Involvement of the distal histidine in the low affinity exhibited by Hb Chico (Lys beta 66----Thr) and its isolated beta chains.
Hemoglobin (Hb) Chico (Lys beta 66----Thr at E10) has a diminished oxygen affinity (Shih, D. T.-b., Jones, R. T., Shih, M. F.-C., Jones, M. B., Koler, R. D., and Howard, J. (1987) Hemoglobin 11, 453-464). Our studies show that its P50 is about twice that of Hb A and that its cooperativity, anion, and Bohr effects between pH 7 and 8 are normal. The Bohr effect above pH 8 is somewhat reduced, indicating a small but previously undocumented involvement of the ionic bond formed by Lys beta 66 in the alkaline Bohr effect. Since the oxygen affinity of the alpha-hemes is likely to be normal, that of the beta-hemes in the tetramer is likely to be reduced by the equivalent of 1.2 kcal/mol beta-heme in binding energy. Remarkably, both initial and final stages of oxygen binding to Hb Chico are of lowered affinity relative to Hb A under all conditions examined. The isolated beta chains also show diminished oxygen affinity. In T-state Hb A, Lys(E10 beta) forms a salt bridge with one of the heme propionates, but comparison with other hemoglobin variants shows that rupture of this bridge cannot be the cause of the low oxygen affinity. X-ray analysis of the deoxy structure has now shown that Thr beta 66 either donates a hydrogen bond to or accepts one from His beta 63 via a bridging water molecule. This introduces additional steric hindrance to ligand binding to the T-state that results in slower rates of ligand binding. We measured the O2/CO partition coefficient and the kinetics of oxygen dissociation and carbon monoxide binding and found that lowered O2 and CO affinity is also exhibited by the R-state tetramers and the isolated beta chains of Hb Chico.
Bonaventura
C
C
Marine Biomedical Center, Duke University Marine Laboratory, Beaufort, North Carolina 28516.
Cashon
R
R
Bonaventura
J
J
Perutz
M
M
Fermi
G
G
Shih
D T
DT
D002248:Carbon Monoxide / Q000378:metabolism; D006455:Hemoglobins, Abnormal / Q000737:chemistry* / Q000378:metabolism; D006639:Histidine / Q000737:chemistry*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008024:Ligands; D008958:Models, Molecular; D010100:Oxygen / Q000378:metabolism; D014961:X-Ray Diffraction
D006455:Hemoglobins, Abnormal; D008024:Ligands; C054742:hemoglobin Chico; D006639:Histidine; D002248:Carbon Monoxide; D010100:Oxygen