Threonine aspartate kinase

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

Azevedo, R.A. Smith, R.J. Lea, P.J. Aspartate kinase regulation in maize Evidence for co-purification of threonine-sensitive aspartate kinase and homoserine dehydrogenase. Phytochemistry, 31, 3731-3734 (1992)... 

Heremans, B. Jacobs, M. A mutant of Arabidopsis thaliana (L.) Heynh. with modified control of aspartate kinase by threonine. Biochem. Genet., 35, 139-153 (1997)... 

Arevalo-Rodriguez, M. Calderon, I.L. Holmberg, S. Mutations that cause threonine sensitivity identify catalytic and regulatory regions of the aspartate kinase of Saccharomyces cerevisiae. Yeast, 15, 1331-1345 (1999)... 

Paris, S. Wessel, P.M. Dumas, R. Overproduction, purification, and characterization of recombinant bifunctional threonine-sensitive aspartate kinase-homoserine dehydrogenase from Arabidopsis thaliana. Protein Expr. Purif., 24, 105-110 (2002)... 

Paris, S. Viemon, C. Curien, G. Dumas, R. Mechanism of control of Arabidopsis thaliana aspartate kinase-homoserine dehydrogenase by threonine. J. Biol. Chem., 278, 5361-5366 (2003)... 

The L-threonine biosynthetic pathway consists of five enzymatic steps from L-aspartate. E. coli has three aspartate kinase isoenzymes, key enzymes which catalyze the first reaction of the L-threonine biosynthetic pathway. The aspartate kinase isoenzymes I, II, and III encoded by the thrA, metL, and lysC genes, respectively, are affected by feedback inhibition by L-threonine, L-methionine, and L-lysine, respectively. C. glutamicum has only one aspartate kinase encoded by the lysC gene, which is subjected to feedback inhibition by L-lysine and... 

Fig. 10. Schematic representation of the split biosynthetic pathway of L-lysine in wildtype Corynebacterium glutamicum including the branch point of aspartate semialdehyde distribution. The metabolites derived from the aldehyde via the synthase activity are D,L-di-aminopimelate and L-lysine, whereas that resulting from dehydrogenase activity are L-threo-nine, L-methionine, and L-isoleucine. The activity of the dehydrogenase is inhibited at elevated L-threonine concentrations and its synthesis is repressed by L-methionine. Accumulating intracellular lysine causes feedback inhibition of aspartate kinase and activates lysE transcription...

Biosynthesis metabolism Asp is formed from oxaloacetic acid by aspartate aminotransferase (EC 2.6.1.1) and serves as starting material in the biosyntheses of threonine, methionine, and lysine. The first step is catalysed by aspartate kinase (EC 2.7.24) which only occurs in plants and microorganisms. This enzyme exists as 3 isozymes in Escherichia coli and exhibits a typical example of feedback regulation. Asp plays a central role in the biosyntheses of pyrimidines and purines. In the urea cycle Asp condenses with " citrulline to aigininosuccinate, a stimulating neuro-transmitter. ... 

Problems associated with identification of specific regulatory mechanisms can be illustrated by a brief consideration of changes in the apparent levels of aspartate kinase during plant growth. Aspartate kinase extracted from whole carrot roots, freshly sliced root tissue, or recently subcultured cell suspensions is predominantly sensitive to inhibition by the pathway product, threonine. However, when root slices are incubated in a growth stimulating medium or when suspension cultures are allowed to grow for a period of time, the bulk of the extractable aspartate kinase activity is sensitive to... 

Inhibitor constants (A j) or the concentration of inhibitor required for half-maximal inhibition under specified assay conditions ffo.s) provide an indication of the quantitative sensitivity of an enzyme. Such indicators of plant aspartate kinase sensitivity are consistently below 1.0 mM and in several instances are less than 100 fiM (Table III). Even though the effective concentration of regulatory metabolites in plant cells is difficult to estimate (Section III,C), the demonstrable inhibitory effects of comparatively low concentrations of lysine or threonine on aspartate kinase i/t vitro suggest that enzyme activity is likely to be regulated by these pathway products in vivo. 

