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

O₂ addition to Fe changes the electronic configuration of the metal and decreases its radius. The oxygenated Fe fits in the porphyrin cavity, and therefore it moves towards the heme plane. Compare the
deoxygenated hemoglobin with
oxygenated hemoglobin . The comparison clearly shows the Fe displacement, as well as the shortening of the Fe-His bond. The movement of the His residue towards Fe affects the rest of the subunit: notice the change in the position of the His-containing helix. The conformational changes induced by O₂ binding to Fe affect the whole tetrameric structre. As the molecule changes from the
deoxygenated structure (form T), to the
oxygenated form (R), the αβ dimers slide relative to each other, decreasing the radius of the central cavity of hemoglobin. This has an important effect in the regulation of affinity towards O₂ : this cavity is lined by
positively charged groups, and in the T form it can hold a 2,3-bisphosphoglycerate (2,3-BPG) molecule. This negatively charged molecule forms very strong ionic bonds with the positively charged groups in the cavity of hemoglobin, and strongly hampers the flexibility of hemoglobin, thereby lowering its ability to bind O₂. BPG lowers Hb affinity to O₂ because it preferentially stabilizes form T. In the R form, the cavity becomes too small to hold BPG, which therefore no longer opposes O₂ binding.