Catalytic Oxidation of p-Cresol by Ascorbate Peroxidase

Abstract

Transient and steady state kinetics, together with a range of chromatographic and spectroscopic techniques, have been used to establish the mechanism and the products of the H2O2-dependent oxidation of p-cresol by ascorbate peroxidase (APX). HPLC, GC-MS, and NMR analyses are consistent with the formation of 2,2′-dihydroxy-5,5′-dimethylbiphenyl (II) and 4α,9β-dihydro-8,9β-dimethyl-3(4H)-dibenzofuranone (Pummerer's ketone, III) as the major products of the reaction. In the presence of cumene hydroperoxide, two additional products were observed which, from GC and MS analyses, were shown to be 1,1-dimethylbenzylalcohol (IV) and bis-(1-methyl-1-phenyl-ethyl)-peroxide (V). The product ratio II:III was dependent on enzyme concentration: at low concentrations Pummerer's ketone (III) predominates and at high concentrations formation of the biphenyl compound (II) is favored. Steady-state data showed a sigmoidal dependence on [p-cresol] that was consistent with the presence of 2.01 ± 0.15 binding sites for the substrate (25.0°C, sodium phosphate, pH 7.0, μ = 2.2 mM) and independent of ionic strength in the range 2.2–500 mM. Single turnover kinetic experiments (pH 7.0, 5.0°C, μ = 0.10 M) yielded second-order rate constants for Compound I reduction by p-cresol, k2, of 5.42 ± 0.10 × 105 M−1 s−1, respectively. Rate-limiting reduction of Compound II by p-cresol, k3, showed saturation kinetics, giving values for Kd = 1.54 ± 0.12 × 10−3 M and k3 = 18.5 ± 0.7 s−1. The results are discussed in the more general context of APX-catalyzed aromatic oxidations.