No abstract

The three-dimensional structure of trypanothione reductase (TR) (EC 1.6.4.8) from Trypanosoma cruzi has been solved at 0.33 nm resolution by molecular replacement using the structure of C. fasciculata TR as a starting model. Elucidation of the T. cruzi TR structure represents the first step in the rational design of a drug against Chagas' disease. The structure of T. cruzi TR is compared with those of C. fasciculata TR as well as human and E. coli glutathione reductase (GR). In the FAD-binding domain, TR has two insertions, each about 10 residues long, which do not occur in GR. The first one is a rigid loop stabilizing the position of helix 91-117 which is responsible for the wider active site of TR as compared to GR. The second insertion does not occur where it is predicted by sequence alignment; rather the residues extend three strands of the 4-stranded beta-sheet by one or two residues each. This increases the number of hydrogen bonds within the sheet structure. The structure of the NADPH.TR complex has been solved at 0.33 nm resolution. The nicotinamide ring is sandwiched between the flavin ring and the side chain of Phe-198 which undergoes the same conformational change upon coenzyme binding as Tyr-197 in GR. In addition to Arg-222 and Arg-228, which are conserved in TR and GR, Tyr-221--the last residue of the second beta-sheet strand of the beta alpha beta dinucleotide binding fold--is in hydrogen bonding distance to the 2' phosphate group of NADPH.

Trypanothionereductase from Trypanosomu cruzi is the most promising target molecule for the rational design of a specific drug against Chagas' disease. The recombinant protein was purified in a single chromatographic step and crystallized. Two crystal forms suitable for X-ray diffraction analysis were obtained. Tetragonal crystals (a=b=87.4 A, ~152.3 A) were grown from 30% polyethylene glycol (average A4, = 8,000) in the presence of 0.2% /3-n-octylglucoside (space group either P42 with one dimer or P4222 with one monomer in the asymmetric umt). Monoclinic crystals (space group P2, a=l36.3 A, b=91.1 A, c=l26.0 A, /3=94") were grown from 1.2 M sodium citrate m the presence of 2% octanoyl-jv-methyl-glucamide.They contain two dimers of the enzyme in the asymmetric unit: both crystal forms diffract to 3 8, resolution.