title: Nature's polyoxometalate chemistry: X-ray structure of the Mo storage protein loaded with discrete polynuclear Mo-O clusters.
abstract: Some N(2)-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 Mo(8), three Mo(5-7), and one Mo(3) 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 Mo(8) clusters (three layers of two, four, and two MoO(n) octahedra) are distinguishable from the [Mo(8)O(26)](4-) cluster formed in acidic solutions by two displaced MoO(n) octahedra implicating three kinetically labile terminal ligands. Stabilization in the covalent Mo(8) cluster is achieved by Mo bonding to Hisα156-N(ε2) and Gluα129-O(ε1). The absence of covalent protein interactions in the noncovalent Mo(8) cluster is compensated by a more extended hydrogen-bond network involving three pronounced histidines. One displaced MoO(n) octahedron might serve as nucleation site for an inhomogeneous Mo(5-7) cluster largely surrounded by bulk solvent. In the Mo(3) cluster located on the 3-fold axis, the three accurately positioned His140-N(ε2) 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.
authors.affiliation: Biochemie I, Fakultät für Chemie, Universität Bielefeld, Universitätsstraße 25, D-33615 Bielefeld, Germany.
authors.forename: Björn
authors.initials: B
authors.lastname: Kowalewski
authors.forename: Juliane
authors.initials: J
authors.lastname: Poppe
authors.forename: Ulrike
authors.initials: U
authors.lastname: Demmer
authors.forename: Eberhard
authors.initials: E
authors.lastname: Warkentin
authors.forename: Thomas
authors.initials: T
authors.lastname: Dierks
authors.forename: Ulrich
authors.initials: U
authors.lastname: Ermler
authors.forename: Klaus
authors.initials: K
authors.lastname: Schneider
mesh_terms: D016948:Azotobacter vinelandii / Q000737:chemistry* / Q000378:metabolism; D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D008982:Molybdenum / Q000737:chemistry* / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation
chemical_list: D001426:Bacterial Proteins; C044659:molybdate; D008982:Molybdenum