National Center for
4NDP: Crystal structure Molybdenum Storage Protein with fully Mo-loaded cavity
Structural diversity of polyoxomolybdate clusters along the three-fold axis of the molybdenum storage protein
J. Inorg. Biochem. (2014) 138 p.122-128
The molybdenum storage protein (MoSto) can store more than 100 Mo or W atoms as discrete polyoxometalate (POM) clusters. Here, we describe the three POM cluster sites along the threefold axis of the protein complex based on four X-ray structures with slightly different polyoxomolybdate compositions between 1.35 and 2 A resolution. In contrast to the Moalpha-out binding site occupied by an Mo3 cluster, the Moalpha-in and Mobeta binding sites contain rather weak and non-uniform electron density for the Mo atoms (but clearly identifiable by anomalous data), suggesting the presence of POM cluster ensembles and/or degradation products of larger aggregates. The "Moalpha-in cluster ensemble" was interpreted as an antiprism-like Mo6 species superimposed with an Mo7 pyramide and the "Mobeta cluster ensemble" as an Mo13 cluster (present mostly in a degraded form) composed of a pyramidal Mo7 and a Mo3 building block linked by three spatially separated MoOx units. Inside the ball-shaped Mo13 cluster sits an occluded central atom, perhaps a metal ion. POM cluster formation at the Moalpha-in and Mobeta sites appears to be driven by filtering out and binding/protecting self-assembled transient species complementary to the protein template.