Uroporphyrinogen decarboxylase decarboxylates the acetic acid groups, converting them to methyl groups.
The physiologically significant substrate is uroporphyrinogen III.
The product is coproporphyrinogen III (pronounce), which is transported back to the mitochondria, where the remainder of heme synthesis occurs.
The substituent pattern in the coproporphyrinogen is MP MP MP PM.
Uroporphyrinogen decarboxylase also acts on uroporphyrinogen I, yielding coproporphyrinogen I. Coproporphyrinogen I has no known function, and its formation is thought to be a blind pathway.