In oxyhemoglobin, the C-terminal residues of all four chains have complete freedom of rotation, and the penultimate tyrosines have partial freedom. These tyrosines spend only a fraction of their time in their bound position, between the H and F helices. In deoxyhemoglobin, each of the C-terminal residues is doubly anchored by salt-bridges. All four penultimate tyrosines are firmly anchored in pockets between the H and F helices. The conformation is such that the tyrosines cannot be displaced from their pockets without also displacing the C-terminal residues and rupturing their salt-bridges. This property is vital for the entire mechanism of interaction between subunits.
Return to tutorial