Studies on Horseradish Peroxidase
IX. KINETICS OF THE OXIDATION OF p-CRESOL BY COMPOUND II
John E. Critchlow 1 and H. Brian Dunford 1
From the
1 From the Department of Chemistry, University of Alberta, Edmonton 7, Canada
The kinetics of the oxidation of p-cresol by Compound II of horseradish peroxidase has been studied by the stopped flow technique at an ionic strength of 0.11 from pH 2 to 11. In acid solution the reaction is kinetically first order in p-cresol, but in the alkaline region a saturation effect attributable to complex formation is observed. At very high pH an additional second order reaction between p-cresol and the complex is detectable. The existence of the complex is confirmed by spectrophotometric determination of its dissociation constant. Since this constant and the over-all second order rate constant do not depend on any common ionizing group in the enzyme, it is argued that the complex is unproductive in nature. Use of the diffusion-controlled limit shows that p-cresol reacts in the unionized form over the experimental pH range, and that electrostatic interactions are therefore not responsible for the degree of complexity of the pH log rate profile.
Submitted on September 20, 1971