Concentration-dependent effects of anions on the anaerobic oxidation of hemoglobin and myoglobin

The redox potentials of hemoglobin and myoglobin and the shapes of their an aerobic oxidation curves are sensitive indicators of globin alterations sur rounding the active site. This report documents concentration-dependent eff ects of anions on the ease of anaerobic oxidation of representative hemoglo bins and myoglobins. Hemoglobin (Hb) oxidation curves reflect the cooperati ve transition from the T state of deoxyHb to the more readily oxidized R-li ke conformation of metHb. Shifts in the oxidation curves for Hb A(0) as Cl- concentrations are increased to 0.2 M at pH 7.1 indicate preferential anio n binding to the T state and destabilization of the R-like state of metHb, leading to reduced cooperativity in the oxidation process. A dramatic rever sal of trend occurs above 0.2 M Cl- as anions bind to lower affinity sites and shift the conformational equilibrium toward the R state. This pattern h as been observed for various hemoglobins with a variety of small anions. St eric rather than electronic effects are invoked to explain the fact that no comparable reversal of oxygen affinity is observed under identical conditi ons. Evidence is presented to show that increases in hydrophilicity in the distal heme pocket can decrease oxygen affinity via steric hindrance effect s while increasing the ease of anaerobic oxidation.