FIG. 6.
The selenium atom accelerates the exchange reaction of the truncated TR enzyme for a number of small molecule substrates. The truncated enzyme could reduce a peptide bearing a selenosulfide bond, whether it was in the context of a ring or not, but would reduce the analogous disulfide bearing compound very poorly. The same is true for small molecule selenium-containing compounds such as selenite, a good substrate for the truncated enzyme, while the analogous sulfur compound, sulfite, is a poor substrate.