Porphyrin

Porphyrins are a "group of compounds containing the porphin structure, four pyrrole rings connected by methine bridges in a cyclic configuration to which a variety of side chains are attached. The nature of the side chain is indicated by a prefix, as uroporphyrin, hematoporphyrin, etc. The porphyrins, in combination with iron, form the heme component in biologically significant compounds such as hemoglobin and myoglobin." Heme is also a component of cytochrome P-450.

Porphyrias are metabolic diseases in the heme biosynthetic pathway characterized by buildup of porphyrin precursors such as porphobilinogen (PBG).