hemoglobin
Hemoglobin is the iron-containing oxygen-transport protein in the red blood cells of all vertebrates. The hemoglobin molecule is an assembly of four globular protein subunits, each tightly associated with a non-protein heme group. The heme group binds to the iron ion. The most common hemoglobin is hemoglobin A, a tetramer consisting of two alpha and two beta subunits, denoted as 22.