Each hemoglobin contains

Hemoglobin is a globular molecule made up of four subunits. Each subunit contains a heme moiety conjugated to a polypeptide. Heme is an iron-containing porphyrin derivative. The polypeptides are referred to collectively as the globin poion of the hemoglobin molecule. There are two pairs of polypeptides in each hemoglobin molecule. Reference: Ganong's review of Medical Physiology; 25th ed; Page: 557