Aspartokinase in Lemna minor L: Studies on the in Vivo and in Vitro Regulation of the Enzyme

Aspartokinase has been isolated from wheat germ and a preliminary survey made of its properties in a partially purified extract. The enzyme has an absolute requirement for ATP and a divalent metal ion. The phosphate donor can be either ATP or GTP, but other nucleotides are ineffective. Both magnesium and manganese will activate the enzyme, whereas calcium shows a trace amount of activity. The enzyme has a Km of 16.7 mm for aspartate, 1.2 mm for ATP, and 3.3 mm for MgCl₂. Lysine inhibits the reaction at fairly low concentrations, and threonine inhibits at high concentrations. Other amino acids which are derived from aspartate (methionine, homoserine, threonine, and isoleucine) have little effect. When lysine and threonine are added together, they show a concerted inhibition of the reaction. The enzyme is also stabilized against heat inactivation by lysine and threonine together but not by either when added separately. It is suggested that aspartokinase from plants is a regulatory enzyme and exhibits a concerted feedback mechanism.     

Interaction of substrates and inhibitors with the homoserine dehydrogenase of kinase-inactivated aspartokinase I.

The aspartokinase activity of the aspartokinase-homoserine dehydrogenase complex of Escherichia coli was affinity labeled with substrates ATP, aspartate, and feedback inhibitor threonine. Exchange-inert ternary adducts of Co(III)-aspartokinase and either ATP, aspartate or threonine were formed by oxidation of corresponding Co(II) ternary complexes with H2O2. The ternary enzyme-Co(III)-threonine adduct (I) had 3.8 threonine binding sites per tetramer, one-half that of the native enzyme. The binding of threonine to I was still cooperative as determined by equilibrium dialysis (nH = 2.2) or by studying inhibition of residual dehydrogenase activity (nH = 2.7). Threonine still protected the SH groups of I against 5,5'-dithiobis(2-nitrobenzoate) (DTNB) reaction but the number of SH groups reacting with thiol reagents (DTNB) was reduced by 1-2 per subunit in the absence of threonine. This suggests either that Co(III) is bound to the enzyme via sulfhydryl groups or that 1-2SH groups are buried or rendered inaccessible in I. The binding of threonine to sites not blocked by the affinity labeling produced changes in the circular dichroism of the complex comparable to changes produced by threonine binding to native enzyme and also protected against proteolytic digestion. The major conformational changes produced by threonine are thus ascribable to binding at this one class of regulatory sites. The interactions of kinase substrates with various aspartokinase-Co(III) complexes containing ATP, aspartate, or threonine and a threonine-insensitive homoserine dehydrogenase produced by mild proteolysis were studied. The inhibition of homoserine dehydrogenase by kinase substrates is not due to binding of these inhibitors at the kinase active site but was shown to be due to binding to sites within the dehydrogenase domain of the enzyme. L-alpha-Aminobutyrate, a presumed threonine analogue, also inhibits the dehydrogenase by binding at the same or similar sites in the dehydrogenase domain and not at threonine regulatory site.     

beta-Aspartokinase from developing endosperm of maize.

β-aspartokinase (EC 2.7.2.4.) has been isolated from the developing endosperm (30 days post-pollination) of Zea mays (cv. Pioneer 3145). Enzyme activity was dependent upon ATP, Mg⁺⁺ or Mn⁺⁺, aspartate, and protein concentration. Double reciprocal plots of velocity vs. aspartate concentrations deviated from a straight line at low aspartate concentration indicating two apparent Km's of 0.5 and 6.6 mM. Enzyme activity was inhibited by lysine but not by methionine or threonine. The endosperm-derived β-aspartokinase behaved similarly to enzyme isolated from 6-day-old etiolated shoot tissue. The presence of β-aspartokinase in developing endosperm provides new insight into the source of the aspartate-derived amino acids in maize endosperm.     

