CHLORIDE ACTS AS A NOVEL NEGATIVE HETEROTROPIC EFFECTOR OF HEMOGLOBINROTHSCHILD (BETA-37 TRP-]ARG) IN SOLUTION

The effects of chloride ion concentration on the rate constants for as sociation of carbon monoxide with human hemoglobin A and a synthetic f orm of the mutant hemoglobin Rothschild (beta37 Trp-->Arg) have been i nvestigated by stopped-flow techniques. Previous studies of the struct ure [Kavanaugh et al. (1992) Biochemistry 31, 4111] and functional pro perties [Rivetti et al. (1993) Biochemistry 32,2888] of hemoglobin Rot hschild crystallized in the T state have demonstrated that the mutant arginine residues create new chloride ion binding sites and that chlor ide ions act to lower the oxygen affinity of hemoglobin Rothschild in these crystals. The studies reported here demonstrate a parallel effec t of chloride ions on the rate of CO association with deoxygenated hem oglobin Rothschild in solution. Although the kinetics of CO binding to this hemoglobin in solution exhibit a Bohr effect, the chloride effec t is independent of pH. In addition, we find that other halide ions ha ve similar effects on the rate constants for the association of CO wit h this hemoglobin variant.