Bacterial thiol peroxidase (Tpx) is the periplasmic antioxidant enzyme widely distributed in most bacterial species, which 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. Although several crystal structures of Tpx are available, no pair of the redox structures reported thus far. Therefore, the conformational changes coupled to the catalytic reaction remain unclear. Herein, we report the solution structures of Bacillus subtilis Tpx in both the reduced and oxidized forms, the first pair of Tpx structures. The overall structures of both forms are very similar, however, significant differences at the active regions around the C-P and C-R residues were observed. In particular, a helix-to-coil transition was observed at the C-R region between the two forms. Our study reveals a dynamic picture of the conformational switch coupled to the redox reaction, thus provides further insights in understanding the catalytic mechanism of bacterial Tpx. (c) 2008 Elsevier Inc. All rights reserved.