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The sedimentation coefficient of the lysine-sensitive aspartokinase of Escherichia coli B in sucrose gradients containing lysine increases with increasing temperature and ionic strength. Sedimentation equilibrium molecular weight determinations provide the basis for interpreting the sedimentation velocity results as being due to a rapidly reversible, lysine-mediated association reaction predominantly of the monomer-dimer type. The dependencies of dimerization and lysine binding on lysine concentration are almost identical, with the half-maximal response occurring around 30 µm. Hill coefficients for lysine inhibition demonstrate a temperature dependence which parallels that for dimerization. Maximum dimerization and maximum Hill coefficients are obtained at temperatures above 20°. Arrhenius plots of enzyme activity are biphasic with a break occurring at 20°. The response of the kinetics of lysine inhibition to the parameters influencing enzyme structure together with the stoichiometry of lysine binding (2 lysines per protein dimer or per 4 subunits) suggest that the association reaction is related to the cooperativity of lysine inhibition.

The lysine-sensitive aspartokinase of Escherichia coli B has a near absolute requirement for a monovalent cation for catalytic activity. This requirement is fulfilled by either K+ or NH4+. S0.5 values for these cations are 25 mm and 7 mm, respectively. Both cations are inhibitory at high concentrations. This inhibition is more marked with NH4+ than with K+. Potassium does not alter the S0.5 value for aspartate, suggesting that the cation effects are on catalysis rather than substrate binding. Both cations antagonize inhibition by lysine and leucine. The I0.5 value for lysine is about 0.17 mm at 0.66 m KCl and is decreased to about 0.05 mm at 0.022 m KCl. There is a slight but probably real decrease in cooperativity of lysine inhibition at the lower cation concentrations.

The apparent specificity of activation of lysine-sensitive aspartokinase (E.C.2.7.2.4) from E. coli by monovalent cations differs depending on the assay used and on the Mg2+ concentration. Activity is nearly absolutely dependent on and is highly specific for a monovalent cation in the aspartate semialdehyde dehydrogenase coupled assay or the adenosine triphosphate-adenosine diphosphate exchange assay. Little specificity for monovalent cations is observed using the aspartyl hydroxamate assay. Activation and specificity are also altered by Mg2+ concentrations at a constant 5 mM nucleotide concentration. At a low (1.25 or 1.6 mM)Mg2+ concentration, monovalent cation activation and specificity are nearly absolute. Less dependence on monovalent cations and less specificity are observed at a higher Mg2+ concentration (6 mM). Li+ inhibits aspartokinase competitively with respect to either K+ or NH4+. Monovalent cations are also thermoprotective and differential thermal inactivation experiments at 56 degrees C reveal that NH4+ and K+, either of which will produce maximum catalytic activity, interact differently with aspartokinase. K+ interacts with positive cooperativity, whereas NH4+ does not. K+, NH4+, and Na+ are about equally effective in enhancing the dissociation of the aspartokinase-aspartylphosphate complex. Li+ is less effective.