Regulatory Structure of the Biosynthetic Pathway for the Aspartate Family of Amino Acids in Lemna paucicostata Hegelm. 6746, with Special Reference to the Role of Aspartokinase

Comprehensive studies were made with Lemna paucicostata Hegelm. 6746 of the effects of combinations of lysine, methionine, and threonine on growth rates, soluble amino acid contents, aspartokinase activities, and fluxes of 4-carbon moieties from aspartate through the aspartokinase step into the amino acids of the aspartate family. These studies show that flux in vitro through the aspartokinase step is insensitive to inhibition by lysine or threonine, and confirm previous in vitro data in establishing that aspartokinase in vivo is present in two orders of magnitude excess of its requirements. No evidence of channeling of the products of the lysine- and threonine-sensitive aspartokinases was obtained, either form of the enzyme alone being more than adequate for the combined in vivo flux through the aspartokinase step. The marked insensitivity of flux through the aspartokinase step to inhibition by lysine or threonine strongly suggests that inhibition of aspartokinase by these amino acids is not normally a major factor in regulation of entry of 4-carbon units into the aspartate family of amino acids. Direct measurement of fluxes of 4-carbon units demonstrated that: (a) Lysine strongly feedback regulates its own synthesis, probably at the step catalyzed by dihydrodipicolinate synthase. (b) Threonine alone does not regulate its own synthesis in vivo, thereby confirming previous studies of the metabolism of [¹⁴C]threonine and [¹⁴C]homoserine in Lemna. This finding excludes not only aspartokinases as an important regulatory determinant of threonine synthesis, but also two other enzymes (homoserine dehydrogenase and threonine synthase) suggested to fulfill this role. Complete inhibition of threonine synthesis was observed only in the combined presence of accumulated threonine and lysine. The physiological significance of this single example of apparent regulation of flux at the aspartokinase step, albeit under unusually stringent conditions of aspartokinase inhibition, remains to be determined. (c) Isoleucine strongly inhibits its own synthesis, probably at threonine dehydratase, without causing compensatory reduction in threonine synthesis. A fundamentally changed scheme for regulation of synthesis of the aspartate family of amino acids is presented that has important implications for improvement of the nutritional contents of these amino acids in plants.     

Site-directed mutagenesis of a residue located in the regulatory site of Escherichia coli aspartate transcarbamoylase. Involvement of lysine 94 in effector binding and the allosteric mechanism

Lysine 94 in the regulatory chain of aspartate transcarbamoylase has been changed to a glutamine residue by site-directed mutagenesis. The resulting enzyme is almost insensitive to the activator ATP and shows a substantially reduced response to the feedback inhibitor CTP. Competition experiments indicate that ATP is still able to bind at low concentrations to the regulatory site of the mutant enzyme, even though no stimulation could be detected. When the nucleosides adenosine or cytidine were used, the saturation curves of the mutant and the wild-type enzyme became indistinguishable. Together these results indicate that lysine 94 is strongly involved in the binding of ATP and CTP by interacting specifically with the triphosphate moiety of these nucleotide effectors. Furthermore, unlike the wild-type enzyme, the inhibitory and stimulatory effects in the mutant enzyme are insensitive to changes in aspartate concentrations, implying that the lysine 94 side chain is also involved in the allosteric mechanism of the enzyme.     

Threonine-sensitive aspartate kinase and homoserine dehydrogenase from Pisum sativum

Aspartate kinase and two homoserine dehydrogenases were partially purified from 4-day-old pea seedlings. A sensitive method for measuring aspartate kinase activity is described. Aspartate kinase activity was dependent upon ATP, Mg²⁺ or Mn²⁺, and aspartate. The aspartate kinase was inhibited in a sigmoidal manner by threonine and K_i for threonine was 0·57 mM. The enzyme could be desensitized to the inhibitor and threonine protected the enzyme against thermal inactivation. Aspartate kinase activity was enhanced by isoleucine, valine and alanine. Homoserine, methionine and lysine were without effect. The homoserine dehydrogenase activity which was associated with aspartate kinase during purification could be resolved into two peaks by gel filtration. The activity of both peaks was inhibited by aspartate and cysteine and one was inhibited by threonine.     

