The role of lysine and leucine binding on the catalytical and structural properties of aspartokinase III of Escherichia coli K 12.

Abstract

The lysine‐sensitive aspartokinase of Escherichia coli K12, a 100000‐molecular weight dimer, possesses four binding sites for L‐lysine, its allosteric inhibitor. The lysine binding is cooperative. In the presence of leucine this cooperative effect disappears, that may explain the synergistic inhibition phenomenon observed between these two amino acids. The four sites are non‐equivalent two by two, with a K of, respectively, 8 μM and 100 μM. The tetramerization of the protein observed in the presence of lysine appears not to be related to the inhibition of enzyme activity. Lysine or leucine binding leads to transconformational changes evidenced by difference spectra and modification in – SH titration rates. All the results are consistent with the hypothesis that the lysine‐sensitive aspartokinase belongs to the class V of allosteric enzymes.