Crystal structure of Trypanosoma cruzi trypanothione reductase in complex with trypanothione, and the structure-based discovery of new natural product inhibitors

Background: Trypanothione reductase (TR) helps to maintain an intracellular reducing environment in trypanosomatids, a group of protozoan parasites th at afflict humans and livestock in tropical areas. This protective function is achieved via reduction of polyamine-glutathione conjugates, in particul ar trypanothione. TR has been validated as a chemotherapeutic target by mol ecular genetics methods. To assist the development of new therapeutics, we have characterised the structure of TR from the pathogen Trypanosoma cruzi complexed with the substrate trypanothione and have used the structure to g uide database searches and molecular modelling studies. Results: The TR-trypanothione-disulfide structure has been determined to 2. 4 Angstrom resolution. The chemical interactions involved in enzyme recogni tion and binding of substrate can be inferred from this structure. Comparis ons with the related mammalian enzyme, glutathione reductase, explain why e ach enzyme is so specific for its own substrate, A CH ... O hydrogen bond c an occur between the active-site histidine and a carbonyl of the substrate. This interaction contributes to enzyme specificity and mechanism by produc ing an electronic induced fit when substrate binds. Database searches and m olecular modelling using the substrate as a template and the active site as receptor have identified a class of cyclic-polyamine natural products that are novel TR inhibitors. Conclusions: The structure of the TR-trypanothione enzyme-substrate complex provides details of a potentially valuable drug target. This information h as helped to identify a new class of enzyme inhibitors as novel lead compou nds worthy of further development in the search for improved medicines to t reat a range of parasitic infections.