Prolidases are peptidases that are specific for dipeptides with proline as the second residue. The structure of recombinant prolidase from the hyperthermophilic archaeonThermococcus sibiricus(Tsprol) was determined at 2.6 Å resolution. The homodimer ofTsprol is characterized by a complete lack of interactions between the N- and C-terminal domains of the two subunits and hence can be considered to be the most open structure when compared with previously structurally studied prolidases. This structure exists owing to intermolecular coordination bonds between cadmium ions derived from the crystallization solution and histidine residues of a His tag and aspartate and glutamate residues, which link the dimers to each other. This linking leads to the formation of a crystal with a loose packing of protein molecules and low resistance to mechanical influence and temperature increase.