title: The threonine-sensitive homoserine dehydrogenase and aspartokinase activities of Escherichia coli K12. Carboxymethylation of the enzyme: threonine binding and inhibition are functionally dissociable.
abstract: The inactivation of the aspartokinase I-homoserine dehydrogenase I by iodoacetic acid and the effect on the sensitivity to its inhibitor, L-threonine, were examined. Both aspartokinase and homoserine dehydrogenase inactivation, as well as the dehydrogenase desensitization toward L-threonine occur as a pseudo-first order process. During its inactivation, the aspartokinase remains sensitive to L-threonine. At 50% inactivation, the inhibition curve of the aspartokinase by L-threonine displays homotropic cooperative effects. This alkylated protein retains eight binding sites for L-threonine. During the carboxymethylation, the protein remains in the tetrameric form until half of the kinase activity is lost. At the end of the inactivation aggregate forms and dimers appear.
mesh_terms: D000478:Alkylation; D001225:Aspartokinase Homoserine Dehydrogenase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D001665:Binding Sites; D004926:Escherichia coli / Q000201:enzymology*; D007461:Iodoacetates; D046911:Macromolecular Substances; D009097:Multienzyme Complexes / Q000378:metabolism*; D013912:Threonine / Q000378:metabolism*
chemical_list: D007461:Iodoacetates; D046911:Macromolecular Substances; D009097:Multienzyme Complexes; D013912:Threonine; D001225:Aspartokinase Homoserine Dehydrogenase