Mechanisms of the reactions of cytochrome c. Rate and equilibrium constants for ligand binding to horse heart ferricytochrome c.

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-‘; imidazole, 15.3 M-l; and pyridine, 2.4 M-l, 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-* for the rupture of the iron-sulfur bond.