Crystalline mitochondrial aspartate aminotransferase exists in only two conformations

The subunits of the alpha 2-dimeric enzyme aspartate aminotransferase are composed of two distinct domains, one large and one small. The active sites are situated close to both the intersubunit and the interdomain interface. Binding of substrate analogues to the active site induces a large conformational change in the enzyme, whereby the small domain rotates by 13 degrees relative to the large domain and completely buries the ligand. We have determined the crystal structures of chicken mitochondrial aspartate aminotransferase (mAATase) in two new crystal forms. A comparison of the structures of mAATase in five crystal forms, including both the unliganded and the liganded enzyme, shows that mAATase exists in either one of two unique conformations, with only minimal adaptations to the crystal lattice. This suggests that both the open, unliganded and closed, liganded structure of mAATase are, to a large extent, stabilized by intramolecular interactions, and are consequently representative of functional states of the protein in solution. A 2-fold-symmetric packing interaction between small domains occurring identically in three crystal forms of mAATase is described.