So we have polypeptide subunit one, subunit two, subunit three and subunit four. So we have two alpha and two beta subunits to form this tetromer molecule. Now, inside each one of these subunits we have a helper prosthetic group we call the heme group. And the heme group is responsible for actually binding the oxygen via an oxidation reduction reaction. So we have heme group one, hemegroup two, heme group three and heme group four. And these heme groups can bind a single oxygen molecule each.