Involvement of the distal histidine in the low affinity exhibited by Hb Chico (Lys beta 66----Thr) and its isolated beta chains.

Abstract

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.

• Authors: C Bonaventura, R Cashon, J Bonaventura, M Perutz, G Fermi, D T Shih
• Journal: The Journal of biological chemistry (J Biol Chem) Vol. 266 Issue 34 Pg. 23033-40 (Dec 05 1991) ISSN: 0021-9258 [Print] United States
• PMID: 1744099 (Publication Type: Journal Article, Research Support, U.S. Gov't, P.H.S.)

Chemical References

• Hemoglobins, Abnormal
• Ligands
• hemoglobin Chico
• Histidine
• Carbon Monoxide
• Oxygen

Topics

• Carbon Monoxide (metabolism)
• Hemoglobins, Abnormal (chemistry, metabolism)
• Histidine (chemistry)
• Humans
• Hydrogen-Ion Concentration
• Kinetics
• Ligands
• Models, Molecular
• Oxygen (metabolism)
• X-Ray Diffraction