The pK, Values of Two Histidine Residues in Human Haemoglobin, the Bohr Effect, and the Dipole Moments of a-Helices

Studies of abnormal and chemically modified haemoglobins indicate that in 0.1 M-NaCl about 40 % of the alkaline Bohr effect of human haemoglobin is contributed by the C-terminal histidine HC3 ( 146)fi. In deoxyhaemoglobin, the imidazole of this histidine forms a salt bridge with aspartate FG1(94)/?, in oxyhaemoglobin or carbonmonoxyhaemoglobin it accepts a hydrogen bond from its own NH group instead. Kilmartin et al. (1973) showed that in 0.2 M-NaCl + 0.2 M-phosphate this change of ligation lowered the pK, of the histidine from 8.0 in Hb $ to 7*1 in HbCO, but Russu et al. (1980) claimed that in bis-Tris buffer without added NaCl its pK, in HbCO dropped no lower than 7.85, and that in this medium the C-terminal histidine made only a negligible contribution to the alkaline Bohr effect. We have compared the histidine resonances of HbCO A with those of three abnormal haemoglobins: HbCO Cowtown (His HC3(146)/? + Leu), HbCO Wood (His FG4(97)fi + Leu) and HbCO Malme (His FG4(97)P + Gln). 0 ur results show that the resonance assigned by Russu et aZ. to His HC3(146)/? in fact belongs to His FG4(97)8. Although in Hb the pK, of His HC3(146 @ is 8.05+0+05 independent of ionic strength, in HbCO its pK, drops sharply with diminishing ionic strength, so that in the buffer employed by Russu et al. it has a pK