From NuGOwiki

A porphyrin is a a cyclic macromolecule made from 4 pyrrole subunits (a five-membered ring with the formula C4H5N) linked on opposite sides through 4 -CH bridges. The molcule is therefore completely aromatic. Porphyrins combine readily with metals, coordinating with them in the central cavity. Some iron-containing porphyrins are called hemes, like hemoglobin. Please find more on porphyrins on the Wikipedia link.