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Nature’s Polyoxometalate Chemistry: X-ray Structure of the Mo Storage Protein Loaded with Discrete Polynuclear Mo–O Clusters
- Kowalewski, Björn, Poppe, Juliane, Demmer, Ulrike, Warkentin, Eberhard, Dierks, Thomas, Ermler, Ulrich, Schneider, Klaus
- Journal of the American Chemical Society 2012 v.134 no.23 pp. 9768-9774
- Azotobacter vinelandii, X-radiation, catalysts, geometry, histidine, hydrogen bonding, ligands, molybdenum, nitrogen-fixing bacteria, nitrogenase, polypeptides, solvents
- Some N₂-fixing bacteria prolong the functionality of nitrogenase in molybdenum starvation by a special Mo storage protein (MoSto) that can store more than 100 Mo atoms. The presented 1.6 Å X-ray structure of MoSto from Azotobacter vinelandii reveals various discrete polyoxomolybdate clusters, three covalently and three noncovalently bound Mo₈, three Mo₅–₇, and one Mo₃ clusters, and several low occupied, so far undefinable clusters, which are embedded in specific pockets inside a locked cage-shaped (αβ)₃ protein complex. The structurally identical Mo₈ clusters (three layers of two, four, and two MoOₙ octahedra) are distinguishable from the [Mo₈O₂₆]⁴– cluster formed in acidic solutions by two displaced MoOₙ octahedra implicating three kinetically labile terminal ligands. Stabilization in the covalent Mo₈ cluster is achieved by Mo bonding to Hisα156–Nε₂ and Gluα129–Oε₁. The absence of covalent protein interactions in the noncovalent Mo₈ cluster is compensated by a more extended hydrogen-bond network involving three pronounced histidines. One displaced MoOₙ octahedron might serve as nucleation site for an inhomogeneous Mo₅–₇ cluster largely surrounded by bulk solvent. In the Mo₃ cluster located on the 3-fold axis, the three accurately positioned His140–Nε₂ atoms of the α subunits coordinate to the Mo atoms. The formed polyoxomolybdate clusters of MoSto, not detectable in bulk solvent, are the result of an interplay between self- and protein-driven assembly processes that unite inorganic supramolecular and protein chemistry in a host–guest system. Template, nucleation/protection, and catalyst functions of the polypeptide as well as perspectives for designing new clusters are discussed.