Bacterial thiol peroxidase (Tpx) is a periplasmic antioxidant enzyme widely distributed in most bacteria species,and catalyzes the reduction of lipid hydroperoxide in vivo.Tpx belongs to the atypical 2-Cys peroxiredoxin (Prx) family and utilizes two active cysteine residues during the redox reaction.The peroxidatic cysteine (CP) attacks the peroxide substrate and forms a sulfenic acid intermediate.Subsequently,the resolving cysteine (CR) functions to resolve the intermediate by forming an intramolecular disulfide bond between Cp and CR.Although a number of crystal structures of Tpx have been reported in different states,the conformational changes coupled to the catalytic reaction,which are essential in understanding the molecular mechanism,remain unclear.