Conformational stability of ferrocytochrome c. Electrostatic aspects of the oxidation by tris(1,10-phenanthroline)cobalt(III) at low ionic strength

At ionic strengths below 0.1 M the oxidation of horse ferrocytochrome c by tris(1,10-phenanthroline)cobalt (III) and tris(2,2'-bipyridine)cobalt(III) proceeds by a pathway which is independent of the transition metal complex concentration. Formation of an activated form of the protein appears to be rate limiting. The rate of oxidation decreases as the ionic strength increases. This dependence of the reaction rate on inert electrolyte concentration indicates that electrostatic association of anions under physiological ionic strength confers stability to the protein. The activated form of the protein, which reacts at least 10(4) times as fast as the predominant form, is thought to be a conformation of the reduced protein with an open heme crevice. Binding of the open form of ferrocytochrome c with the redox-inactive cationic transition metal complexes hexamminecobalt(III) and tris(1,10-phenanthroline)chromium(III) inhibits the oxidation by tris(1,10-phenanthroline)cobalt(III). Reactions of tris(1,10-phenanthroline)cobalt(III) with 4-carboxy-2,5-dinitrophenyllysine 13 and 72 ferrocytochromes c show no dependence on ionic strength. NMR studies at pH 7 demonstrate that ferricytochrome c is partly (15%) in the open conformation at low ionic strength. Furthermore, the interaction of redox-inert tris (1,10-phenanthroline)chromium(III) with ferricytochrome c under conditions identical to those of the kinetic studies demonstrates that the transition metal complex binds only to the open form of the protein. Titration with increasing amounts of tris(1,10-phenanthroline) chromium(III) shows changes in the NMR spectrum that are inconsistent with a single binding site.