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

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 angstrom X-ray structure of MoSto from Azotobacter vinelandii reveals various discrete polyoxomolybdate clusters, three covalently and three noncovalently bound Mo-8, three Mo5-7, and one Mo-3 clusters, and several low occupied, so far undefinable clusters, which are embedded in specific pockets inside a locked cage-shaped (alpha beta)(3) protein complex. The structurally identical Mo-8 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 Mo-8 cluster is achieved by Mo bonding to His alpha 156-N-epsilon 2 and Glu alpha 129-O-epsilon 1. 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 MoOn octahedron might serve as nucleation site for an inhomogeneous Mo5-7 cluster largely surrounded by bulk solvent. In the Mo-3 cluster located on the 3-fold axis, the three accurately positioned His140-N-epsilon 2 atoms of the alpha 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.