A Kinetic Study of the Horse-Radish Peroxidase-Catalyzed Oxidation of Iodide

Reaction steps in which H+ and I− are added to horse-radish peroxidase have been studied by examining the steady-state kinetics of the peroxidase-catalyzed oxidation of I−. For this purpose spectrophotometric methods for following the reaction were developed. Kinetic data were interpreted by extensions of methods proposed by Wong and Hanes in 1962 and Cleland in 1963. It is shown that H+ and I− are transferred to both ferriperoxidase (the free enzyme) and compound II in separate steps. Reaction with undissociated HI is not important. The ferriperoxidase-H+-I− complex is similar to complexes known to be produced in the presence of singly charged anions other than I−. Addition of both H+ and I− is required for the reduction of compound II to ferriperoxidase. The need for H+ brings support to George's suggestion made in 1953 that compound II contains a ferryl FeO2+ structure.