Hemes and Hemoproteins. 5: Kinetics of the Peroxidatic Activity of Microperoxidase-8: Model for the Peroxidase Enzymes

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Abstract

The peroxidatic activity of the heme octapeptide from cytochrome c, microperoxidase-8 (MP-8), was assayed at 25°C under conditions where formation of Compound I is rate limiting. In the pH range 6–9, the reaction rate increased linearly with a slope close to unity. The active form of the substrate is the hydroperoxide anion, HO2, and an extrapolated second-order rate constant was obtained for the reaction of aquoMP-8 with HO2 of 3.7 × 108M sec−1, which is close to the second-order rate constants reported for reaction of the peroxidase enzymes with H2O2. Comparison with published data shows that the Fe3+ ion of MP-8 reacts as expected with simple anions, electrons, and HO2, while the analogous reactions of the enzymes all show a requirement for one H +. We conclude that the peroxidase enzymes activate H2O2 under physiological conditions through a pH-independent, H+-coupled binding of the required H2O2. The peroxidase activity of MP-8 can be increased more than tenfold by the presence of the guanidinium ion, which is ascribed to formation of the ion-pair GuaH+HO2; this suggests a role for the invariant distal Arg in the enzymes.

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    2

    Address reprint requests to Dr. D.A. Baldwin, National Chemical Research Laboratory, CSIR, P.O. Box 395, Pretoria, 0001, South Africa.

    1

    Present address: Department of Chemistry, University of Surrey, Guildford, England, GU2 5XH.

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