Global conformational changes control the reactivity of methane monooxygenase

We present here X-ray scattering data that yield new structural information on the multicomponent enzyme methane monooxygenase and its components: a h ydroxylase dimer, and two copies each of a reductase and regulatory protein B. Upon formation of the enzyme complex, the hydroxylase undergoes a drama tic conformational change that is observed in the scattering data as a fund amental change in shape of the scattering particle such that one dimension is narrowed (by 25% or 24 Angstrom) while the longest dimension increases ( by 20% or 25 Angstrom). These changes also are reflected in a 13% increase in radius of gyration upon complex formation. Both the reductase and protei n B are required for inducing the conformational change. We have modeled th e scattering data for the complex by systematically modifying the crystal s tructure of the hydroxylase and using ellipsoids to represent the reductase and protein B components. Our model indicates that protein B plays a role in optimizing the interaction between the active centers of the reductase a nd hydroxylase components, thus, facilitating electron transfer between the m. In addition, the model suggests reasons why the hydroxylase exists as a dimer and that a possible role for the outlying gamma-subunit may be to sta bilize the complex through its interaction with the other components. We fu rther show that proteolysis of protein B to form the inactive B' results in a conformational change and B' does not bind to the hydroxylase. The trunc ation thus could represent a regulatory mechanism for controlling the enzym e activity.