Thiol-specific peroxidase that catalyzes the reduction of hydrogen peroxide and organic hydroperoxides to water and alcohols, respectively. Plays a role in cell protection against oxidative stress by detoxifying peroxides and as sensor of hydrogen peroxide-mediated signaling events. Might participate in the signaling cascades of growth factors and tumor necrosis factor-alpha by regulating the intracellular concentrations of H(2)O(2) (By similarity). Reduces an intramolecular disulfide bond in GDPD5 that gates the ability to GDPD5 to drive postmitotic motor neuron differentiation (PubMed:19766572). [thioredoxin]-dithiol + a hydroperoxide = [thioredoxin]-disulfide + an alcohol + H2O Homodimer; disulfide-linked, upon oxidation. 5 homodimers assemble to form a ring-like decamer (By similarity). Interacts with GDPD5; forms a mixed-disulfide with GDPD5 (By similarity). Interacts with SESN1 and SESN2 (By similarity). Interacts with FAM107A (PubMed:21969592). Found in various tissues; high concentration in liver. By oxidative and sulfhydryl-reactive agents. Phosphorylated on Thr-90 during the M-phase, which leads to a decrease in enzymatic activity. Mice embryos loss approximately 50% of Islet1/Islet2+ and HB9+ motor neurons, whereas dorsal-ventral patterning events and the numbers of Olig2+ progenitors are normal. Toward the end of the cell death phase they have equivalent numbers of motor neurons as wild type embryos. The active site is a conserved redox-active cysteine residue, the peroxidatic cysteine (C(P)), which makes the nucleophilic attack on the peroxide substrate. The peroxide oxidizes the C(P)-SH to cysteine sulfenic acid (C(P)-SOH), which then reacts with another cysteine residue, the resolving cysteine (C(R)), to form a disulfide bridge. The disulfide is subsequently reduced by an appropriate electron donor to complete the catalytic cycle. In this typical 2-Cys peroxiredoxin, C(R) is provided by the other dimeric subunit to form an intersubunit disulfide. The disulfide is subsequently reduced by thioredoxin. Belongs to the peroxiredoxin family. AhpC/Prx1 subfamily.