Coproporphyrinogen I
Coproporphyrinogen I is an isomer of coproporphyrinogen III, a metabolic intermediate in the normal biosynthesis of heme. The compound is not normally produced by the human body; its production and accumulation causes a type of porphyria.
The difference between coproporphyrinogen I and III is the arrangements of the four carboxyethyl and the four methyl groups. The I isomer has the sequence MP-MP-MP-MP, whereas in the III isomer it is MP-MP-MP-PM, with the last two side chains reversed.Biosynthesys
Coproporphyrinogen I is not produced in the normal porphyrin biosynthesis pathway. However, if the enzyme uroporphyrinogen-III cosynthaseis missing or inactive, the compound uroporphyrinogen I is produced instead of uroporphyrinogen III. The enzyme uroporphyrinogen III decarboxylase will also act on the I isomer, producing coproporphyrinogen I:
The reaction entails the conversion of the four carboxymethyl side chains to methyl groups, with release of four molecules of carbon dioxide.
Unlike the III isomer, coproporphyrinogen I is not further processed by the body, and accumulates. This situation occurs in the pathological condition called congenital erythropoietic porphyria.