Threonine Overproduction in Transgenic Tobacco Plants Expressing a Mutant Desensitized Aspartate Kinase of Escherichia coli

In higher plants, the synthesis of the essential amino acid threonine is regulated primarily by the sensitivity of the first enzyme in its biosynthetic pathway, aspartate kinase, to feedback inhibition by threonine and lysine. We aimed to study the potential of increasing threonine accumulation in plants by means of genetic engineering. This was addressed by the expression of a mutant, desensitized aspartate kinase derived from Escherichia coli either in the cytoplasm or in the chloroplasts of transgenic tobacco (Nicotiana Tabacum cv Samsun NN) plants. Both types of transgenic plants exhibited a significant overproduction of free threonine. However, threonine accumulation was higher in plants expressing the bacterial enzyme in the chloroplast, indicating that compartmentalization of aspartate kinase within this organelle was important, although not essential. Threonine overproduction in leaves was positively correlated with the level of the desensitized enzyme. Transgenic plants expressing the highest leaf aspartate kinase activity also exhibited a slight increase in the levels of free lysine and isoleucine, both of which share a common biosynthetic pathway with threonine, but showed no significant change in the level of other free amino acids. The present study proposes a new molecular biological approach to increase the limiting content of threonine in higher plants.     

Cobalt(III) affinity-labeled aspartokinase. Formation of substrate and inhibitor adducts.

The kinase active site of the aspartokinase-homoserine dehydrogenase enzyme complex of Excherichia coli has been affinity labeled both with substrates aspartate and adenosine triphosphate and feedback inhibitor threonine. Co(III) exchange-inert adducts of aspartokinase and inhibitor or substrates were produced in situ by oxidation of Co(II) with H2O2. Emzyme-Co(III)-adenosine 5'-triphosphate (ATP), enzyme-Co(III)-aspartate, and enzyme-Co(III)-threonine ternary adducts were produced in this manner. The formation of the enzyme-Co(III)-threonine adduct leads us to conclude that threonine inhibits the kinase activity of this enzyme complex by binding in the first coordination sphere of the catalytic metal ion cofactor, a conclusion which is consistent with evidence derived from previous nuclear magnetic resonance data obtained in this laboratory. The quaternary adducts formed by H2O2 oxidation in the presence of aspartokinase, Co(II), ATP, aspartate, and threonine comprised a mixture of both ezyme-Co(III)-ATP-aspartate and enzyme-Co(III)-ATP-threonine adducts. The formation of the quaternary aspartate-containing adduct was unexpected, since the presence of threonine was expected to prevent access of the aspartate to the active site; most significantly however, the the sum of the numbers of aspartate plus threonine molecules incorporated per active site is one. We believe that this shows direct steric overlap between the metal-adjacent binding sites for aspartate and threonine. Aspartate or threonine can not occupy the kinase active site simultaneously; this conclusion is consistent with the direct competitive inhibition of aspartate by threonine observed in steady-state kinetic studies.     

The ATP binding site of the yeast plasma membrane proton-translocating ATPase

Photoaffinity labeling of the active site of the yeast plasma membrane H(+)-ATPase has been studied with 2-azido-AMP and 2-azido-ATP. The ATPase activity of the enzyme decreases as the time of photolysis of the photoactive nucleotides in the presence of the enzyme increases. The covalent incorporation of [alpha-32P]2-azido-AMP into the enzyme and the inhibition of ATPase activity have comparable time courses. ATP protects the ATPase from incorporation of and photoinactivation by 2-azido-ATP or 2-azido-AMP. In the dark, 2-azido-ATP inhibits the ATPase at concentrations comparable to the apparent Michaelis constant for MgATP. After photolysis and proteolysis of the protein, three overlapping peptides labeled by the nucleotide analogues were purified by reversed-phase high performance liquid chromatography and sequenced. The peptides are derived from a region of the ATPase that is highly conserved in related cation pumps forming a phosphorylated intermediate during the catalytic cycle. Labeling with both nucleotide analogues occurs in peptides containing residues from aspartate 560 to lysine 566. The amino acids in this region conform to a consensus sequence for ATP binding derived from phosphofructokinase.     

