The activity of eukaryotic PRDX1 gradually decreases with time, which is due to the overoxidation of the catalytic cysteine C52. Normally, oxidized cysteine C52-SOH is generated as a catalytic intermediate, which is subsequently reduced by thioredoxin. Occasionally, further oxidation happens, generating C52-SOOH , where the catalytic cysteine is converted to cysteine-sulfinic acid. This over-oxidation cannot be reversed by thioredoxin (Yang et al. 2002, Budanov et al. 2004). Bacterial peroxiredoxin AhpC does not undergo over-oxidation due to structural difference (Wood et al. 2003).