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Studies on Cytochrome c Peroxidase

IV. A COMPARISON OF PEROXIDE-INDUCED COMPLEXES OF HORSERADISH AND CYTOCHROME c PEROXIDASES

Takashi Yonetani 1

From the 1 From the Johnson Research Foundation, University of Pennsylvania, Philadelphia, Pennsylvania 19104

The stoichiometry between horseradish peroxidase (HRP) and H2O2 in the formation of Complexes I and II was spectrophotometrically studied by titrating HRP with H2O2 in the presence and absence of added reducing agents. The molar stoichiometry between H2O2 added and Complex II formed was found to be 1:1 in the presence and absence of added reducing agents. No evidence was obtained that more than 1 mole of Complex II was formed per mole of H2O2 added. The titration of Complex II with ferrocyanide confirmed George's finding that Complex II retained only 1 oxidizing equivalent per enzyme hematin unit. The oxidation of ascorbate and ferrocytochrome c upon additions of stoichiometric quantities of H2O2 in the presence of HRP exhibited two-phased kinetics: a very rapid initial oxidation followed by a slower secondary oxidation. These results were consistent with the currently accepted mechanism of the HRP reaction of Equations 2, 5, and 6.

The titration of an enzyme-H2O2 complex (Complex ES) of yeast cytochrome c peroxidase with ferrocyanide indicated that Complex ES retained 2 oxidizing equivalents per enzyme hematin unit. No reducing equivalent of added donors was consumed in the process of the formation of Complex ES from cytochrome c peroxidase. The added donors were oxidized only in the process of the conversion of Complex ES to cytochrome c peroxidase. These results were consistent with the cytochrome c peroxidase mechanism of Equations 7 and 8.

Complex ES of cytochrome c peroxidase was compared with Complex II of HRP.

Submitted on December 10, 1965


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M Sivaraja, D. Goodin, M Smith, and B. Hoffman
Identification by ENDOR of Trp191 as the free-radical site in cytochrome c peroxidase compound ES
Science, August 18, 1989; 245(4919): 738 - 740.
[Abstract] [PDF]


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