Tryparedoxin peroxidases (TXNPx) are peroxiredoxintype
enzymes that detoxify hydroperoxides in trypanosomatids.
Reduction equivalents are provided by
trypanothione [T(SH)2] via tryparedoxin (TXN). The
T(SH)2-dependent peroxidase system was reconstituted
from TXNPx and TXN of T. brucei brucei (TbTXNPx
and TbTXN). TbTXNPx efficiently reduces organic
hydroperoxides and is specifically reduced by TbTXN,
less efficiently by thioredoxin, but not by glutathione
(GSH) or T(SH)2. The kinetic pattern does not comply
with a simple rate equation but suggests negative cooperativity
of reaction centers. Gel permeation of oxidized
TbTXNPx yields peaks corresponding to a decamer
and higher aggregates. Electron microscopy
shows regular ring structures in the decamer peak.
Upon reduction, the rings tend to depolymerise forming
openchain oligomers. Co-oxidation of TbTXNPx
with TbTXNC43S yields a dead-end intermediate mimicking
the catalytic intermediate. Its size complies
with a stoichiometry of one TXN per subunit of TXNPx.
Electron microscopy of the intermediate displays pentangular
structures that are compatible with a model
of a decameric TbTXNPx ring with ten bound TbTXN
molecules. The redox-dependent changes in shape
and aggregation state, the kinetic pattern and molecular
models support the view that, upon oxidation of a
reaction center, other subunits adopt a conformation
that has lower reactivity with the hydroperoxide.
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