 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.
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