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Opening image: Human deoxyHb. The C-terminal residues are shown in magenta, the β subunits in blue tones.
The hemoglobin molecule exists in two conformational states, T and R, which are in dynamic equilibrium. As this lesson illustrates, C-terminal ion pairs are required to maintain the T (deoxy) state.
Click to view down the twofold rotation axis. The four C-termini are located at the outside edge of each subunit interface.
Each C-terminal residue forms two ion pairs when hemoglobin is in the T (deoxy) state. The binding of one oxygen molecule to one of the αβ units causes a movement that disrupts these ion pairs, allowing the tetramer to shift from the deoxy to the oxy conformation. Release of oxygen allows the hemoglobin molecule to reform the ion pairs and resume the deoxy conformation.
We shall begin with His 146β, the C-terminal residue of the β chain. This residue forms two ion pairs, one with Asp 94β of the same chain, and the other with Lys 40α.
The intrachain ion pair between the side chains of His 146β and Asp 94β. spacefill.
Cross-linking of α and β chains by an ion pair between the ε amino group of Lys 40α and the carboxyl group of His 146β. spacefill.
Toggle between and interfaces. By symmetry, both interfaces have the same arrangement of ion pairs.
Click to return to 100% zoom. Toggle between and . The shift from the deoxy- to oxy- conformation requires rupture of the ion pairs to His 146β.
The role of Arg 141α, the C-terminal residue of the α chain, is similar to that played by His 146β. The main difference is that Arg 141α forms cross-links between the two α chains whereas His146β forms cross-links between the α and β chains.
of one α chain forms ion pairs with and of the other α chain.
Note how neatly the guanidino group of Arg141α is tucked away from the surface of the hemoglobin molecule.
the oxy conformation. Note that the guanidino group of Arg141α projects into the solvent when the hemoglobin molecule is in the oxy- conformation. You can locate its former ion-pair partners (shown in magenta) by carefully rocking the molecule.