Hemoglobin has two conformational states. In this lesson, we explore how the two quaternary structures arise from two different ways to assemble the hemoglobin tetramer from two αβ dimers.

Click here to view down the exact twofold symmetry (Y) axis. You can see more of the central channel by rocking the molecule gently.

The backbone of the α subunits (heme groups are shown in spacefill). Click here to view down the exact twofold rotation axis.

Toggle spacefill. Note that the α subunits barely touch.

Click here to rotate the molecule. The heme groups face the solvent.

The β subunits are also related by rotational symmetry.

Toggle spacefill. There is no contact between β subunits in deoxyHb. Click here to rotate.

Adult hemoglobin is a dimer of αβ heterodimers.

Note the extensive interface between the α₁ and β₁ subunits. The hydrophobic cores of each subunit merge to form a continuous core. Consequently, the contacts between the α₁ and the β₁ subunits do not change when one oxygen molecule binds to one of the αβ dimers. Instead, a slight change in conformation at the corner of one subunit allows the α₁β₁ dimer to rotate about 15° with respect to the α₂β₂ dimer when the hemoglobin molecule shifts from the deoxy to the oxy conformation.

Human oxyHb. Click here to view along the twofold symmetry axis. OxyHb's quaternary structure differs from that of deoxyHb in two ways:

1. Rotation of one αβ dimer with respect to the other closes the central channel,
2. The β subunits come in contact with each other.

The β subunits are still related by rotational symmetry in oxyHb, as are the α subunits.

The structures of the αβ dimers do not change very much in the allosteric transition between the two quaternary structures.

Animate the superposition of the β chain from the two quaternary structures: the R state and T state.

Toggle rotation of β subunits as the tetramer switches between the deoxy and oxy conformations. The position of the iron atom on the left is held constant. Upon oxygen binding the β subunit on the right rotates towards you. The C-terminal residues are shown in red.