Crystallization and properties of aspartate aminotransferase from Escherichia coli B

Bioanalysis for biocatalysis: multiplexed capillary electrophoresis-mass spectrometry assay for aminotransferase substrate discovery and specificity profiling

In this work, we introduce an entirely automated enzyme assay based on capillary electrophoresis coupled to electrospray ionization mass spectrometry termed MINISEP-MS for multiple interfluent nanoinjections-incubation-separation-enzyme profiling using mass spectrometry. MINISEP-MS requires only nanoliters of reagent solutions and uses the separation capillary as a microreactor, allowing multiple substrates to be assayed simultaneously. The method can be used to rapidly profile the substrate specificity of any enzyme and to measure steady-state kinetics in an automated fashion. We used the MINISEP-MS assay to profile the substrate specificity of three aminotransferases (E. coli aspartate aminotransferase, E. coli branched-chain amino acid aminotransferase, and Bacillus sp. YM-1 D-amino acid aminotransferase) for 33 potential amino acid substrates and to measure steady-state kinetics. Using MINISEP-MS, we were able to recapitulate the known substrate specificities and to discover new amino acid substrates for these industrially relevant enzymes. Additionally, we were able to measure the apparent K(M) and k(cat) parameters for amino acid donor substrates of these aminotransferases. Because of its many advantages, the MINISEP-MS assay has the potential of becoming a useful tool for researchers aiming to identify or create novel enzymes for specific biocatalytic applications.

UPLC-MS-based urine metabolomics reveals indole-3-lactic acid and phenyllactic acid as conserved biomarkers for alcohol-induced liver disease in the Ppara-null mouse model

Since the development and prognosis of alcohol-induced liver disease (ALD) vary significantly with genetic background, identification of a genetic background-independent noninvasive ALD biomarker would significantly improve screening and diagnosis. This study explored the effect of genetic background on the ALD-associated urinary metabolome using the Ppara-null mouse model on two different backgrounds, C57BL/6 (B6) and 129/SvJ (129S), along with their wild-type counterparts. Reversed-phase gradient UPLC-ESI-QTOF-MS analysis revealed that urinary excretion of a number of metabolites, such as ethylsulfate, 4-hydroxyphenylacetic acid, 4-hydroxyphenylacetic acid sulfate, adipic acid, pimelic acid, xanthurenic acid, and taurine, were background-dependent. Elevation of ethyl-β-d-glucuronide and N-acetylglycine was found to be a common signature of the metabolomic response to alcohol exposure in wild-type as well as in Ppara-null mice of both strains. However, increased excretion of indole-3-lactic acid and phenyllactic acid was found to be a conserved feature exclusively associated with the alcohol-treated Ppara-null mouse on both backgrounds that develop liver pathologies similar to the early stages of human ALD. These markers reflected the biochemical events associated with early stages of ALD pathogenesis. The results suggest that indole-3-lactic acid and phenyllactic acid are potential candidates for conserved and pathology-specific high-throughput noninvasive biomarkers for early stages of ALD.

Stereoselective chemoenzymatic synthesis of the four stereoisomers of l-2-(2-carboxycyclobutyl)glycine and pharmacological characterization at human excitatory amino acid transporter subtypes 1, 2, and 3

The four stereoisomers of l-2-(2-carboxycyclobutyl)glycine, l-CBG-I, l-CBG-II, l-CBG-III, and l-CBG-IV, were synthesized in good yield and high enantiomeric excess, from the corresponding cis and trans-2-oxalylcyclobutanecarboxylic acids 5 and 6 using the enzymes aspartate aminotransferase (AAT) and branched chain aminotransferase (BCAT) from Escherichia coli. The four stereoisomeric compounds were evaluated as potential ligands for the human excitatory amino acid transporters, subtypes 1, 2, and 3 (EAAT1, EAAT2, and EAAT3) in the FLIPR membrane potential assay. While the one trans-stereoisomer, l-CBG-I, displayed weak substrate activity at all three transporters, EAAT1-3, we found a particular pharmacological profile for the other trans-stereoisomer, l-CBG-II, which displayed EAAT1 substrate activity and inhibitory activity at EAAT2 and EAAT3. Whereas l-CBG-III was found to be a weak inhibitor at all three EAAT subtypes, the other cis-stereoisomer l-CBG-IV was a moderately potent inhibitor with 20-30-fold preference for EAAT2/3 over EAAT1.

