A proton nuclear magnetic resonance investigation of histidyl residues in sickle hemoglobin.

Abstract

Using high-resolution proton nuclear magnetic resonance spectroscopy at 250 MHz, we have determined the individual pK values of 22 surface histidyl residues (11 per alpha beta dimer) of sickle hemoglobin in both deoxy and carbon monoxy forms. Seven histidyl residues in the deoxy form and three in the carbon monoxy form are found to have pK values and chemical shifts different from the corresponding ones in human normal adult hemoglobin. Two of these histidyl residues are the beta 2 histidine and the beta 146 histidine, indicating that the conformations of the amino- and carboxyl-terminal regions of the beta chain in sickle hemoglobin are altered compared to those in human normal adult hemoglobin. The differences in the pK values of the additional surface histidyl residues between sickle and normal hemoglobins suggest that the effect of the amino acid substitution at the sixth position of the beta chain in sickle hemoglobin, namely, glutamic acid replaced by valine, is not restricted to the region around the mutation site but can extend to other regions in the protein molecule. In the deoxy form, the histidyl residues of sickle hemoglobin that have altered pK values and chemical shifts compared to the corresponding ones in human normal adult hemoglobin have been found to be sensitive to the early stages of the polymerization process [Russu, I.M., & Ho, C. (1980) Proc. Natl. Acad. Sci. U.S.A. 77, 6577-6581].