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.