Crystal structure of Escherichia coli thiol peroxidase in the oxidized state -: Insights into intramolecular disulfide formation and substrate binding in atypical 2-Cys peroxiredoxins

Thioredoxin-dependent thiol peroxidase (Tpx) from Escherichia coli represents a group of antioxidant enzymes that are widely distributed in pathogenic bacterial species and which belong to the peroxiredoxin (Prx) family. Bacterial Tpxs are unique in that the location of the resolving cysteine (C-R) is different from those of other Prxs. E. coli Tpx (EcTpx) shows substrate specificity toward alkyl hydroperoxides over H2O2 and is the most potent reductant of alkyl hydroperoxides surpassing AhpC and BCP, the other E. coli Prx members. Here, we present the crystal structure of EcTpx in the oxidized state determined at 2.2-Angstrom resolution. The structure revealed that Tpxs are the second type of atypical 2-Cys Prxs with an intramolecular disulfide bond formed between the peroxidatic (C-P, Cys(61)) and resolving (Cys(95)) cysteine residues. The extraordinarily long N-terminal chain of EcTpx folds into a beta-hairpin making the overall structure very compact. Modeling suggests that, in atypical 2-Cys Prxs, the C-R-loop as well as the C-P-loop may alternately assume the fully folded or locally unfolded conformation depending on redox states, as does the C-P-loop in typical 2-Cys Prxs. EcTpx exists as a dimer stabilized by hydrogen bonds. Its substrate binding site extends to the dimer interface. A modeled structure of the reduced EcTpx in complex with 15-hydroperoxyeicosatetraenoic acid suggests that the size and shape of the binding site are particularly suited for long fatty acid hydroperoxides consistent with its greater reactivity.