Leishmaniasis is a
neglected disease caused by Leishmania, an intracellular protozoan parasite which possesses a unique
thiol metabolism based on
trypanothione.
Trypanothione is used as a source of electrons by the
tryparedoxin/
tryparedoxin peroxidase system (TXN/TXNPx) to reduce the hydroperoxides produced by macrophages during
infection. This detoxification pathway is not only unique to the parasite but is also essential for its survival; therefore, it constitutes a most attractive
drug target. Several forms of TXNPx, with very high sequence identity to one another, have been found in Leishmania strains, one of which has been used as a component of a potential anti-leishmanial
polyprotein vaccine. The structures of cytosolic TXN and TXNPx from L. major (LmTXN and LmTXNPx) offer a unique opportunity to study
peroxide reduction in Leishmania parasites at a molecular level, and may provide new tools for multienzyme inhibition-based
drug discovery. Structural analyses bring out key structural features to elucidate LmTXN and LmTXNPx function. LmTXN displays an unusual N-terminal α-helix which allows the formation of a stable domain-swapped dimer. In LmTXNPx, crystallized in reducing condition, both the locally unfolded (LU) and fully folded (FF) conformations, typical of the oxidized and reduced
protein respectively, are populated. The structural analysis presented here points to a high flexibility of the loop that includes the peroxidatic
cysteine which facilitates Cys52 to form an inter-chain
disulfide bond with the resolving
cysteine (Cys173), thereby preventing over-oxidation which would inactivate the
enzyme. Analysis of the electrostatic surface potentials of both LmTXN and LmTXNPx unveils the structural elements at the basis of functionally relevant interaction between the two
proteins. Finally, the structural analysis of TXNPx allows us to identify the position of the
epitopes that make the
protein antigenic and therefore potentially suitable to be used in an anti-leishmanial
polyprotein vaccine.