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. The left panel shows a deoxygenated hemoglobin subunit, while the right panel shows an oxygenated hemoglobin subunit in the same orientation. 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.
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