Chloride Acts as a Novel Negative Heterotropic Effector of Hemoglobin Rothschild (β37 Trp→Arg) in Solution

The effects of chloride ion concentration on the rate constants for association of carbon monoxide with human hemoglobin A and a synthetic form of the mutant hemoglobin Rothschild (β37 Trp→Arg) have been investigated by stopped-flow techniques. Previous studies of the structure [Kavanaugh et al. (1992) Biochemistry 57, 4111] and functional properties [Rivetti et al. (1993) Biochemistry 32, 2888] of hemoglobin Rothschild crystallized in the T state have demonstrated that the mutant arginine residues create new chloride ion binding sites and that chloride ions act to lower the oxygen affinity of hemoglobin Rothschild in these crystals. The studies reported here demonstrate a parallel effect of chloride ions on the rate of CO association with deoxygenated hemoglobin Rothschild in solution. Although the kinetics of CO binding to this hemoglobin in solution exhibit a Bohr effect, the chloride effect is independent of pH. In addition, we find that other halide ions have similar effects on the rate constants for the association of CO with this hemoglobin variant.