The Stability of the Lattice Structure in Low Salt T-State Haemoglobin Crystals

The reconstruction of the electron density of molecules in crystals from X-ray diffraction measurements depends on the exactness of the packing of the molecules in the unit cell and the crystal lattice. Crystals of T-state haemoglobin, the low affinity form of the molecule, grow from high salt solutions or from low salt solutions in the presence of polyethylene glycol. The low salt lattice has the special property that it allows the haemoglobin molecule to bind oxygen and other ligands without the crystal breaking up. The stability of the low salt T-state crystals appears to arise from a small number of well-defined salt bridges and hydrogen bonds that are concentrated in specific lattice directions. These together form a framework within which the molecule can make adjustments which are sufficient to accommodate ligand binding but in which the larger quaternary movements normally associated with oxygenation are prevented. In these crystals therefore interactions with ligands can be studied directly by X-ray diffraction and the structural basis of the T-state's low affinity for oxygen can be analysed.