Structural features of uranium-protein complexes

Uranium toxicity depends on its chemical properties rather than on its radioactivity and involves its interaction with macromolecules. Here, a systematic survey of the structural features of the uranyl sites observed in protein crystal structures deposited in the Protein Data Bank is reported. Beside the two uranyl oxygens, which occupy the axial positions, uranium tends to be coordinated by five other oxygen atoms, which occupy the equatorial vertices of a pentagonal bipyramid. Even if one or more of these equatorial positions are sometime empty, they can be occupied only by oxygen atoms that belong to the carboxylate groups of Glu and Asp side-chains, usually acting as monodentate ligands, to water molecules, or to acetate anions. Although several uranium sites appear undefined or unrefined, with a single uranium atom that lacks the two uranyl oxygen atoms, this problem seems to become less frequent in recent years. However, it is clear that the crystallographic refinements of the uranyl sites are not always well restrained and a better parametrization of these restraints seems to be necessary.