A physical characterization of sulfane sulfurtransferase

The bacterial enzyme sulfane sulfurtransferase has been studied using spectroscopic techniques. The enzyme was characterized in terms of its near-UV absorption spectrum, molar ellipticity, intrinsic fluorescence spectra and the effects of general and ionic quenching reagents upon its fluorescence. Fluorescence model studies are consistent with sulfane sulfurtransferase having only a single tryptophan residue, which accounts for its low UV absorption coefficient and suggested that this residue is at least partially exposed to solvent. Second derivative absorption spectroscopy studies revealed that most of the bacterial enzyme's tyrosine residues are exposed to solvent. Unlike the better known sulfurtransferase, bovine liver rhodanese, sulfane sulfurtransferase does not undergo a detectable increase in quantum yield when shifting from the sulfur-containing covalent enzyme intermediate to the free enzyme form (which lacks sulfur) during catalysis. CD studies suggest that sulfane sulfurtransferase has a significantly higher proportion of alpha-helix than rhodanese. The renaturation of sulfane sulfurtransferase denatured in 6 M guanidine was shown to be rapid and complete provided that the enzyme had not been oxidized while in the denatured state. Sulfane sulfurtransferase, like rhodanese, catalyzes the transfer of sulfur from thiosulfate to cyanide via a persulfide intermediate, and displays remarkably similar kinetics in this process (Aird, B.A., Heinrikson, R.L. and Westley, J. (1987) J. Biol. Chem 262, 17327-17335). In light of this, the results of the structural studies with sulfane sulfurtransferase are compared and contrasted to data from similar experiments with rhodanese in hopes that they would provide insight about which phenomena observed with rhodanese are intrinsic to the process of transferring sulfur atoms.