Bacterial defenses against oxidants: mechanistic features of cysteine-based peroxidases and their flavoprotein reductases.

Antioxidant defenses include a group of ubiquitous, non-heme peroxidases, designated the peroxiredoxins, which rely on an activated cysteine residue at their active site to catalyze the reduction of hydrogen peroxide, organic hydroperoxides, and peroxynitrite. In the typical 2-Cys peroxiredoxins, a second cysteinyl residue, termed the resolving cysteine, is also involved in intersubunit disulfide bond formation during the course of catalysis by these enzymes. Many bacteria also express a flavoprotein, AhpF, which acts as a dedicated disulfide reductase to recycle the bacterial peroxiredoxin, AhpC, during catalysis. Mechanistic and structural studies of these bacterial proteins have shed light on the linkage between redox state, oligomeric state, and peroxidase activity for the peroxiredoxins, and on the conformational changes accompanying catalysis by both proteins. In addition, these studies have highlighted the dual roles that the oxidized cysteinyl species, cysteine sulfenic acid, can play in eukaryotic peroxiredoxins, acting as a catalytic intermediate in the peroxidase activity, and as a redox sensor in regulating hydrogen peroxide-mediated cell signaling.