HOMOSERINE DEHYDROGENASE OF ESCHERICHIA COLI K 12LAMBDA .I. FEEDBACK INHIBITION BY L-THREONINE AND ACTIVATION BY POTASSIUM IONS

The isolation of homoserine dehydrogenase from Escherichia coli K 12λ is described and the results of a detailed study of the homoserine dehydrogenase catalyzed oxidation of L-homoserine to aspartic β-semialdehyde at pH 8.9 are presented. Based on the results of the kinetic study, a tentative two-substrate-two-modifier model is proposed which appears to be consistent with the experimental data, including that for the activation of the enzyme by K+ and the inhibition of the enzyme by L-threonine. The salient features of the tentative model are (1) the activation of the enzyme by K+ is essential and involves two highly cooperative K+ binding sites, (2) the activation of the enzyme by K+ must precede the binding of homoserine by the enzyme, and (3) both the K+activated enzyme-nicotinamide-adenine dinucleotide phosphate-homoserine complex and the K+activated enzyme-nicotinamide-adenine dinucleotide phosphatehomoserine-threonine complex are capable of forming products. An initial velocity equation is derived by a rapid equilibrium treatment of the model and the estimated values of all of the enzyme-ligand dissociation constants are presented. © 1969, American Chemical Society. All rights reserved.