Transient kinetic measurements show that cytochrome c peroxidase reacts with excess of hydroperoxides to produce compound ES in two phases. The activation energies for the fast and slow phases are calculated to be 6.3 and 20.5 kcal·mol−1, respectively. The fast phase is assigned to the reaction of native active (pulsed) cytochrome c peroxidase with peroxides, whereas the slow phase is due to the presence of an inactive (aged, resting) enzyme. As the active species is exhausted, the equilibrium between the active and inactive enzymes is shifted by a slow conformational change to replenish the active enzyme. Since the rate-limiting step of the reaction of the inactive enzyme with peroxides is the conformation change, the overall reaction rate is independent of the nature and concentration of peroxides.
