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Nature’s Polyoxometalate Chemistry: X-ray Structure of the Mo Storage Protein Loaded with Discrete Polynuclear Mo−O Clusters

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Poppe,  Juliane
Department of Molecular Membrane Biology, Max Planck Institute of Biophysics, Max Planck Society;

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Demmer,  Ulrike
Department of Molecular Membrane Biology, Max Planck Institute of Biophysics, Max Planck Society;

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Warkentin,  Eberhard
Department of Molecular Membrane Biology, Max Planck Institute of Biophysics, Max Planck Society;

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Ermler,  Ulrich
Department of Molecular Membrane Biology, Max Planck Institute of Biophysics, Max Planck Society;

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Citation

Kowalewski, B., Poppe, J., Demmer, U., Warkentin, E., Dierks, T., Ermler, U., et al. (2012). Nature’s Polyoxometalate Chemistry: X-ray Structure of the Mo Storage Protein Loaded with Discrete Polynuclear Mo−O Clusters. The Journal of Physical Chemistry A, 134(23), 9768-9774. doi:10.1021/ja303084n.


Cite as: http://hdl.handle.net/11858/00-001M-0000-0024-D560-0
Abstract
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 MoOη octahedra) are distinguishable from the [Mo8O26]4− cluster formed in acidic solutions by two displaced MoOη 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 MoOη 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.