-!- Peroxiredoxins (Prxs) are a ubiquitous family of antioxidant proteins. -!- They can be divided into three classes: typical 2-Cys, atypical 2-Cys and 1-Cys peroxiredoxins. -!- The peroxidase reaction comprises two steps centered around a redox- active cysteine called the peroxidatic cysteine. -!- All three peroxiredoxin classes have the first step in common, in which the peroxidatic cysteine attacks the peroxide substrate and is oxidized to S-hydroxycysteine (a sulfenic acid). -!- The second step of the peroxidase reaction, the regeneration of cysteine from S-hydroxycysteine, distinguishes the three peroxiredoxin classes. -!- For typical 2-Cys Prxs, in the second step, the peroxidatic S-hydroxycysteine from one subunit is attacked by the 'resolving' cysteine located in the C-terminus of the second subunit, to form an intersubunit disulfide bond, which is then reduced by one of several cell-specific thiol-containing reductants (R'-SH) (e.g. thioredoxin, AhpF, tryparedoxin or AhpD), completing the catalytic cycle. -!- In the atypical 2-Cys Prxs, both the peroxidatic cysteine and its resolving cysteine are in the same polypeptide, so their reaction forms an intrachain disulfide bond. -!- To recycle the disulfide, known atypical 2-Cys Prxs appear to use thioredoxin as an electron donor. -!- The 1-Cys Prxs conserve only the peroxidatic cysteine, so that its oxidized form is directly reduced to cysteine by the reductant molecule.