Thermodynamic and kinetic evidence for a two-step reaction between methane monooxygenase compound Q and substrates

Methane monooxygenase (MMO) contains a binuclear iron cluster that forms a bis-mu-oxo FC(IV)(2) intermediate termed compound Q (Q) which has previously been shown to be the species that oxidizes methane to form methanol. This report summarizes the results from transient kinetic studies that were used to study the temperature dependence of the rate of Q decay as a function of substrate type, concentration, and deuteration. The Arrhenius plots for the rate of Q decay in the presence of methane were found to be nonlinear with both the temperature and rate at which the break is observed dependent upon methane concentration. In contrast, the Arrhenius plots for the reaction of Q with alternate substrates such as ethane and propane are linear. An analysis of the thermodynamic activation parameters in combination with the deuterium kinetic isotope effects (KIE) suggests that the reaction between Q and the substrate occurs in two steps, one being substrate binding and the other C-H bond cleavage. (C) 2002 Elsevier Science B.V. All rights reserved.