Protoporphyrin is a cyclic tetrapyrrole that consists of porphyrin bearing four methyl substituents at positions 3, 8, 13 and 17, two vinyl substituents at positions 7 and 12 and two 2-carboxyethyl substituents at positions 2 and 18. The parent of the class of protoporphyrins. It has a role as a photosensitizing agent, a metabolite, an Escherichia coli metabolite and a mouse metabolite. It is a conjugate acid of a protoporphyrinate and a protoporphyrin(2-).