May act as a chaperone rather than a peroxidase. Has no thioredoxin-dependent peroxidase activity. Shows weak chaperone activity. Homodimer; disulfide-linked, upon oxidation. 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 the typical 2-Cys Prx5 family, C(R) is provided by the other dimeric subunit to form an intersubunit disulfide. Pmp20 lacks the resolving cysteine residue. Belongs to the peroxiredoxin family. Prx5 subfamily.