SUMMARY
2-Cys peroxiredoxins (Prxs) play two different roles depending on the physiological status of the cell. They are thioredoxin-dependent peroxidases under low oxidative stress, and ATP-independent chaperones upon exposure to high peroxides concentrations. These alternative functions have been associated with changes in the oligomerization state from low (LMW) to high (HMW) molecular weight species. Here we present the structures of Schistosoma mansoni PrxI in both states: the LMW decamer and the HMW 20-mer, formed by two stacked decamers. The latter is the first structure of a 2-Cys Prx chaperonic form. Comparison of the structures sheds light on the mechanism by which chemical stressors, such as high H2O2 concentration and acidic pH, are sensed and translated into a functional switch in this protein family. We also propose a model to account for the in vivo formation of long filaments of stacked Prx rings.
Schistosomiasis is the second most widespread human parasitic disease. It is principally treated with one drug, praziquantel, that is administered to 100 million people each year; less sensitive strains of schistosomes are emerging. One of the most appealing drug targets against schistosomiasis is thioredoxin glutathione reductase (TGR). This natural chimeric enzyme is a peculiar fusion of a glutaredoxin domain with a thioredoxin selenocysteine (U)-containing reductase domain. Selenocysteine is located on a flexible C-terminal arm that is usually disordered in the available structures of the protein and is essential for the full catalytic activity of TGR. In this study, we dissect the catalytic cycle of Schistosoma mansoni TGR by structural and functional analysis of the U597C mutant. The crystallographic data presented herein include the following: the oxidized form (at 1.9 Å resolution); the NADPH-and GSH-bound forms (2.3 and 1.9 Å , respectively); and a different crystal form of the (partially) reduced enzyme (3.1 Å ), showing the physiological dimer and the entire C terminus of one subunit. Whenever possible, we determined the rate constants for the interconversion between the different oxidation states of TGR by kinetic methods. By combining the crystallographic analysis with computer modeling, we were able to throw further light on the mechanism of action of S. mansoni TGR. In particular, we hereby propose the putative functionally relevant conformational change of the C terminus after the transfer of reducing equivalents from NADPH to the redox sites of the enzyme.
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