Chloride masks effects of opposing positive charges in Hb A and Hb Hinsdale (beta 139 Asn-->Lys) that can modulate cooperativity as well as oxygen affinity.

In the human hemoglobin variant Hb Hinsdale, lysine is substituted for asparagine at position beta 139 (H17), which lies in the water-filled cavity that runs through the center of the molecule. This substitution adds two extra cationic residues to the excess of four cationic residues normally lining this cavity. Moo-Penn and colleagues who discovered this hemoglobin, found its oxygen affinity in 0.5 M bis-Tris buffer to be lower than that of Hb A. Their finding conflicted with our prediction that additional cationic groups lining the central cavity would destabilize the T-structure by increased electrostatic repulsion and thereby increase the oxygen affinity. We have, therefore, remeasured the ligand-binding properties of Hb Hinsdale. In chloride-free Hepes buffer, Hb Hinsdale has greatly increased oxygen affinity and lower cooperativity than Hb A. A comparison of the properties of Hb A, Hb Hinsdale, Hb Deer Lodge (beta 2 His-->Arg) and Hb Abruzzo (beta 143 His-->Arg) in 0.05 M Hepes versus 0.05 M bis-Tris buffers shows that very low chloride concentrations can significantly alter cooperativity as well as oxygen affinity. The apparent conflict between the findings of Moo-Penn and colleagues and our prediction arises from the enhanced chloride effects exhibited by Hb Hinsdale. On going from 0.05 M Hepes to 0.05 M bis-Tris at pH 7.0, log P50 values for Hb A and Hb Hinsdale are increased by 0.28 and 1.12, respectively. The Bohr effect, the kinetics of oxygen dissociation, the second-order rate constant of CO binding and the rate of CO recombination after flash photolysis were also determined for Hb Hinsdale. The enhanced chloride sensitivity of Hb Hinsdale is consistent with the allosteric mechanism of chloride interactions proposed by Perutz et al. in the accompanying paper.