Hemoglobin
Pechik, I., Ji, X., Fidelis, K., Karavitis, M., Moult, J., Brinigar, W.S., Fronticelli, C., Gilliland, G.L. (1996) Biochemistry 35: 1935-1945

The deoxyhemoglobin structure is shown in the right panel. As in the study of secondary structure, its inspection can be made easier by eliminating the sidechains and observing only its
backbone.
Helixes schematical drawing Hemoglobin contains two α chains and two β chains, i.e. four
monomers. Each monomer carries one
heme close to its surface. O₂ binds to these hemes.

Detailed view of the heme

Heme contains an organic moiety (protoporphyrin IX) and an iron atom. In the deoxygenated heme shown, the iron atom is above the heme plane, rather than aligned with it. This happens because its radius is larger than the inner cavity in the protoporphyrin.

The heme is anchored to the subunit through a histidine residue. Four other coordination positions are occupied by the porotoporphyrin nitrogen atoms. Since iron has six coordinating positions, this leaves still one vacant position available for oxygen binding.