A comparison of two independently determined structures of trypanothione reductase from Crithidia fasciculata

The enzyme trypanothione reductase (TR) is unique to trypanosomes and leishmania parasites, the causal agents of several important medical and veterinary tropical diseases. TR helps regulate the intracellular reducing environment of the parasite and it has been identified as a target for developing novel chemotherapeutic agents by structure-aided drug design. For this purpose it is essential to have confidence in the structural detail of the molecular target. Two independent studies of Crithidia fasciculata TR at medium resolution, in different space groups have afforded an opportunity to assess the reliability of the models. We summarize the important methodological details of each analysis and present a comparison of the geometry, thermal parameters and three-dimensional structure of the models. Particular attention has been paid to the disulfide substrate-binding site which is the area of most interest with respect to enzyme inhibition. The comparison has shown that the structures agree closely with Calpha atoms superposing with an r.m.s. of less than 0.5 A. The consistency of the models gives a high level of confidence that they are suitable for computer-aided drug design. The conformation of many side chains in the active site, in particular the catalytic residues, are well conserved in both structures. However, the comparison indicates a difference in the conformation of Trp21 and Met113 which together form a hydrophobic patch on the rim of the active-site cleft and interact with the spermidine moiety of the substrate. Consideration of the electron-density maps together with the structural comparison indicates that there is some conformational flexibility in this region of the active site. This heterogeneity may be used in the recognition of the substrate by the enzyme and should be considered when mapping out the size, shape and chemical properties of the active site.