Kinetic and Molecular Properties of Lysine-sensitive Aspartokinase

Lysine or Mg2+ alone at high concentrations brings about partial dimerization of the lysine-sensitive aspartokinase of Escherichia coli B. The native enzyme, which has a molecular weight of 75,300 and consists of 2 subunits, responds to a combination of lysine and Mg 2+ at much lower concentrations by dimerizing to a tetrameric molecule with a molecular weight approaching 150,000. This dimerization is also observed in the presence of substrates at lysine concentrations known to be inhibitory. The substrates MgATP and aspartate also produce dimerization at high concentrations, and substrate inhibition is observed with MgATP. Aging of aspartokinase results in a loss of cooperativity for lysine inhibition with lysine saturation curves becoming hyperbolic. Aging also results in a loss of lysine-binding capacity and catalytic activity and decreases the extent of dimerization induced by lysine and Mg2+.