Chemoenzymatic synthesis of a series of 4-substituted glutamate analogues and pharmacological characterization at human glutamate transporters subtypes 1-3

A series of nine L-2,4-syn-4-alkylglutamic acid analogues (1a-i) were synthesized in high yield and high enantiomeric excess (>99% ee) from their corresponding 4-substituted ketoglutaric acids (2a-i), using the enzyme aspartate aminotransferase (AAT) from pig heart or E. coli. The synthesized compounds were evaluated as potential ligands for the glutamate transporters EAAT1, EAAT2, and EAAT3 (excitatory amino acid transporter, subtypes 1-3) in the FLIPR membrane potential (FMP) assay. We found a distinct change in the pharmacological profile when the 4-methyl group (compound 1a, an EAAT1 substrate and EAAT2,3 inhibitor) was extended to a 4-ethyl group, compound 1b, as this analogue is an inhibitor at all three subtypes, EAAT1-3. Furthermore, we conclude that both large and bulky hydrophobic substituents in the 4-position of L-2,4-syn Glu are allowed by all three glutamate transporter subtypes EAAT1-3 while maintaining inhibitory activity.

Biosynthesis of Glutamate, Aspartate, Asparagine, L-Alanine, and D-Alanine

Glutamate, aspartate, asparagine, L-alanine, and D-alanine are derived from intermediates of central metabolism, mostly the citric acid cycle, in one or two steps. While the pathways are short, the importance and complexity of the functions of these amino acids befit their proximity to central metabolism. Inorganic nitrogen (ammonia) is assimilated into glutamate, which is the major intracellular nitrogen donor. Glutamate is a precursor for arginine, glutamine, proline, and the polyamines. Glutamate degradation is also important for survival in acidic environments, and changes in glutamate concentration accompany changes in osmolarity. Aspartate is a precursor for asparagine, isoleucine, methionine, lysine, threonine, pyrimidines, NAD, and pantothenate; a nitrogen donor for arginine and purine synthesis; and an important metabolic effector controlling the interconversion of C3 and C4 intermediates and the activity of the DcuS-DcuR two-component system. Finally, L- and D-alanine are components of the peptide of peptidoglycan, and L-alanine is an effector of the leucine responsive regulatory protein and an inhibitor of glutamine synthetase (GS). This review summarizes the genes and enzymes of glutamate, aspartate, asparagine, L-alanine, and D-alanine synthesis and the regulators and environmental factors that control the expression of these genes. Glutamate dehydrogenase (GDH) deficient strains of E. coli, K. aerogenes, and S. enterica serovar Typhimurium grow normally in glucose containing (energy-rich) minimal medium but are at a competitive disadvantage in energy limited medium. Glutamate, aspartate, asparagine, L-alanine, and D-alanine have multiple transport systems.

Kinetics of the repression of tylosin biosynthesis by ammonium ion in Streptomyces fradiae

Nitrogen regulation of tylosin synthesis in Streptomyces fradiae NRRL 2702 was studied in batch and chemostat cultures using a soluble synthetic medium. In batch cultures, valine dehydrogenase (VDH; EC 1.4.1.8), threonine dehydratase (TDT; EC 4.2.1.16) and aspartate aminotransferase (ASAT; EC 2.6.1.1) reached their highest specific activities at 120 h. The specific activities of the three enzymes showed close correlation with the value of specific tylosin formation rate (qTYL). In chemostat cultures, the maximum value of qTYL was 1.14 tylosin per mycelial mass per h (mg g-1 h-1) at the specific growth rate of 0.05 h-1, and after reaching a rate of 0.1 h-1, qTYL decreased with increasing levels of the specific growth rate. This value of qTYL was 3.5-times as large as that of maximum qTYL observed in the batch culture. The specific formation rates of VDH, TDT, ASAT and tylosin were repressed by high levels of specific ammonium ion uptake rate.

