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Structure of haemoglobin in the deoxy quaternary state with ligand bound at the α haems

Journal of Molecular Biology, ISSN: 0022-2836, Vol: 206, Issue: 4, Page: 723-736
1989
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Article Description

We report the X-ray crystal structure of two analogues of human haemoglobin in the deoxy quaternary (T) state with ligand bound exclusively at the α haems. These models were prepared from symmetric, mixed-metal hybrid haemoglobin molecules. The structures of α Fe(II) β Co(II), its carbonmonoxy derivative α Fe(II) CO β CoII, and α FeII O 2 β Ni(II) are compared with native deoxy haemoglobin by difference Fourier syntheses at 2·8, 2·9 and 3·5 Å resolution, respectively, and the refined α Fe(II) CO β Co(II) structure is analysed. In both the native deoxy and liganded T molecules, the mean plane of the α-subunit haem is parallel with the axis of the F helix, but this plane is tilted with respect to the helix axis in the oxy-quaternary R state. The side-chains of LeuFG3 and ValFG5 sterically restrict haem tilting in the T state. We propose that strain energy develops at the contact between the haem and these residues in the liganded T-state haemoglobin, and that the strain is, in part, responsible for the low affinity of the T-state α haem.

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