Georges N. Cohen

Rétro-inhibition et répression de l'homosérine déshydrogénase d'Escherichia coli

Abstract The homoserine dehydrogenase of Escherichia coli K12 is under the control of the two types of known regulatory negative feedback mechanisms: repression and end-product inhibition. The regulatory agent is l -threonine. End-product inhibition by l -threonine is highly specific and strictly non-competitive. The inhibition site of the enzyme can be selectively destroyed by heat. The stability of the enzyme and of its capacity to be inhibited have been studied. The sedimentation constant of the inhibitable enzyme is higher than that of the enzyme having lost its inhibition site. The synthesis of E. coli homoserine dehydrogenase is greatly and specifically repressed by l -threonine in the culture medium. The genetic control of the synthesis of the aspartic acid family of amino acids is discussed.

Regulation by methionine of the synthesis of third aspartokinase and of a second homoserine dehydrohenase in Escherichia coli K 12

Abstract 1. 1. Evidence is presented that, in addition to the lysine-sensitive aspartokinase III and to the threonine-sensitive association of aspartokinase I and homoserine dehydrogenase I, there exist in Escherichia coli K 12 two heretofore undetected enzymes: aspartokinase II and homoserine dehydrogenase II. The synthesis of these two enzymes is under repressive control by methionine. 2. 2. Some of their properties are described: it should be noted that their activity is not end-product regulated. 3. 3. A constitutive mutant for the two novel activities has been isolated. 4. 4. In E. coli B, homoserine dehydrogenase II is absent. 5. 5. The overall features of repression and end-product inhibition in branched biosynthetic pathways are discussed.

The Aspartokinases and Homoserine Dehydrogenases of Escherichia coli

Publisher Summary This chapter discusses the aspartokinases and homoserine dehydrogenases of Escherichia coli . The repression of the synthesis of an early enzyme of the biosynthetic pathway—such as aspartokinase—or its efficient feedback inhibition by any one of the essential metabolites should create insuperable difficulties for the bacterial cell, because it would limit the supply of the synthesis of the common intermediates necessary for the synthesis of the other essential metabolites of the pathway. When E. coli K12 is grown under one or the other repressive conditions, only one type of aspartokinase is found, which can then be totally inhibited by its specific feedback inhibitor. The synthesis of homoserine dehydrogenase, as in the case of the threonine-sensitive aspartokinase, is repressed when threonine and isoleucine are present in excess and are derepressed under the limitation of either of the two amino acids.

La β-aspartokinase sensible a la lysine d'escherichia coli; purification et propriétés

Summary Lysine-sensitive aspartokinase (ATP: L -aspartate 4-phosphotransferase, EC 2.7.2.4) of Escherichia coli has been obtained, with a specific activity about 4000 times higher than in crude extracts from non-derepressed cells. Some of its properties (kinetic constants, approximate molecular mass, activation energy, characteristics of the inhibition by lysine and analogues, protection by inhibitor against thermal in-activation) are described. The importance of the regulation of the enzyme activity by lysine in vivo is discussed.

The threonine-sensitive homoserine dehydrogenase and aspartokinase activities of Escherichia coli: III. Inactivation at pH 9

1. 1. Homoserine dehydrogenase I (l-homoserine:NADP+ oxidoreductase, EC 1.1.1.3) of Escherichia coli is inactivated with apparent first-order kinetics by exposure at pH 9 in Tris buffer. This inactivation is accompanied by desensitization of the enzyme towards threonine. 2. 2. L-Aspartate and ATP, the substrates of the associated activity, β-aspartokinase I (ATP:L-aspartate 4-phosphotransferase, EC 2.7.2.4) protect against the desensitization of homoserine dehydrogenase.