Hemoglobin. Molecular graphic of the hemoglobin molecule with a hem group highlighted in color. Most of the molecule appears transparent. The carbon chain around the hem group is colored red, nitrogen atoms in the porphyrin ring are yellow and the central iron atom is silver. Hemoglobin is the oxygen-carrying pigment in red blood cells. It consists of four globin polypeptides, each of which carries a hem group with a central iron atom, which binds reversibly to oxygen. The porphyrin ring around the iron atom is flat to give oxygen molecules easy access.