Targeting Apicomplexan Parasites: Structural and Functional Characterization of <i>Cryptosporidium</i> Thioredoxin Reductase as a Novel Drug Target

Cryptosporidiosis poses a significant health threat to young children and immunocompromised individuals due to the lack of effective therapies. Here, we demonstrate that the <i>Cryptosporidium parvum</i> redox system is fundamentally different from their human host. Humans possess independent glutathione (GSH) and thioredoxin (Trx) pathways. <i>Cryptosporidium</i> lacks authentic glutathione reductase (GR), and we hypothesize that it most likely utilizes the Trx reductase (TrxR) plus Trx couple to maintain GSH in its reduced state. Given the central role of CpTrxR in the parasite's redox homeostasis, we focus on its functional and structural characterization. We find that the combination of CpTrxR and<i>C. parvum</i> Trx efficiently reduces oxidized GSH, in effect functioning as a GR. Auranofin, a gold-containing compound, is known to kill parasites in culture, and here we demonstrate that CpTrxR is irreversibly inhibited by this compound. The crystallographic structures of CpTrxR, a type II TrxR characterized by the distinctive C-terminal -CGGGKCG motif found exclusively in apicomplexan parasites, including <i>Plasmodium</i> spp., the causative agents of malaria, are presented. Our study characterizes three unprecedented catalytically competent intermediates of the C-terminal tail in the so-called "in" conformations, providing insights into the structural and functional properties of type II TrxR. These findings offer valuable information for the design of CpTrxR inhibitors, addressing the pressing need for new therapeutic options against cryptosporidiosis, particularly in populations where current treatments are insufficiently effective.