Aspartokinase-homoserine dehydrogenases

Starnes, W.L. Munk, P. Maul, S.B. Cunningham, G.N. Cox, D.J. Shive, W. Threonine-sensitive aspartokinase-homoserine dehydrogenase complex, amino acid composition, molecular weight, and subunit composition of the complex. Biochemistry, 11, 677-687 (1972)... 

Purification of a threonine-sensitive, aspartokinase-homoserine dehydrogenase complex comparisons of the adsorptivities of agarose derivatives as a function of the length of the alkyl chain 60... 

Veron, M. Falcoz-Kelly E Cohen, G.N. The threonine-sensitive homoserine dehydrogenase and aspartokinase activities of Escherichia coli K12. The two catalytic activities are carried by two independent regions of the polypeptide chain. Eur. J. Biochem., 28, 520-527 (1972)... 

Overproduction of E (isoleucine) inhibits enzyme E6 (threonine deaminase), and the consequent rise of D (threonine) reduces the rate of production of C (homoserine) via enzyme E3 (homoserine dehydrogenase). The concentration of B (aspartate semialdehyde) rises, and this in turn inhibits Ej (aspartokinase). It is therefore obvious why the control system is called a negative feedback network, or sequential feedback system. 

The pathway of biosynthesis of L-lysine and L-threonine in Corynebacterium glutamicum is shown in Fig. 1. The first step, the formation of phosphoaspartate from aspartate, is catalyzed by aspertokinase and this enzyme is susceptible to the concerted feedback inhibition by L-lysine and L-threonine. The auxotrophic mutant of homoserine (or threonine plus methionine), lacking homoserine dehydrogenase, was constructed and found to produce L-lysine in the culture medium. Second, the mutants which show the threonine or methionine sensitive phenotype caused by the mutation on homoserine dehydrogenase (low activity) was also found to produce appreciable amounts of L-lysine in the culture medium. Furthermore, a lysine analogue (S-aminoethylcysteine) resistant mutant was obtained as an L-lysine producer and in this strain aspartokinase was insensitive to the feedback inhibition. 

In E. coli there are three aspartokinases that catalyze the conversion of aspartate to p-aspartyl phosphate. All three catalyze the same reaction, but they have very different regulatory properties, as is indicated in Fig. 24-13. Each enzyme is responsive to a different set of end products. The same is true for the two aspartate semialdehyde reductases which catalyze the third step. Both repression of transcription and feedback inhibition of the enzymes are involved. Two of the aspartokinases of E. coli are parts of bifimctional enzymes, which also contain the homoserine dehydrogenases that are needed to reduce aspartate semialdehyde in the third step. These aspar-tokinase-homoserine dehydrogenases 1 and 11 (Fig. 

The Aspartokinases and Homoserine Dehydrogenases of Escherichia coli Georges N. Cohen... 

In E. coli there are three aspartokinases I (coded by thrA), II (coded by met L), and III (coded by lysC) and two homoserine dehydrogenases I and II that are inhibited or repressed by only one or two amino acids of the aspartate family which ensures that pathway is not shut down with the excess of one product. Each amino acid regulates the first enzyme in its branch to maintain the proper ratio of the amino acids. In Corynebacterium, the regulation is much simpler with only one aspartokinase and here the amino acid biosynthesis is controlled by the synergistic action of the end products. 

Fazel A, Mueller K, Le Bras G, Garel JR, Veron M, Cohen GN (1983) A triglobular model for the polypeptide chain of aspartokinase I-homoserine dehydrogenase I of Escherichia coli. Biochemistry 22 158-165... 

The intermediate of L-lysine biosynthesis, L-aspartyl-semialdehyde, is also a precursor for the biosynthesis of L-threonine, L-isoleucine, and L-methionine. Homoserine dehydrogenase catalyses NADPH-dependent reduction of L-aspartyl-semialdehyde to L-homoserine. Flux from L-aspartyl-semialdehyde toward L-homoserine could be reduced by introducing alleles for less active homoserine dehydrogenase variants. In addition, L-lysine production increased as the prevailing threonine concentrations in such horn mutants were too low for feedback inhibition of aspartokinase [67, 68]. 

The forward and reverse reactions were verified by showing a stoichiometric relation between decrease of 3-aspartyl phosphate and TPNH and the increase of aspartyl semialdehyde in the forward reaction and an equivalent formation of /3-aspartyl phosphate and TPN when the components of the reverse reaction were incubated. Participation of phosphate in the reverse reaction was demonstrated by the dependence of the equilibrium level of TPNH on the phosphate concentration. The product of aspartyl phosphate reduction was established to be L-aspartyl /3-semialdehyde by its subsequent reaction with DPNH and homoserine dehydrogenase to yield homoserine. /3-Aspartyl phosphate formed by aspartyl semi-aldehyde oxidation was identified by its reaction with ADP in the (8-aspartokinase system. 

Phosphofructokinase ( . coli) Phosphofructokinase (yeast) Aspartokinase I-Homoserine dehydrogenase I (E. coli) Pyruvate kinase (yeast)... 

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