heme /hēm/ [Gk, haima, blood] , the pigmented iron-containing nonprotein part of the hemoglobin molecule. There are four heme groups in a hemoglobin molecule, each consisting of a cyclic structure of four pyrrole residues, called protoporphyrin, and an iron ion in the center. Heme binds and carries oxygen in the red blood cells, releasing it to tissues. Also spelled haeme. See also hemoglobin, porphobilinogen, protoporphyrin.
