The Mechanism of the Rhodanese-catalyzed Thiosulfate-Cyanide Reaction

The spontaneous reaction between thiosulfate and cyanide and the rhodanese catalysis of that same reaction have been studied as a function of temperature and dielectric constant, and the activation parameters for the rate-determining steps of both reactions have been determined. Thermodynamic parameters for the dissociation of the rhodanese-thiosulfate complex have also been obtained. From a comparison of the values for the enzyme-catalyzed reaction with those for the spontaneous process, it has been possible to propose a more detailed mechanism for rhodanese catalysis. This mechanism involves binding of thiosulfate to a metal ion in the enzyme, with complex formation requiring a conformational change in the protein. In the metal ion-thiosulfate complex there is an electronic shift away from the planetary sulfur atom of thiosulfate, with resultant stretching and weakening of the sulfur-sulfur bond. This bond is then more susceptible to the attack by a strong enzymic nucleophil which effects the cleavage. Rupture of the sulfur-sulfur bond appears to be accompanied by a second conformational change.