Revisiting the Mechanism of Dioxygen Activation in Soluble Methane Monooxygenase from M. capsulatus (Bath): Evidence for a Multi-Step, Proton-Dependent Reaction Pathway

Stopped-flow kinetic investigations Of Soluble methane monooxygenase (sMMO) from M. capsulatus (Bath) have clarified discrepancies that exist in the literature regarding several aspects of catalysis by this enzyme. The development of thorough kinetic analytical techniques has led to the discovery of two novel oxygenated iron species that accumulate in addition to the well-established intermediates H-peroxo and Q. The first intermediate, P*, is a precursor to H-peroxo and Was identified when the reaction of reduced MMOH and MMOB With O-2 was carried out in the presence of >= 540 mu M methane to suppress the dominating absorbance signal due to Q. The optical properties of P* are similar to those of H-peroxo with epsilon(420) = 3500 M-1 cm(-1) and epsilon(720) = 1250 M-1 cm(-1). These values are Suggestive Of a peroxo-to-iron(III) charge-transfer transition and resemble those of peroxodiiron(III) intermediates characterized in other carboxylate-bridged diiron proteins and synthetic model complexes. The second identified intermediate, Q*, forms on the pathway of Q decay when reactions are performed in the absence of hydrocarbon substrate. Q* does not react with methane, forms independently of buffer composition, and displays a unique shoulder at 455 nm in its optical spectrum. Studies conducted at different pH Values reveal that rate constants corresponding to P* decay/H-peroxo formation and H-peroxo decay/Q formation are both significantly retarded at high pH and indicate that both events require proton transfer. The processes exhibit normal kinetic solvent isotope effects (KSIEs) of 2.0 and 1.8, respectively, when the reactions are performed in D2O. Mechanisms are proposed to account for the observations of these novel intermediates and the proton dependencies of P* to H-peroxo and H-peroxo to Q conversion.