Hemes and hemoproteins. 5: Kinetics of the peroxidatic activity of microperoxidase-8: model for the peroxidase enzymes.
The peroxidatic activity of the heme octapeptide from cytochrome c, microperoxidase-8 (MP-8), was assayed at 25 degrees 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 X 10(8) M-1 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.
Baldwin
D A
DA
Department of Chemistry, University of the Witwatersand, Johannesburg, South Africa.
Marques
H M
HM
Pratt
J M
JM
D000818:Animals; D003574:Cytochrome c Group / Q000378:metabolism*; D006420:Hemeproteins / Q000378:metabolism*; D006736:Horses; D007700:Kinetics; D009206:Myocardium / Q000378:metabolism; D010544:Peroxidases / Q000378:metabolism*
D003574:Cytochrome c Group; D006420:Hemeproteins; D010544:Peroxidases; C021799:microperoxidase