As we have seen, the molecular basis of allosterism in haemoglobin and of cooperativity in oxygen binding and release by its 4 subunits is on the alteration of structure resulting from binding and release of oxygen.
We can specify how the interaction among subunits is altered by looking at the example we just studied: the salt bridges in deoxyhaemoglobin, which break as a result of oxygen binding; let's see for instance the bridge between His146 and Asp94:
(α2Lys40 :: β1His146 :: β1Asp94)
In the T form, as we have already seen, the conformation of beta chains allows His146 and Asp94 to form a salt bridge. Binding of oxygen produces a conformational change that breaks that bridge. A consequence is the decrease in pK of the imidazole group in His146, which leads to release of a proton (Bohr's effect).
Note that the bridge between the carboxy group in Asp94 and the imidazole ring in His146 requires the latter to be protonated.