Crystal structure of T state haemoglobin with oxygen bound at all four haems.
The cooperative binding of oxygen by haemoglobin results from restraints on ligand binding in the T state. The unfavourable interactions made by the ligands at the haems destabilise the T state and favour the high affinity R state. The T <==> R equilibrium leads, in the presence of a ligand, to a rapid increase in the R state population and therefore generates cooperative binding. There is now considerable understanding of this phenomenon, but the interactions that reduce ligand affinity in the T state have not yet been fully explored, owing to the difficulties in preparing T state haemoglobin crystals in which all the subunits are oxygenated. A protocol has been developed to oxygenate deoxy T state adult human haemoglobin (HbA) crystals in air at 4 C at all four haems without significant loss of crystalline order. The X-ray crystal structure, determined to 2.1 A spacing, shows significant changes in the alpha and beta haem pockets as well as changes at the alpha(1)beta(2) interface in the direction of the R quaternary structure. Most of the shifts and deviations from deoxy T state HbA are similar to, but larger than, those previously observed in the T state met and other partially liganded T state forms. They provide clear evidence of haem-haem interaction in the T state.
Paoli
M
M
Department of Chemistry, University of York, Heslington, York, UK.
Liddington
R
R
Tame
J
J
Wilkinson
A
A
Dodson
G
G
D000328:Adult; D000494:Allosteric Regulation; D001665:Binding Sites; D003196:Computer Graphics; D003460:Crystallization; D018360:Crystallography, X-Ray; D006418:Heme / Q000737:chemistry / Q000378:metabolism*; D006441:Hemoglobin A / Q000737:chemistry* / Q000378:metabolism; D006454:Hemoglobins / Q000737:chemistry* / Q000378:metabolism; D006801:Humans; D007501:Iron / Q000378:metabolism; D008958:Models, Molecular; D010100:Oxygen / Q000378:metabolism*; D011485:Protein Binding; D011487:Protein Conformation; D017434:Protein Structure, Tertiary
D006454:Hemoglobins; D006418:Heme; C100003:deoxyhemoglobin; D006441:Hemoglobin A; D007501:Iron; D010100:Oxygen