Component interactions in the soluble methane monooxygenase system from Methylococcus capsulatus (Bath).

The soluble methane monooxygenase system of Methylococcus capsulatus (Bath) includes three protein components: a 251-kDa non-heme dinuclear iron hydroxylase (MMOH), a 39-kDa iron-sulfur- and FAD-containing reductase (MMOR), and a 16-kDa regulatory protein (MMOB). The thermodynamic stability and kinetics of formation of complexes between oxidized MMOH and MMOB or MMOR were measured by isothermal titration calorimetry and stopped-flow fluorescence spectroscopy at temperatures ranging from 3.3 to 45 degrees C. The results, in conjunction with data from equilibrium analytical ultracentrifugation studies of MMOR and MMOB, indicate that free MMOR and MMOB exist as monomers in solution and bind MMOH with 2:1 stoichiometry. The role of component interactions in the catalytic mechanism of sMMO was investigated through simultaneous measurement of oxidase and hydroxylase activities as a function of varied protein component concentrations during steady-state turnover. The partitioning of oxidase and hydroxylase activities of sMMO is highly dependent on both the MMOR concentration and the nature of the organic substrate. In particular, NADH oxidation is significantly uncoupled from methane hydroxylation at MMOR concentrations exceeding 20% of the hydroxylase concentration but remains tightly coupled to propylene epoxidation at MMOR concentrations ranging up to the MMOH concentration. The steady-state kinetic data were fit to numerical simulations of models that include both the oxidase activities of free MMOR and of MMOH/MMOR complexes and the hydroxylase activity of MMOH/MMOB complexes. The data were well described by a model in which MMOR and MMOB bind noncompetitively at distinct interacting sites on the hydroxylase. MMOB manifests its regulatory effects by differentially accelerating intermolecular electron transfer from MMOR to MMOH containing bound substrate and product in a manner consistent with its activating and inhibitory effects on the hydroxylase.