A heme (American English) or haem (British English) is a prosthetic group that consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin. Not all porphyrins contain iron, but a substantial fraction of porphyrin-containing metalloproteins have heme as their prosthetic subunit; these are known as hemoproteins. Types Major hemes There are several biologically important kinds of heme: Heme a Heme b Heme c Heme o PubChem number 7888115 444098… (More on Heme)