A Mechanistic Study of Self-inactivation of the Peroxidase Activity in Prostaglandin H Synthase-1

Prostaglandin H synthase (PGHS) is a self-activating and self-inactivating enzyme. Both the peroxidase and cyclooxygenase activities have a limited number of catalytic turnovers. Sequential stopped-flow measurements were used to analyze the kinetics of PGHS-1 peroxidase self-inactivation during reaction with several different hydroperoxides. The inactivation followed single exponential kinetics, with a first-order rate constant of 0.2-0.5 s-1 at 24 degrees C. This rate was independent of the peroxide species and concentration used, strongly suggesting that the self-inactivation process originates after formation of Compound I and probably with Intermediate II, which contains an oxyferryl heme and a tyrosyl radical. Kinetic scan and rapid scan experiments were used to monitor the heme changes during the inactivation process. The results from both experiments converged to a simple, linear, two-step mechanism in which Intermediate II is first converted in a faster step (0.5-2 s-1) to a new compound, Intermediate III, which undergoes a subsequent slower (0.01-0.05 s-1) transition to a terminal species. Rapid-quench and high pressure liquid chromatography analysis indicated that Intermediate III likely retains an intact heme group that is not covalently linked with the PGHS-1 protein.