β4 Hemoglobin
(based on 1cbl.pdb)
The nonfunctional β4 hemoglobin is shown above. Its proximal histidine residues are colored in cyan and the heme groups are colored in the cpk format.
The β4 hemoglobin is unable to form these salt bridges and therefore is stuck in a state of high oxygen affinity. In this protein His 146 interacts weakly with the other subunits and no bond can be formed with Asp 94 (pink). The His 146 and Lys 40 salt bridge between the beta and alpha subunits cannot be formed either in β4 hemoglobin because Gln 39 in the β4 hemoglobin takes the place of Lys 40 in the normal hemoglobin. Gln 39 and His 146 (pink) are incapable of forming a bond. The β4 hemoglobin, therefore, is stuck in a state of high oxygen affinity.
In β4 hemoglobin, the exposure of the residue at the C3 position would be unfavorable because Trp 37 (hot pink) obtains the C3 position. Tryptophan is a hydrophobic residue. Therefore, the transition of the β4 hemoglobin to a T state is inhibited by the potential exposure of a hydrophobic residue.
In the β4 hemoglobin, Val 67 (green) does not block the ligand binding pocket. This increases the β4 hemoglobin's affinity for oxygen.
Here is a simplified version of the previous model.
In the β4 hemoglobin, a T state could not be stabilized due to the alternative conformations of Arg 40 and Arg 99. Arg 40 and Arg 99 (salmon/orange) are too close to one another to form an ionic interaction.
Here is a simplified version of the previous model.

Primary Citation: Borgstahl, G.e.o., and A. Arnone. "The 1.8 Angstrom Structure Of Carbonmonoxy- β4 Hemoglobin: Analysis Of A Homotetramer With The R Quaternary Structure Of Liganded α2β2 Hemoglobin." Journal of Molecular Biology 236 (1994): 831-43. Print.

    HbA Hemoglobin
(based on 2hhb.pdb)
The normal α2β2 hemoglobin is shown above in a deoxygenated T state. Its proximal histidine residues are colored in cyan and the heme groups are colored in the cpk format.
The Bohr effect is essential to the functionality of hemoglobin in transporting oxygen. The normal α2β2 hemoglobin can switch between a state of high oxygen affinity (R state) and a state of low oxygen affinity (T state) with changes in the environment's pH and carbon dioxide concentration. A low pH and high carbon concentration encourage the formation of salt bridges in the hemoglobin which transition the protein from a state of high oxygen affinity to low oxygen affinity. The protein may release the bound oxygen as a result. The model above shows the salt bridges between Asp 94 and His 146 of the β subunits and between His 146 of the β subunits and Lys 40 of the α subunits (pink).
In the normal hemoglobin the transition from R state to T state causes the exposure of the residue at the C3 position on the β subunits. In normal hemoglobin, Thr 38 (hot pink), a hydrophilic residue takes the C3 position. Therefore, the transition to a T state is favored by the exposure of a hydrophilic residue.
In normal hemoglobin, Val 67 (green) on the β subunits inhibits the binding of oxygen by blocking the ligand binding pocket. This helps to decrease the oxygen affinity of the T state hemoglobin.
Here is a simplified version of the previous model.
In normal hemoglobin, the T state is stabilized by ionic bonds between Arg 40 and Arg 99 on the β subunits (salmon).
Here is a simplified version of the previous model.