Peroxiredoxins are a class of ubiquitously expressed redox active enzymes that utilize a conserved peroxidatic cysteine to reduce peroxides. They are involved in both protection against oxidative stress and in hydrogen peroxide signaling. Some peroxiredoxins have been shown to be sensitive to hyperoxidation of their peroxidatic cysteine. This inactivates the peroxidase activity, but has been suggested to be important for their role in hydrogen peroxide signaling. Furthermore, hyperoxidation has been shown to induce a switch to chaperone function for some peroxiredoxins.
During the reaction cycle, the peroxidatic cysteine cycles between a reduced state and a disulfide state. Here, the structure of a mutant mimicking the reduced state of the S. cerevisiae peroxiredoxin thiol-specific antioxidant protein 2 (TSA2) was determined. The structure reveals TSA2 in the decameric form with two of the ten active sites in a locally unfolded conformation, which is normally associated with the disulfide state. In these subunits a novel conformation of a strictly conserved proline is observed. This novel conformation is expected to affect reactivity and protect against hyperoxidation of the peroxidatic cysteine. The structure also provides a plausible explanation for the previously observed pKa difference between the peroxidatic cysteines of TSA2 and its close homologue TSA1.
A low-resolution structure of TSA2 in the disulfide state was also determined. This structure indicates that disulfide formation in TSA2 not only requires a local unfolding of the active site, but also requires a tilting of the α-helix (α2) containing the peroxidatic cysteine.
Since the C-terminal conformation has been found to affect sensitivity to hyperoxidation, it was investigated whether changes in pH affected the C-terminal conformation of TSA2. Tryptophan fluorescence and CD spectroscopy showed no indications of such changes with changes in pH in the range from 5.8-9.0.