INVESTIGATION OF FREE ASPARTATE-AMINOTRANSFERASE CRYSTALS

Data are presented on the structure of free aspartate aminotransferase from chicken heart cytosol at a resolution of 2.7 angstrom. The three -dimensional structure of the entire molecule and the structure of the active site were established by X-ray diffraction studies. It is conc luded that the three-dimensional structure of the entire molecule and of the active site of the free enzyme resemble the structures of the f ree enzyme obtained after 2-methylaspartate is removed from the crysta l. The subunits of the crystalline enzyme's dimeric molecule have diff erent conformations: one subunit is in the open conformation, while th e other is in the closed. A hypothesis is proposed that the position o f the O5'-phosphate ''behind'' the pyrimidine ring is the decisive fac tor determining the transamination rate.