Multivalent Feedback Inhibition of Aspartokinase in Bacillus polymyxa

Abstract Several nonpolar amino acids protect aspartokinase from inactivation by the cationic detergent trimethyloctadecylammonium chloride and by heat. The active agents are l-leucine, l-phenylalanine, l-methionine, l-tryptophan, l-alanine, glycine, l-valine, and l-isoleucine. d-Amino acids and other l-amino acids are inactive. The same nonpolar l-amino acids also reverse the inhibition of aspartokinase caused by the feedback inhibitors, l-threonine and l-lysine. They differ in their specificity toward the feedback inhibitors. Amino acids of Group I (l-tryptophan, l-methionine, and l-norleucine) counteract the inhibition by l-lysine only, amino acids of Group II (l-leucine, l-valine, l-isoleucine, and l-norvaline) counteract the inhibition by l-threonine only, and those of Group III (l-phenylalanine, l-alanine, glycine, and allylglycine) counteract the inhibition by both l-lysine and l-threonine. Cooperativity is not observed between amino acids belonging to different groups in overcoming the inhibition by combinations of the feedback inhibitors or in protecting the enzyme from inactivation by trimethyloctadecylammonium chloride. Group I amino acids antagonize the ability of those of Groups II and III for overcoming the inhibition by l-threonine. Similarly, Group II amino acids counteract those of Groups I and III in overcoming the inhibition by l-lysine. Group I amino acids act cooperatively with l-threonine to inhibit aspartokinase, and those of Group II act cooperatively with l-lysine. There is no correlation between the ability of an amino acid to overcome feedback inhibition and its ability to antagonize the effects of amino acids belonging to other groups. These results can be accounted for by a model that assumes a single binding site for all nonpolar l-amino acids and different patterns of nonexclusive binding of these amino acids to the site in four allosteric states of the enzyme. Some biological implications of the effect of nonpolar l-amino acids on the feedback inhibition of aspartokinase are discussed.