Structure of the liganded T state of haemoglobin identifies the origin of cooperative oxygen binding

A molecular description of haemoglobin's cooperative oxygen binding and release was founded on the X-ray crystal structures of the deoxy-T and oxy-R states. Since the R state's oxygen affinity is close to that of an isolated subunit, the crucial allosteric phenomena are (1) the reduced affinity of the T state and (2) the kinetic pathway between the two quaternary structures. To investigate these phenomena directly, we have determined at high resolution (dmin = 2.1 A) the crystal structures of two liganded T-state haemoglobins. In the liganded T-state alpha subunit, both the tight packing of the haem and the intersubunit contacts inhibit a conformational change between the F helix and FG corner which would allow the haem to become planar and the iron to assume symmetrical R-like coordination. In the beta subunit, by contrast, we find no strain on the proximal side, but the intersubunit contacts prevent the haem from tilting about an axis parallel to the F helix which would open up the binding site to oxygen. In both subunits, ligand binding in the T state induces structural changes towards the tertiary conformation of the R state.