Structure of the Mo‐Fe protein component of Azotobacter vinelandii nitrogenase

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Abstract
The Mo-Fe protein of nitrogenase from A. vinelandii and Klebsiella pneumoniae (Av1 and Kp1, respectively) consists of 4 subunits of similar, but not identical, relative molecular mass. The hydrodynamic properties of Av1 (sedimentation and diffusion coefficient) and its total relative molecular mass are very similar to those of Kp1 and catalase from bovine liver, a tetramer of 4 identical subunits. By EM the Av1, Kp1 and catalase tetramers are seen as protein particles of diameter 9.0-10.0 nm; no details of the subunit structure can be observed. Av1 (but not Kp1) forms regular polymers of variable length at low ionic strength in the presence of MgCl2. The structure of these polymers, of diameter 21.2 nm, is complex. Optical diffraction studies give a smallest repeating distance of 8.4 nm (corresponding to the diameter of the Av1 tetramer) and indicate a 4-start helix. The latter structure is incompatible with a flat, square subunit arrangement of the Av1 tetramer as proposed by Stasny et al. It is proposed that the subunit arrangement of the Av1 tetramer is of the tetrahedral type. This was also proposed for the catalase tetramer from optical diffraction studies of electron micrographs of catalase tubes, indicating a 222 symmetry. The proposal is in agreement with the recent finding that Av1 protein crystals belong to the P21 space group.