title: Peroxidase activity of the hemeoctapeptide N-acetylmicroperoxidase-8.
abstract: The pH dependence of the peroxidase activity (guaiacol assay) of the ferric hemeoctapeptide N-acetylmicroperoxidase-8 (N-AcMP8) was studied under conditions where formation of the Compound I analogue of the peroxidase enzymes is rate limiting. The pH profile of the reaction rate is consistent with a mechanism where both H2O2 and HO2- can displace H2O coordinated trans to neutral His but where the hydroxo complex of the hemepeptide (OH- trans to His) is kinetically inert. At pH > 11, where the proximal His ligand of Fe(III) ionizes to form a histidinate, the hydroxo complex (OH- trans to His-) becomes kinetically labile. A comparison of DeltaH(double dagger) and DeltaS(double dagger) values for the reaction of H2O2 and HO2-, obtained from the temperature dependence of the rate constants, with values for CN- and cysteine reported previously, shows that the activation parameters depend on the identity of the incoming ligand. This suggests that ligand substitution at Fe(III) in N-AcMP8 proceeds through an interchange mechanism.
authors.affiliation: Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand, P.O. Wits, Johannesburg, 2050 South Africa. hmarques@aurum.chem.wits.ac.za
authors.forename: Helder M
authors.initials: HM
authors.lastname: Marques
mesh_terms: D004789:Enzyme Activation; D005290:Ferric Compounds / Q000737:chemistry / Q000378:metabolism; D006418:Heme / Q000031:analogs & derivatives* / Q000378:metabolism*; D006420:Hemeproteins / Q000378:metabolism*; D006861:Hydrogen Peroxide / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D008956:Models, Chemical; D010446:Peptide Fragments / Q000378:metabolism*; D010544:Peroxidases / Q000378:metabolism*
chemical_list: D005290:Ferric Compounds; D006420:Hemeproteins; D010446:Peptide Fragments; D006418:Heme; D006861:Hydrogen Peroxide; C071067:N(alpha)-acetylmicroperoxidase-8; D010544:Peroxidases