And the purple circle is that metal atom. That Fe atom. So on one side of the protoporphine plane, the iron atom is bound to the histodine residue of that polypeptide chain. That polypeptide chain can be part of the myoglobin or it can be part of the hemoglobin molecule. So because each polypeptide chain contains a single heme group, myoglobin contains a single heme group. But Hemoglobin contains four different heme groups.