Specificity of Aspartokinase III fromEscherichia coliand an Examination of Important Catalytic Residues

Aspartokinase III (AK III) has been purified from a plasmid-containing strain ofEscherichia coli.The enzyme shows broad specificity for the phosphoryl acceptor substrate. Structural analogs of aspartic acid with a derivatized α-carboxyl group are accepted as alternative substrates by the enzyme. Derivatives at the α-amino group are also tolerated by AK III but with diminished catalytic activity. As has been previously observed with aspartokinase I (T. S. Angeles and R. E. Viola, 1992,Biochemistry31, 799), derivatization of the β-carboxyl group, which serves as the phosphoryl acceptor, does not prevent catalytic activity. These β-derivatized analogs are capable of productive binding to these enzymes through a reversal of regiospecificity, making the α-carboxyl group available as the phosphoryl acceptor. Chemical modification and pH profile studies have identified the functional groups of cysteine and histidine as being involved in the catalytic activity of AK III.