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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.