A comparison of the properties of the phosphofructokinases of the fat body and flight muscle of the adult male desert locust

1. Phosphofructokinase was isolated, and partially purified by ammonium sulphate fractionation, from the fat body and flight muscle of the desert locust. 2. Ammonium sulphate appears to stabilize the enzymes, but does not activate them. 3. Both flight-muscle and fat-body enzymes give sigmoidal hexose monophosphate concentration-activity curves, which are characteristic of regulatory enzymes. 4. At low ATP concentrations both the enzyme activities increase rapidly with increasing ATP concentrations, but above an optimum concentration ATP becomes inhibitory. This optimum concentration is 0.2mm for the fat-body enzyme and 0.1mm for the flight-muscle enzyme. 5. AMP activates both the enzymes; half-maximal activation occurs at 10mum in each case, the effect being independent of substrate concentration. 6. 3',5'-(cyclic)-AMP (0.5mm) and P(i) (1mm) activate the flight-muscle enzyme, but have no effect on the fat-body enzyme. 7. FDP (1mm) inhibits both enzymes, and with the flight-muscle enzyme this inhibition is increased by increasing the ATP concentration. 8. Citrate, phosphoenolpyruvate and alpha-glycerophosphate have no effect on either enzyme under the assay conditions used. 9. The properties of phosphofructokinases from the locust are compared with those of phosphofructokinases from other sources.     

Lysine- and lysine-plus-threonine-inhibitable aspartokinases in Bacillus brevis.

Two aspartokinase (ATP:L-aspartate 4-phosphotrasferase, EC 2.7.2.4) enzyme activities have been identified and partially purified from Bacillus brevis. Aspartokinase I is subject to both inhibition and repression by lysine, and has a molecular weight in the region of 110 000. Aspartokinase II is a lysine-stabilised enzyme, inhibited multivalently by lysine plus theonine and has a molecular weight in the region of 95 000. This attern of aspartokinase activity has not been described previously and is unusual in that one end product (lysine) regulates two isoenzymes catalysing the first reaction of a branced biosynthetic pathway. In the absence of lysine, aspartokinase II changes to a more unstable non-inhibitable enzyme. Both enzymes are stabilised by sulphydryl reducing agents and have similar affinities for ATP, aspartate and lysine. However, there is no evidence for a view that they are products of a common gene. Problem concerned with the regulation of aspartokinase activities in Bacillus species are discussed.     

Inhibition of growth and aspartokinase activity of Salmonella typhimurium by thialysine.

Thialysine (S-2-aminoethyl cysteine) is an analog of lysine and has been reported to inhibit the lysyl-tRNA synthetase activity of Escherichia coli. This analog inhibits the growth of Salmonella typhimurium when added to glucose minimal medium at concentrations of 1.25 mM or greater. The addition of lysine with thialysine restores the normal growth rate, whereas, methionine, valine, or leucine each enhances the growth inhibition casued by thialysine. Enzyme assays demonstrate that thialysine inhibits not only the lysyl-tRNA synthetase from S. typhimurium, but also the aspartokinase activity. Lysine and thialysine appear to inhibit the same 40% of the total aspartokinase because simultaneous addition of the two compounds to the reaction mixture does not increase the inhibition caused by either alone. Furthermore, the slow growth of cells in the presence of 2.5 mM thialysine decreases the level of aspartokinase activity, suggesting that thialysine causes repression of enzyme synthesis as well as inhibition of activity.     

Lysine overproducing Corynebacterium glutamicum is characterized by a robust linear combination of two optimal phenotypic states

A homoserine auxotroph strain of Corynebacterium glutamicum accumulates storage compound trehalose with lysine when limited by growth. Industrially lysine is produced from C. glutamicum through aspartate biosynthetic pathway, where enzymatic activity of aspartate kinase is allosterically controlled by the concerted feedback inhibition of threonine plus lysine. Ample threonine in the medium supports growth and inhibits lysine production (phenotype-I) and its complete absence leads to inhibition of growth in addition to accumulating lysine and trehalose (phenotype-II). In this work, we demonstrate that as threonine concentration becomes limiting, metabolic state of the cell shifts from maximizing growth (phenotype-I) to maximizing trehalose phenotype (phenotype-II) in a highly sensitive manner (with a Hill coefficient of 4). Trehalose formation was linked to lysine production through stoichiometry of the network. The study demonstrated that the net flux of the population was a linear combination of the two optimal phenotypic states, requiring only two experimental measurements to evaluate the flux distribution. The property of linear combination of two extreme phenotypes was robust for various medium conditions including varying batch time, initial glucose concentrations and medium osmolality.     

A heat-sensitive lysis mutant of Bacillus subtilis 168 with a low activity of pyruvate carboxylase.

