The Interdependence of Substrate and Protein Transformations in Rhodanese Catalysis

Circular dichroism, optical rotatory dispersion, and absorption studies allowed direct observation of the functioning enzyme protein and confirmed the major transformations of rhodanese indicated by study of the thermodynamic parameters. In these experiments, the catalytic sequence was treated as separate half reactions by omission of the second substrate. Based on the values of the kinetic constants reported in the preceding paper, substrate to product ratios were altered to interconvert the conformers of the enzyme-sulfur intermediate and the free enzyme. Detailed comparison of the spectra for the two forms yields a picture of the transformation involving some 20 amino acid residues in a change from a disordered coil-type structure to an ordered β-type structure as the enzyme binds thiosulfate ion and discharges sulfite ion. The inability of sulfate ion to produce a conformational change suggests that, in addition to the cationic site, another enzymic group is involved in substrate binding.