzyme deficiency as in porphyria cutanea tarda; and HEPATOERYTHROPOIETIC PORPHYRIA.