Aspartate aminotransferase and tylosin biosynthesis in Streptomyces fradiae

Aspartate aminotransferase as well as valine dehydrogenase and threonine dehydratase was required for the biosynthesis of tylosin in Streptomyces fradiae NRRL 2702. The biosynthesis of these enzymes and tylosin production were repressed by high concentrations of ammonium ions. The change in specific tylosin production rates in batch cultures with different initial concentrations of ammonium ions showed patterns similar to those of the specific production rates of aspartate aminotransferase, valine dehydrogenase, and threonine dehydratase. Aspartate aminotransferase has been purified by acetone precipitation, DEAE-cellulose, hydroxyapatite, and preparative electrophoresis chromatographies. The purified enzyme (120 kDa) consisted of two subunits identical in molecular mass (54 kDa) and showed homogeneity, giving one band with a pI of 4.2 upon preparative isoelectric focusing. The enzyme was specific for L-aspartate in the forward reaction; the Km values were determined to be 2.7 mM for L-aspartate, 0.7 mM for 2-oxyglutarate, 12.8 mM for L-glutamate, and 0.15 mM for oxaloacetate. The enzyme was somewhat thermostable, having a maximum activity at 55 degrees C, and had a broad pH optimum that ranged from 5.5 to 8.0. The mode of action was a ping-pong-bi-bi mechanism.

Tyrosine 70 fine-tunes the catalytic efficiency of aspartate aminotransferase

The aspartate aminotransferase mutant Y70F exhibits kcat = 8% and kcat/KM = 2% of the wild type values for the transamination of aspartate and alpha-ketoglutarate. The affinity of the enzyme for the noncovalently bound inhibitor maleate is reduced 17-fold by the mutation, while only a 2.5-fold reduction is observed for alpha-methylaspartate, which forms a stable, covalent external aldimine. The high population of the quinonoid intermediate formed in the reaction of the wild type with beta-hydroxyaspartate is more than 75% diminished by the mutation. The values of the Y70F C alpha-H kinetic isotope effects for the aspartate reaction are larger than those of wild type (DV = 2.4 vs 1.52; D(V/K) = 2.5 vs 1.7). Conversely, the Y70F value of D(V/K) for the glutamate reaction is decreased compared to wild type (1.75 vs 2.5). These results, combined with previous studies of Lys258 mutants, eliminate Tyr70 as an essential component of the catalytic apparatus, with the caveat that the functionally of the deleted hydroxyl group is possibly replaced by a water molecule.

Pre-steady-state kinetics of Escherichia coli aspartate aminotransferase catalyzed reactions and thermodynamic aspects of its substrate specificity

