FIG. 3.

Mechanisms of catalysis by Prxs. The three main reactions universal to the catalytic cycle of Prxs are [1] peroxidation, [2] resolution, and [3] recycling. During reaction 2, a local unfolding event occurs to facilitate the formation of disulfide bonds, with S_P and S_R designating the sulfur atoms of the peroxidatic and resolving cysteines, respectively. For the typical 2-Cys Prxs, the resolving cysteine, C_R, is located in the C-terminus of the protein and the C_R from one subunit forms a disulfide bond with the peroxidatic cysteine, C_P, on its partner subunit. For atypical 2-Cys Prxs, the C_R is found in an alternate location, frequently in the same subunit as the corresponding C_P. 1-Cys Prxs do not contain a C_R and are presumably recycled by a low-molecular-weight thiol or are directly reduced by thioredoxin. 2 R′SH in reaction 3 represents a thioredoxin-like protein or domain. Overoxidation of C_P (reaction 4) and ATP-dependent reduction of Cys-S_PO₂H by sulfiredoxin (reaction 5) depict redox regulation and repair occurring in some eukaryotic typical 2-Cys Prxs.