Crystal structure of triosephosphate isomerase from Trypanosoma cruzi in hexane

To gain insight into the mechanisms of enzyme catalysis in organic solvents , the x-ray structure of some monomeric enzymes in organic solvents was det ermined. However, it remained to be explored whether the structure of oligo meric proteins is also amenable to such analysis, The field acquired new pe rspectives when it was proposed that the x-ray structure of enzymes in nona queous media could reveal binding sites for organic solvents that in princi ple could represent the starting point for drug design. Here, a crystal of the dimeric enzyme triosephosphate isomerase from the pathogenic parasite T rypanosome cruzi was soaked and diffracted in hexane and its structure solv ed at 2-Angstrom resolution. Its overall structure and the dimer interface were not altered by hexane. However, there were differences in the orientat ion of the side chains of several amino acids, including that of the cataly tic Glu-168 in one of the monomers. No hexane molecules were detected in th e active site or in the dimer interface. However, three hexane molecules we re identified on the surface of the protein at sites, which in the native c rystal did not have water molecules. The number of water molecules in the h exane structure was higher than in the native crystal. Two hexanes localize d at <4 Angstrom from residues that form the dimer interface; they were in close proximity to a site that has been considered a potential target for d rug design.