[Mechanism of Thiocyanate Dehydrogenase Functioning Based on Structural Data].

Thiocyanate dehydrogenase is enzyme catalyzing transformation of a thiocyanate ion into a cyanate ion with outcome of two electrons, two protons and a neutral atom of sulphur. Earlier structures of thiocyanate dehydrogenase from Thioalkalivibrio paradoxus were solved. Despite not perfect quality of the structures (twinning and pronounced anisotropy of the crystals, incomplete occupancy of the copper ions, absence of data for complexes with analogues of the substrate), there was suggested a mechanism of the enzyme functioning based on those structures. Recently at atomic resolution there have been solved structures of a gene-modified copy of relative enzyme from Ptlomicrobium methylotrophicum for free protein and its complex with thiourea. In the new structures copper ions of the active site possess complete occupancy. In these structures it is possible to reliably identify two conformations of the protein molecule with opened and closed active sites. The new structural high resolution data also allowed us to determine the presence of the superposition of different states of the copper ions for each of the two conformations. In each state the copper ions have different oxidation degrees, different corresponding ligands and partial occupancies. The ion charges were determined according to the ions coordination. In the protein molecule with the closed active site the complexes with inhibitor (thiourea ion) and molecular oxygen are observed. The complex with thiourea allows us to model binding of thiocyanate ion to the enzyme molecule. Taking into account the changes of the structures in the opened and closed conformations, a mechanism of the attacking oxygen ligand activation is suggested. A new scheme of the enzymatic reaction is discussed.