COMPARISON OF BRANCHED-CHAIN AND TIGHTLY COUPLED REACTION-MECHANISMS FOR PROSTAGLANDIN-H SYNTHASE

Two types of mechanisms have been proposed to account for the combinat ion of peroxidase and cyclooxygenase activities in prostaglandin H syn thase (PGHS). One, a branched-chain mechanism [Dietz, R., et al. (1988 ) Eur. J. Biochem. 171, 321-328], postulates that the cyclooxygenase r eaction propagates essentially independently of peroxidase catalysis. The second, a tightly coupled mechanism [Bakovic, M., & Dunford, H. B. (1994) Biochemistry 33, 6475-6482], postulates that peroxidase cataly sis is an integral part of cyclooxygenase propagation. Qualitative and quantitative predictions from the two mechanisms have been compared w ith several observed characteristics of the PGHS reaction with arachid onate, including the ability to accumulate PGG(2) and oxidized enzyme intermediates, the stoichiometry between cosubstrate and fatty acid co nsumption, and the hydroperoxide activator requirement. The observed c haracteristics, particularly the accumulation of micromolar levels of PGG(2) even in the presence of cosubstrate and the stoichiometry betwe en cosubstrate oxidation and fatty acid oxygenation of less than 1.3 ( compared to a theoretical maximum of 2.0), were largely consistent wit h predictions from the branched-chain mechanism, but contradicted impo rtant predictions of the tightly coupled mechanism. These results indi cate that PGHS catalysis is more accurately described by the branched- chain mechanism than by the tightly coupled mechanism.