A mutant of Bacillus subtilis which grew in complex medium at 30 degrees C but lysed at 45 degrees C has been isolated. It could only grow on minimal medium at 45 degrees C with added aspartate (20 microgram ml-1) but lysed if lysine (20 microgram ml-1) was also present. The requirement for aspartate was due to a low activity of pyruvate carboxylase; the site of the mutation (pyc) was linked (16% cotransducible using phage PBSI) to the pyrD locus, and the order of markers deduced was: pyrD-cysC-pyc. This defect appeared to lead to decreased synthesis of mesodiaminopimelic acid (mesoA2pm), an amino acid unique to peptidoglycan and its precursors. At the restrictive temperature the mutant accumulated uridine-5'-diphosphate N-acetylmuramyl-L-alanyl-D-glutamate, since meso A2pm is the next amino acid to be added to the growing peptide chain of peptidoglycan. This resulted in an inhibition of peptidoglycan synthesis, determined as a reduced incorporation of N-acetyl[14C]glucosamine. Peptidoglycan synthesis was not decreased if the mutant was grown in media containing aspartate but lacking lysine. The sensitivity to lysine may arise because (i) at 45 degrees C the mutant was starved for aspartate and hence mesoA2pm even when aspartate was present, since aspartate utilization, as estimated by the incorporation of [3H]aspartate into trichloroacetic acid precipitable material, was relatively inefficient; and (ii) this diminished level of mesoA2pm synthesis from aspartate was further curtailed since lysine inhibits one of the aspartokinases in B. subtilis. Thus, addition of lysine allowed protein synthesis and hence autolysin production to proceed whilst peptidoglycan synthesis remained inhibited. When autolysis was blocked, either indirectly by stopping protein synthesis through starvation of aspartate and lysine, or directly by introducing a lyt mutation, then shifting the mutant to 45 degrees C did not result in lysis but growth still ceased.     

Genetic and amino-acid analysis of two maize threonine-overproducing,lysine-insensitive aspartate kinase mutants

The aspartate-derived amino-acid pathway leads to the production of the essential amino-acids lysine, methionine, threonine and isoleucine. Aspartate kinase (AK) is the first enzyme in this pathway and exists in isoforms that are feedback inhibited by lysine and threonine. Two maize (Zea mays L.) threonine-overproducing, lysine-insensitive AK mutants (Ask1-LT19 and Ask2-LT20) were previously isolated. The present study was conducted to determine the map location of Ask2 and to examine the amino-acid profiles of the Ask mutants. The threonine-overproducing trait conferred by Ask2-LT20 was mapped to the long arm of chromosome 2. Both mutants exhibited increased free threonine concentrations (nmol/mg dry weight) over wild-type. The percent free threonine increased from approximately 2% in wild-type kernels to 37–54% of the total free amino-acid pool in homozygous mutant kernels. Free methionine concentrations also increased significantly in homozygous mutants. Free lysine concentrations were increased but to a much lesser extent than threonine or methionine. In contrast to previous studies, free aspartate concentrations were observed to decrease, indicating a possible limiting factor in threonine synthesis. Total (free plus protein-bound) amino-acid analyses demonstrated a consistent, significant increase in threonine, methionine and lysine concentrations in the homozygous mutants. Significant increases in protein-bound (total minus free) threonine, methionine and lysine were observed in the Ask mutants, indicating adequate protein sinks to incorporate the increased free amino-acid concentrations. Total amino-acid contents (nmol/kernel) were approximately the same for mutant and wild-type kernels. In five inbred lines both Ask mutations conferred the threonine-overproducing phenotype, indicating high expressivity in different genetic backgrounds. These analyses are discussed in the context of the regulation of the aspartate-derived amino-acid pathway.     

Differential effects of metal chelators on Na plus, K plus-ATPase activity

Concentrations of EDTA greater than 1 mm inhibited the enzymatic activity of a Na⁺, K⁺-ATPase preparation isolated from beef brain by competing with ATP for Mg²⁺. EGTA, which does not chelate Mg²⁺ significantly at pH 7.4, had no such effect. Low concentrations of either EDTA or EGTA stimulated enzymatic activity. This effect was maximal at 5–6 μm of chelator. Stimulation of the reaction was observed only if the chelator was added to the incubation medium before initiation of the reaction by the addition of enzyme. If low concentrations of the chelators were added after enzyme, i.e., after the turnover cycle had begun, no effect was seen.     

