Coproporphyrinogen III is a metabolic intermediate in the biosynthesis of many compounds that are critical for living organisms, such as hemoglobin and chlorophyll.

The coproporphyrinogens have the outermost hydrogen atoms of the core replaced by four methyl groups −CH3 (M) and four propionic acid groups −CH2−CH2−COOH (P).

In coproporphyrogen III, the order around the outer ring is MP-MP-MP-PM.

In the main porphyrin biosynthesis pathway, coproporphyrinogen III is derived from uroporphyrinogen III by the action of the enzyme uroporphyrinogen III decarboxylase: The conversion entails four decarboxylations, which turn the four acetic acid groups −CH2−COOH into methyl groups −CH3, with release of four carbon dioxide molecules.

[1][2] Coproporphyrinogen III is further used as a substrate for the enzyme coproporphyrinogen III oxidase which oxidizes and further decarboxylates it to protoporphyrinogen IX.