title: pH and temperature dependence of the rate of compound I formation from the reaction of prostaglandin endoperoxide synthase with hydrogen peroxide.
abstract: The formation of primary oxidized compound of prostaglandin endoperoxide synthase, compound I, was studied as a function of pH and temperature using hydrogen peroxide as a substrate. Analysis of the results indicates that compound I formation is influenced by an ionizable group with a pKa of 4.06 +/- 0.04. The protonated form of hydrogen peroxide preferentially reacts with the unprotonated form of the enzyme over the pH range of 3.5 to 9.1, suggesting the importance of acid-base catalysis for compound I formation. The second-order rate constant for the reaction of the enzyme with hydrogen peroxide in the pH-independent region is (4.6 +/- 0.2) x 10(5) M-1 S-1 at an ionic strength of 0.1 M and temperature of 4.0 +/- 0.2 degrees C. The effect of temperature on the rate of compound I formation was studied from 3.4 to 24.1 degrees C in the pH-independent region (pH 6.98) and at a constant ionic strength of 0.1 M. The kinetic parameters obtained from the temperature dependence are the following: Arrhenius activation energy, Ea = 102 +/- 5 kJ/mol; free energy of activation, delta G++, 36 +/- 3 kJ/mol; enthalpy of activation, delta H++, 100 +/- 5 kJ/mol; entropy of activation, delta S++, 215 +/- 9 J/mol K. These activation values are very different from those obtained for the reactions of other peroxidases and catalases with hydrogen peroxide, indicating profound differences in active site structure.
authors.affiliation: Department of Chemistry, University of Alberta, Edmonton, Canada.
authors.forename: M
authors.initials: M
authors.lastname: Bakovic
authors.forename: H B
authors.initials: HB
authors.lastname: Dunford
mesh_terms: D000085:Acetates / Q000737:chemistry; D000818:Animals; D006861:Hydrogen Peroxide / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008297:Male; D011451:Prostaglandin-Endoperoxide Synthases / Q000737:chemistry*; D012669:Seminal Vesicles / Q000737:chemistry; D012756:Sheep; D013057:Spectrum Analysis; D013696:Temperature
chemical_list: D000085:Acetates; D006861:Hydrogen Peroxide; D011451:Prostaglandin-Endoperoxide Synthases