A Fluorescence study of conformational changes induced by substrate and temperature in bovine liver thiosulfate sulfurtransferase

Structural transitions occurring in the range of 0-50°C have been detected and studied in the enzyme thiosulfate sulfurtransferase (thiosulfate:cyanide sulfurtransferase, EC 2.8.1.1) by investigating both the intrinsic protein fluorescence and the fluorescence of covalently bound probes. The intrinsis fluorescence of the enzyme decreases sharply at 27°C and the magnitude of this quenching is smaller for the sulfur-substituted enzyme (ES) than for the free enzyme (E), both of which are obligatory catalytic intermediates. The effect with ES is fully reversible and almost completely so with E. Fluorescence depolarization studies with thiosulfate sufurtransferase labeled at a number of different sites with the fluorosphore dimethylaminonaphthalene show a sharp increase in the polarization starting at 27°C when the temperature/viscosity ratio is varied with temperature and no transition when the ratio is varied with glycerol. Enzyme activity shows no transition at 27°C but falls abruptly ahove 40°C. Under the appropriate conditions, the 27°C transition will lead to association of thiosulfate suifurtransferase molecules and the appearance of turbidity. Both activity and fluorescence measurements support the idea that the ES form is significantly more stable than the E form. These results may result from changes in the interactions between the structural domains into which the single polypeptide chain of thiosulfate sulfurtransferase is folded.