Conformations of lysine-sensitive aspartokinase.

1. The technique of differential thermal and proteolytic inactivation has been employed as a conformational probe for the lysine-sensitive aspartokinase (EC 2.7.2.4) of Escherichia coli B. 2. L-Amino acid inhibitors of this enzyme each induce a characteristic enzyme conformation. This is evidenced by rates of thermal and proteolytic inactivation and Arrhenius activation energies for thermal inactivation which are characteristic of the amino acid present. 3. Phenylalanine and leucine binding are mutually exclusive as evidenced by competitive behavior in thermal inactivation experiments, suggesting a hydrophobic amino acid binding site with broad specificity. 4. The phenylalanine-dependent conformation and the leucine-dependent conformation differ considerably. In comparison with the native enzyme, the former is more labile to proteolysis by trypsin whereas the latter is more stable. First-order rate constants for thermal inactivation of the phenylalanine- and leucine-dependent conformations are, respectively, about one-half and one-tenth that of the native enzyme. 5. Items 3 and 4 taken together suggest that the conformations are ligand induced and do not arise via ligand stabilization of spontaneously arising conformers.