protoporphyrin type III

pro·to·por·phy·rin type III

(prō'tō-pōr'fi-rin),
The principal protoporphyrin found in nature (one of 15 possible isomers), characterized by the presence of four methyl groups, two vinyl groups, and two propionic acid side chains; a porphyrin derivative that, with iron, forms the heme of hemoglobin and the prosthetic groups of myoglobin, catalase, cytochromes, etc.