... mecha-nism of transphosphorylation, which implies the ABFig. 3. Stereo diagram of the active center of HtA. (A) Superposition of the active-siteresidues of the 20 conformers of the solu-tion structure ... electrostatically with the othercharged groups. In the NMR structures, this sidechain of D40 is a short distance (£ 3A˚) from the sidechains of H42 and R95. Finally, the aromatic ring of F126, placed ... second-ary structure elements. As in other ribotoxins, theseABCFig. 2. Representation of the 3D structure of HtA in solution. (A) Superposition of the 20 best structures obtained in this work(PDB...