Aspartokinase threonine sensitive

LT-aspartokinase <10> (<10> lysine-threonine-sensitive isoenzyme [3]) [3] aspartate kinase (phosphorylating) aspartate kinase III <11> [33] aspartic kinase... 

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)... 

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)... 

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... 

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. 

Isoenzymes of enzymes involved in the first step of a branched biosynthetic pathway may differ in their sensitivity to inhibitors. Thus the enzyme aspartate kinase catalyses in Escherichia coli the first step in the synthesis of lysine, methionine and threonine. Three isoenzymes occur, the synthesis and activity of one is suppressed by L-lysine, the activity of the second is depressed by L-threonine and the activity of the third by homoserine (an intermediate in methionine biosynthesis). Thus accumulation of L-lysine or L-threonine suppresses their own further S3mthesis but does not prevent the activity of the aspartokinase isoenzyme involved in methionine synthesis. Again peroxidase isoenzymes differ in their activity in destroying indol-3yl-acetic acid (lAA) and the pattern of peroxidase isoenzymes can be altered by feeding lAA or gibberellic acid (GA) to plant tissues. 

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