Escherichia coli protein Tpx []is a bacterial, periplasmic antioxidant protein with a thiol peroxidase activity. It is a small protein of 18kDa whose sequence is well conserved in other bacterial species. Tpx which differs from other peroxidases (PRXs) in that it shows substrate specificity toward alkyl hydroperoxides over hydrogen peroxide. As with all other PRXs, the peroxidatic cysteine (N-terminal) of Tpx is oxidized into a sulfenic acid intermediate upon reaction with peroxides. Tpx is able to resolve this intermediate by forming an intramolecular disulfide bond with a conserved C-terminal cysteine (the resolving cysteine), which can then be reduced by thioredoxin []. This differs from the typical 2-cys PRX which resolves the oxidized cysteine by forming an intermolecular disulfide bond with the resolving cysteine from the other subunit of the homodimer. Atypical 2-cys PRX homodimers have a loop-based interface (A-type for alternate), in contrast with the B-type interface of typical 2-cys and 1-cys PRXs [].
This subfamily of peroxiredoxin (known as 1-cys PRX) composed of PRXs containing only one conserved cysteine, which serves as the peroxidatic cysteine. They are homodimeric thiol-specific antioxidant (TSA) proteins that confer a protective role in cells by reducing and detoxifying hydrogen peroxide, peroxynitrite, and organic hydroperoxides []. As with all other PRXs, a cysteine sulfenic acid intermediate is formed upon reaction of 1-cys PRX with its substrates. Having no resolving cysteine, the oxidized enzyme is resolved by an external small-molecule or protein reductant such as thioredoxin or glutaredoxin. Similar to typical 2-cys PRX, 1-cys PRX forms a functional dimeric unit with a B-type interface, as well as a decameric structure which is stabilized in the reduced form of the enzyme. Other oligomeric forms, tetramers and hexamers, have also been reported [, , ]. Mammalian 1-cys PRX is localized cellularly in the cytosol and is expressed at high levels in brain, eye, testes and lung []. The seed-specific plant 1-cys PRXs protect tissues from reactive oxygen species during desiccation and are also called rehydrins [, ].
