Higher oxidation states of prostaglandin H synthase

Abstract

The reaction of prostaglandin H synthase with prostaglandin G 2 , the physiological substrate for the peroxidase reaction, was examined by rapid reaction techniques at 1 °C. Two spectral intermediates were observed and assigned to higher oxidation states of the enzymes. Intermediate I was formed within 20 ms in a bimolecular reaction between the enzyme and prostaglandin G 2 with k 1 = 1.4 × 10 7 M −1 s −1 . From the resemblance to compound I of horseradish peroxidase, the structure of intermediate I was assigned to [(protoporphyrin IX) +· Fe IV O). Between 10 ms and 170 ms intermediate II was formed from intermediate I in a monomolecular reaction with k 2 = 65 s −1 . Intermediate II, spectrally very similar to compound II of horseradish peroxidase or complex ES of cytochrome‐ c peroxidase, was assigned to a two‐electron oxidized state [(protoporphyrin IX)Fe IV O] Tyr +· which was formed by an intramolecular electron transfer from tyrosine to the porphyrin‐π‐cation radical of intermediate I. A reaction scheme for prostaglandin H synthase is proposed where the tyrosyl radical of intermediate II activates the cyclooxygenase reaction.