Three-Dimensional Structure Determination of a Protein Supercomplex That Oxidizes Methane to Formaldehyde inMethylococcus capsulatus(Bath)

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Abstract
The oxidation of methane to methanol in methanotrophs is catalyzed by the enzyme methane monooxygenase (MMO). Two distinct forms of this enzyme exist, a soluble cytoplasmic MMO (sMMO) and a membrane-bound particulate form (pMMO). The active protein complex termed pMMO−C was purified recently from Methylococcus capsulatus (Bath) (1). The complex consists of pMMO hydroxylase and an additional component pMMO−R, which was proposed to be the reductase for the pMMO complex. Further study of this complex has led here to the proposal that the pMMO−R is in fact methanol dehydrogenase, the subsequent enzyme in the methane oxidation pathway by methanotrophs. We describe here the biochemical and biophysical characterization of a stable purified complex of pMMO hydroxylase (pMMO−H) with methanol dehydrogenase (MDH) and report the first three-dimensional (3D) structure, determined by cryoelectron microscopy and single particle analysis to ∼16 Å resolution. The 3D structure reported here provides the first insights into the supramolecular organization of pMMO with MDH. These studies of pMMO−MDH complexes have provided further understanding of the structural basis for the particular functions of the enzymes in this system which might also be of relevance to the complete process of methane oxidation by methanotrophs under high copper concentration in the environment.