Several plant aspartate kinases are activated by other amino acids such as valine, alanine, and isoleucine (Table HI). Activation does not appear to be a general effect of hydrophobic amino acids, since neither methionine nor leucine influence the activity of the maize enzyme yet, leucine has been reported to activate the enzyme isolated from Sinapsis alba and inhibit the enzyme from Helianthus annus. Interaction of these secondary effectors with the various aspartate kinases has not been fully explored, but several observations suggest a considerable degree of complexity. Alanine partially relieves threonine inhibition of the enzyme isolated from pea seedlings (Aames and Rognes, 1974). Lysine inhibition of maize aspartokinase is diminished in the presence of isoleucine, alanine, or valine (Bryan e/ al., 1970) and threonine, even though it does not inhibit the maize enzyme, counteracts... 

Aspartate kinase is usually subject to very strong inhibition by lysine and/or threonine (Miflin et ai, 1979). Even if the worker can be certain that the activity that is measured is due to the action of asparagine synthetase, there is still no information on whether glutamine or ammonia is acting as the amino donor. It is for these reasons that the hydroxamate assay is not recommended. 

Lysine plus threonine severely inhibits growth of maize in a synergistic manner. Growth inhibition could result from combined effects of lysine on aspartate kinase and threonine on homoserine dehydrogenase, resulting in starvation for methionine. Growth inhibition by lysine -i- threonine can be overcome by supplying methionine. Bryan [1980) Miflin [1977). 

Fig. 1. Bio thetic pathways for the essential aspartate-family amino adds. The numbers represent enzymes catalyzing the reaction 1, aspartate kinase 2, homoserine dehydrogenase 3, homoserine kinase 4, threonine thase 5, threonine dehydrogenase 6, acetolactate thase 7, dihydrodipicolinate thase 8, diaminopimelate decarboiylase.

Aspartate kinase, aspartate semialdehyde dehydrogenase 2 homoserine dehydrogenase 3 homoserine kinase, threonine synthase 4 aspartate 4-decarboxylase... 

Fig. 14.1 The biosynthesis pathway of L-threonine. The pathway consists of centeral metabolic pathways and the threonine terminal pathways. The centeral metabolic pathways involve glycolysis, phosphate pentose pathway, TCA cycle and anaplerotic pathways. The threonine terminal pathway consists of five enzymetic steps. The first, third, and fourth reactions are catalyzed by the three key enzymes aspartate kinase, homoserine dehydrogenase, tmd homoserine kinase, respectively. There are four competing pathways that affect the biosynthesis of L-threonine, leading to formation of L-lysine, L-metMonine, L-isoleucdne, and glycine...

Starting from the building block of L-aspartate, the biosynthesis of L-threonine comprises five successive reactions sequencially catalyzed by aspartate kinase, aspartyl semialdehyde dehydrogenase, homoserine dehydrogenase, homoserine kinase and threonine synthase. 

Fig. 14.3 L-threonine and L-lysine binding sites in C. glutamicum aspartate kinase, (a) The dimer structure of ap-subunits is shown. The regulatory domain from a-subunit is shown in green and P-subunit is shown in blue. The bound L-threonine and L-lysine molecules are shown in orange and pink, respectively, (b) The bound L-threonine molecule and the amino acid residues directly involved in its binding are shown in the manner of sticks, (c) The bound L-lysine molecule and the amino acid residues directly involved in its binding are shown in the manner of sticks. These structural models are built by using the PyMOL software, Protein Data Bank (accession number 3AAW) and the published information by Yoshida et al. (2010)...

Kato C, Kurihara T, Kobashi N, Yamane H, Nishiyama M (2004) Conversion of feedback regulation in aspartate kinase by domain exchange. Biochem Bioph Res Co 316 802-808 Kim YH, Park JS, Cho JY, Cho KM, Park YH, Lee J (2004) Proteomic response analysis of a threonine-overproducing mutant of Escherichia coli. Biochem J 381 823-829 Klaffl S, Eikmanns BJ (2010) Genetic and functional analysis of the soluble oxaloacetate decarboxylase from Corynebacterium glutamicum. J Bacterid 192 2604-2612 Komatsubara S, Kisumi M, Murata K, Chibata 1 (1978) Threonine production by regulatory mutants of Serratia marcescens. Appl Environ Microbiol 35 834-840 Kotaka M, Ren J, Lockyer M, Hawkins AR, Stammers DK (2006) Structures of R- and T-state Escherichia coli aspartokinase 111 mechanisms of the allosteric transition and inhibition by lysine. J Biol Chem 281 31544-31552... 

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