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) X 10(7) M-1 s(-1) at pH 5.0, 4 degrees 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, k(2)', spanned S orders of magnitude and correlated linearly with bond dissociation enthalpies (BDEs) of strong C-H bond substrates with a log k(2)' vs BDE slope of similar to 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 Bronsted-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 (FeO)-O-IV-H BDE was estimated to be similar to 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.