Structural and functional characterization of Schistosoma mansoni Thioredoxin

Abstract

Schistosomiasis, the human parasitosis caused by various species of the blood-fluke Schistosoma, is a debilitating disease affecting 200 million people in tropical areas. The massive administration of the only effective drug, praziquantel, leads to the appearance of less sensitive parasite strains, thus, making urgent the search for new therapeutic approaches and new suitable targets. The thiol-mediated detoxification pathway has been identified as a promising target, being essential during all the parasite developmental stages and sufficiently different from the host counterpart. As a part of a project aimed at the structural characterization of all the proteins involved in this pathway, we describe hereby the high-resolution crystal structure of Schistosoma mansoni Thioredoxin (SmTrx) in three states, namely: the wild-type oxidized adult enzyme and the oxidized and reduced forms of a juvenile isoform, carrying an N-terminal extension. SmTrx shows a typical thioredoxin fold, highly similar to the other components of the superfamily. Although probably unlikely to be a reasonable drug target given its high similarity with the human counterpart, SmTrx completes the characterization of the whole set of thiol-mediated detoxification pathway components. Moreover, it can reduce oxidized glutathione and is one of the few defence proteins expressed in mature eggs and in the hatch fluid, thus confirming an important role in the parasite. We believe its crystal structure may provide clues for the formation of granulomas and the pathogenesis of the chronic disease.

Figure 1

Stereo ribbon representation of the overall fold of oxidized wt-SmTrx. Residues of the active site (Asp28, Trp33, Cys34, and Cys37) are in stick representation, together with the water molecule putatively involved in catalysis (see text). The electron density 2F_o – F_c contoured at 1.5σ is also shown for the same residues. An interactive view is available in the electronic version of the article.

Figure 2

Structure comparison. Panel A: Electrostatic surface of wt-SmTrx (left) and HsTrx (1ERT) (right). The view is rotated 90° on x-axis from Fig. 1, looking at the molecule through the active site. Side chains of the active site are shown as green sticks, residues of the second sphere are in grey. Panel B: Sequence alignment between wt-SmTrx (SCHMA) and HsTrx (HUMAN). The residues shown in Panel A are highlighted with the same colour scheme. Stars represent identity, colons represent high similarity; dots represent low similarity. Panel C: Superposition of oxidized wt-SmTrx (blue) and oxidized K3E-SmTrx (cyan). Panel D: Enlarged view of the active site of superposed oxidized (cyan) and reduced (magenta) K3E-SmTrx. An interactive view is available in the electronic version of the article.