Pseudomonas aeruginosa aspartate transcarbamoylase. Characterization of its catalytic and regulatory properties

Aspartate transcarbamoylase from Pseudomonadaceae is a class A enzyme consisting of six copies of a 36-kDa catalytic chain and six copies of a 45-kDa polypeptide of unknown function. The 45-kDa polypeptide is homologous to dihydroorotase but lacks catalytic activity. Pseudomonas aeruginosa aspartate transcarbamoylase was overexpressed in Escherichia coli. The homogeneous His-tagged protein isolated in high yield, 30 mg/liter of culture, by affinity chromatography and crystallized. Attempts to dissociate the catalytic and pseudo-dihydroorotase (pDHO) subunits or to express catalytic subunits only were unsuccessful suggesting that the pDHO subunits are required for the proper folding and assembly of the complex. As reported previously, the enzyme was inhibited by micromolar concentrations of all nucleotide triphosphates. In the absence of effectors, the aspartate saturation curves were hyperbolic but became strongly sigmoidal in the presence of low concentrations of nucleotide triphosphates. The inhibition was unusual in that only free ATP, not MgATP, inhibits the enzyme. Moreover, kinetic and binding studies with a fluorescent ATP analog suggested that ATP induces a conformational change that interferes with the binding of carbamoyl phosphate but has little effect once carbamoyl phosphate is bound. The peculiar allosteric properties suggest that the enzyme may be a potential target for novel chemotherapeutic agents designed to combat Pseudomonas infection.     

Use of Firefly Luciferase for ATP Measurement: Other Nucleotides Enhance Turnover

Firefly luciferase utilizes only ATP and a few closely related nucleotides as substrates for the formation of luciferyl adenylate which is an intermediate in the bioluminescent reaction sequence that oxidizes firefly luciferin. The enzyme shows two different time courses of light production depending on ATP concentration used: a flash with high concentrations of ATP (>8μM) or a fairly constant production of light with lower concentrations of ATP (< 1 μM). Many nucleotides, nucleotide-containing substances and other compounds, when added either prior to or 1 min after the addition of ATP, change the time course of light production. When added before ATP, these compounds yield a reaction mixture in which light production is fairly constant (at the level characteristic of the flash observed with that ATP concentration). When the compounds are added after ATP addition, light production is markedly stimulated and the higher rate of light production is maintained for several minutes. There is an increase in quanta of light produced per luciferase dimer from 1 to 5/min with the addition of any of several nucleotide analogues. These results are consistent with a stimulated release of the inhibitory product oxyluciferin, allowing turnover of the enzyme. This enzyme turnover permits more light output at high ATP concentrations, thus enhancing the sensitivity of enzyme determination.     

Improved L-lysine yield with Corynebacterium glutamicum: use of dapA resulting in increased flux combined with growth limitation

The amino acid L-lysine is produced on a large scale using mutants of Corynebacterium glutamicum. However, as yet recombinant DNA techniques have not succeed in improving strains selected for decades by classic mutagenesis for high productivity. We here report that seven biosynthetic enzymes were assayed and oversynthesis of the dihydrodipicolinate synthase resulted in an increase of lysine accumulation from 220 mM to 270 mM. The synthase, encoded by dapA, is located at the branch point of metabolite distribution to either lysine or threonine and competes with homoserine dehydrogenase for the common substrate aspartate semialdehyde. When graded dapA expression was used, as well as quantification of enzyme activities, intracellular metabolite concentrations and flux rates, a global response of the carbon metabolism to the synthase activity became apparent: the increased flux towards lysine was accompanied by a decreased flux towards threonine. This resulted in a decreased growth rate, but increased intracellular levels of pyruvate-derived valine and alanine. Therefore, modulating the flux at the branch point results in an intrinsically introduced growth limitation with increased intracellular precursor supply for lysine synthesis. This does not only achieve an increase in lysine yield but this example of an intracellularly introduced growth limitation is proposed as a new general means of increasing flux for industrial metabolite overproduction. Received: 8 August 1997 / Received revision: 2 October 1997 / Accepted: 14 October 1997