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. And that means Myoglobin can only bind onto one oxygen, while Hemoglobin can bind four different oxygen atoms, as we'll see in just a moment. So on the bottom of that fe, we have the bond form between the nitrogen of this histogine residue and this metal atom found in that heme group. Now, in this particular state, the electron density around that fe is simply too large for that fe to actually fit inside the center of that plane.