Journal
BIOCHIMICA ET BIOPHYSICA ACTA-PROTEINS AND PROTEOMICS
Volume 1699, Issue 1-2, Pages 235-243Publisher
ELSEVIER SCIENCE BV
DOI: 10.1016/j.bbapap.2004.03.003
Keywords
horseradish peroxidase compound II; phenol; aniline; oxidation; kinetics
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The catalytic constant (k(cat)) and the second-order association constant of compound 11 with reducing substrate (k(5)) of horseradish peroxidase C (HRPC) acting on phenols and anilines have been determined from studies of the steady-state reaction velocities (V-0 vs. [S-0]). Since k(cat)=k(2)k(6)/k(2)+k(6), and k(2) (the first-order rate constant for heterolytic cleavage of the oxygen-oxygen bond of hydrogen peroxide during compound I formation) is known, it has been possible to calculate the first-order rate constant for the transformation of each phenol or aniline by HRPC compound II (k(6)). The values of k(6) are quantitatively correlated to the sigma values (Hammett equation) and can be rationalized by an aromatic substrate oxidation mechanism in which the substrate donates an electron to the oxyferryl group in HRPC compound 11, accompanied by two proton additions to the ferryl oxygen atom, one from the substrate and the other the protein or solvent. k(6) is also quantitatively correlated to the experimentally determined C-13-NMR chemical shifts (delta(1)) and the calculated ionization potentials, E (HOMO), of the substrates. Similar dependencies were observed for k(cat)., and k(5). From the kinetic analysis, the absolute values of the Michaelis constants for hydrogen peroxide and the reducing substrates (K-M(H2O2) and K-M(S)), respectively, were obtained. (C) 2004 Elsevier B.V. All rights reserved.
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