Detection and kinetic characterization of a highly reactive heme-thiolate peroxygenase compound I

Abstract

The extracellular heme-thiolate peroxygenase from Agrocybe aegerita (AaeAPO) has been shown to hydroxylate alkanes and numerous other substrates using hydrogen peroxide as the terminal oxidant. We describe the kinetics of formation and decomposition of AaeAPO compound I upon its reaction with mCPBA. The UV-vis spectral features of AaeAPO-I (361, 694 nm) are similar to those of chloroperoxidase-I and the recently described cytochrome P450-I. The second-order rate constant for AaeAPO-I formation was 1.0 (±0.4) × 10(7) M(-1) s(-1) at pH 5.0, 4 °C. The relatively slow decomposition rate, 1.4 (±0.03) s(-1), allowed the measurement of its reactivity toward a panel of substrates. The observed rate constants, k2', spanned 5 orders of magnitude and correlated linearly with bond dissociation enthalpies (BDEs) of strong C-H bond substrates with a log k2' vs BDE slope of ∼0.4. However, the hydroxylation rate was insensitive to a C-H BDE below 90 kcal/mol, similar to the behavior of the tert-butoxyl radical. The shape and slope of the Brønsted-Evans-Polanyi plot indicate a symmetrical transition state for the stronger C-H bonds and suggest entropy control of the rate in an early transition state for weaker C-H bonds. The AaeAPO-II Fe(IV)O-H BDE was estimated to be ∼103 kcal/mol. All results support the formation of a highly reactive AaeAPO oxoiron(IV) porphyrin radical cation intermediate that is the active oxygen species in these hydroxylation reactions.

Figure 1

UV–vis transients observed upon 1:1 mixing of 13 µM of ferric enzyme with 25 µM of mCPBA at pH 5.0, 4°C. Maximum yield of AaeAPO–I reached 70% at 30 ms. Blue line indicates the spectrum of AaeAPO–I (361 nm) obtained by global analysis.

Figure 2

(a) UV–vis spectra obtained during the reaction of 10 µM AaeAPO–I with 16 µM p–ethylbenzoic acid at pH 5.0, 4°C. Inset: Observed first-order decay rates versus p–ethylbenzoic acid concentration. The apparent second-order rate constant, k₂, was obtained from the slope. (b) Observed first–order decay rates versus concentrations of THF or THF–d₈. A KIE of 4.3 for THF and THF–d₈ was determined from the ratio of the slopes.

Figure 3

Plot of log k₂’ vs. substrate C–H BDE, compound numbers are from Table S1.

Scheme 1

Scheme 2