Ionic Strength Dependence of the Oxidation of Iodide and Ferrocyanide by Compound I of Horseradish Peroxidase

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Abstract
The kinetics of the oxidation of iodide and ferrocyanide by compound I of horseradish peroxidase were studied at 25.degree. C as a function of ionic strength and pH. The ionic strength dependencies of the 2nd-order rate constants are tested with an extended form of the Debye-Huckel equation for the activity coefficients of the reacting species. For the reaction of iodide with compound I it is shown that the pH variation of the rate constant at zero ionic strength is caused mainly by titrating a catalytically important acid group and not mainly by the varying charge of the protein. The ferrocyanide reaction rate with compound I does not correlate with enzyme net charge, at all pH values. The influence of electrostatic interactions on reaction rates is discussed.