[Structural basis for differences in substrate specificity of aspartate aminotransferase isoenzymes]

X-Ray structural data concerning the substrate binding site of cytosolic chicken aspartate aminotransferase (AspAT) are reported. The structure of the complex of AspAT with the substrate-like inhibitor maleate has been refined at 2.2 A resolution. The lengths of hydrogen bonds between a bound molecule of maleate and side chains of amino acid residues in the active site are presented as well as other interatomic distances in the substrate binding site. The data obtained for the cytosolic AspAT have been compared with those for the mitochondrial chicken AspAT. It has been inferred that differences in substrate specificity of the AspAT isoenzymes are determined by interactions involving amino acid residues which are situated in the immediate vicinity of the active site and influence ionization or orientation of functional groups interacting with substrate. An explanation is suggested for different rates of transamination of aromatic amino acids in the active sites of the cytosolic and mitochondrial isoenzymes.