Subunit Structure of Aspartokinase III of Escherichia coli K12

The lysine‐sensitive aspartokinase III of Escherichia coli K12 is composed of subunits of 50000 molecular weight. By determination of terminal residues and inspection of tryptic hydrolysis products, the subunits appear identical. The native enzyme is a dimer of 105000 molecular weight in the presence of 0.15 M KCl in the absence of lysine. Two types of equilibrium can be evidenced: an association at high ionic strength in the presence of lysine leading to a tetrameric form and a dissociation towards monomer at low ionic strength in the absence of lysine. The two equilibria are dependent on protein concentration.