Electron microscopy of the Mo‐Fe‐protein from Azotobacter vinelandii nitrogenase

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Abstract
The quaternary structure of the Mo‐Fe‐protein from Azotobacter vinelandii has been studied by electron microscopy. A model of the molecule of the Mo‐Fe‐protein has been proposed: two α subunits are displaced relative to two β subunits along a twofold axis, so the molecule can be characterized by the point‐group pseudosymmetry 222. Computer averaging of the images showed that one of the projections of the molecule could be characterized by twofold rotational symmetry. Micrographs of nitrogenase recombined complex (Mo‐Fe‐protein + Fe‐protein) have been obtained. They showed particles close in size and form to the Mo‐Fe‐protein molecule. Therefore, it has been proposed that the Fe‐protein could be situated in the central cavity of Mo‐Fe‐protein.