Three-dimensional structure of abnormal human haemoglobins M Hyde Park and M Iwate

Abstract

Two mutant haemoglobins in which the haem-linked histidine is changed to tyrosine have been studied by X-ray crystallographic techniques. Deoxyhaemoglobin M Hyde Park ( α 2 β 2 92 His→Tyr) crystallizes isomorphously with normal human deoxyhaemoglobin. The difference Fourier at 3.5 Å resolution between these two forms shows principally that 20 to 30% of the haemoglobin M Hyde Park molecules in the crystal have lost their β-haem groups. The resulting structural distortions are restricted to the environment of the β-haem group and do not cause significant changes in the rest of the molecule. The partial loss of the β-haem group explains the unusual instability of the mutant haemoglobin in vitro and the presence of slight haemolytic anaemia in vivo. The structure shows no evidence for a link between the β-haem and the distal histidine E7(63)β in those molecules which retain the β-haems. Under the usual crystallizing conditions, deoxyhaemoglobin M Iwate ( α 2 87 His→Tyr β 2) crystallizes in a new, orthorhombic space group that is closely related to the normal monoclinic one. In the two forms observed at 5.5 Å resolution, the α-chains are retained in the met state but the β-chains are deoxy or met. In both forms, the α-haem groups are probably bonded to both the distal histidine E7(58)α and the proximal, mutation-introduced tyrosine F8(87)α. Both the met and β-deoxy forms exist in the quaternary deoxy conformation, yet structural changes do occur in the β-chains when ligand is added to this chain only. The existence of methaemoglobin M Iwate in the quaternary deoxy structure while methaemoglobin M Hyde Park is probably in the oxy conformation explains the differences in the physiological properties of these two mutants. An analysis of the quaternary structure conformations of the various liganded states of these two mutants shows that the haemoglobin structure probably remains in the deoxy conformation and does not switch to the oxy conformation until after the α-chains have combined with an external ligand. The change of quaternary structure appears to be independent of the ligand state of the β-chains.