Mechanisms of the Reactions of Cytochrome c

Abstract

Abstract Kinetic and equilibrium data for the binding of azide, imidazole, and pyridine to horse heart ferricytochrome c have been obtained by conventional spectrophotometric and stopped flow techniques. The stability constants of the 1:1 complexes formed between ferricytochrome c and the ligands are: azide, 4.5 m-1; imidazole, 15.3 m-1; and pyridine, 2.4 m-1, at 25°, pH 7, and 1.0 m ionic strength. These measurements are consistent with a model in which the added ligands bind to the iron by displacing the coordinated methionine-80. The kinetic data indicate a rate constant of about 60 s-1 for the rupture of the iron-sulfur bond.