The Molybdenum Storage Protein

Abstract For synthesing the FeMo cofactor of nitrogenase N 2 ‐fixing bacteria continuously demand molybdate nonconstantly available in their habitats. Significant selection advantages have those species containing the molybdenum storage protein (MoSto) due to its capability to deposit ca. 130 Mo compactly as polynuclear Mo(VI)‐O or polyoxometalate (POM) clusters. Due to similar physicochemical properties, W can be stored in a related fashion. Heterododecameric MoSto is built up of two heterohexameric (αβ) 3 cages each containing 5 distinct locations for POM clusters. The complex interplay of self‐assembly and protein‐promoted processes creates a great diversity of POM clusters varying between 2 and 16 Mo atoms. Starting from smaller core species multiply linked with protein templates larger aggregates are synthesized that contain as a component an archetypical POM cluster found in solution. Mechanistically, POM cluster formation proceeds within a two‐step process. Molybdate is, at first, pressed through the cage shell in an ATP‐consuming fashion and then spontaneously polymerizes in the cage to POM clusters. Molybdate appears to be activated in the first step by phosphorylation in an ATP kinase reaction. The formed instable phosphoric‐molybdic anhydride intermediate is afterward, instantaneously, hydrolyzed in a locked chamber and the energized molybdate penetrates the protein shell at a thin place.