The four half-transamination reactions [the pyridoxal form of Escherichia coli aspartate aminotransferase (AspAT) with aspartate or glutamate and the pyridoxamine form of the enzyme with oxalacetate or 2-oxoglutarate] were followed in a stopped-flow spectrometer by monitoring the absorbance change at either 333 or 358 nm. The reaction progress curves in all cases gave fits to a monophasic exponential process. Kinetic analyses of these reactions showed that each half-reaction is composed of the following three processes: (1) the rapid binding of an amino acid substrate to the pyridoxal form of the enzyme; (2) the rapid binding of the corresponding keto acid to the pyridoxamine form of the enzyme; (3) the rate-determining interconversion between the two complexes. This mechanism was supported by the findings that the equilibrium constants for half- and overall-transamination reactions and the steady-state kinetic constants (Km and kcat) agreed well with the predicted values on the basis of the above mechanism using pre-steady-state kinetic parameters. The significant primary kinetic isotope effect observed in the reaction with deuterated amino acid suggests that the withdrawal of the alpha-proton of the substrates is rate determining. The pyridoxal form of E. coli AspAT reacted with a variety of amino acids as substrates. The Gibbs free energy difference between the transition state and the unbound state (unbound enzyme plus free substrate), as calculated from the pre-steady-state kinetic parameters, showed a linear relationship with the accessible surface area of amino acid substrate bearing an uncharged side chain.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of replacement of tryptophan-140 by phenylalanine or glycine on the function of Escherichia coli aspartate aminotransferase

Trp140 of E. coli aspartate aminotransferase has been converted to Phe or Gly by site-directed mutagenesis. As compared to the wild-type enzyme, either of the mutant enzymes showed 10- to 100-fold increase in Km's for natural dicarboxylic substrates, but did not show appreciable changes in Km's for aromatic substrates. Teh kcat values for dicarboxylic and aromatic substrates were greatly decreased by [Trp140----Gly] mutation, but were decreased to lesser extents by [Trp140----Phe] mutation. These findings suggested that N(1) of Trp140 may not be essential for catalysis, but may be partly involved in the binding of the distal carboxylate group of the dicarboxylic substrates.

Purification and characterization of thermostable aspartate aminotransferase from a thermophilic Bacillus species

Aspartate aminotransferase (EC 2.6.1.1) was purified to homogeneity from cell extracts of a newly isolated thermophilic bacterium, Bacillus sp. strain YM-2. The enzyme consisted of two subunits identical in molecular weight (Mr, 42,000) and showed microheterogeneity, giving two bands with pIs of 4.1 and 4.5 upon isoelectric focusing. The enzyme contained 1 mol of pyridoxal 5'-phosphate per mol of subunit and exhibited maxima at about 360 and 415 nm in absorption and circular dichroism spectra. The intensities of the two bands were dependent on the buffer pH; at neutral or slightly alkaline pH, where the enzyme showed its maximum activity, the absorption peak at 360 nm was prominent. The enzyme was specific for L-aspartate and L-cysteine sulfinate as amino donors and alpha-ketoglutarate as an amino acceptor; the KmS were determined to be 3.0 mM for L-aspartate and 2.6 mM for alpha-ketoglutarate. The enzyme was most active at 70 degrees C and had a higher thermostability than the enzyme from Escherichia coli. The N-terminal amino acid sequence (24 residues) did not show any similarity with the sequences of mammalian and E. coli enzymes, but several residues were identical with those of the thermoacidophilic archaebacterial enzyme recently reported.

Reversible dissociation and unfolding of aspartate aminotransferase from Escherichia coli: characterization of a monomeric intermediate

The unfolding and dissociation of the dimeric enzyme aspartate aminotransferase (D) from Escherichia coli by guanidine hydrochloride have been investigated at equilibrium. The overall process was reversible, as judged from almost complete recovery of enzymic activity after dialysis of 0.7 mg of denatured protein/mL against buffer. Unfolding and dissociation were monitored by circular dichroism and fluorescence spectroscopy and occurred in three separate phases: D in equilibrium 2M in equilibrium 2M* in equilibrium 2U. The first transition at about 0.5 M guanidine hydrochloride coincided with loss of enzyme activity. It was displaced toward higher denaturant concentrations by the presence of either pyridoxal 5'-phosphate or pyridoxamine 5'-phosphate and toward lower denaturant concentrations by decreasing the protein concentration. Therefore, bound coenzyme stabilizes the dimeric state, and the monomer (M) is inactive because the shared active sites are destroyed by dissociation of the dimer. M was converted to M* and then to the fully unfolded monomer (U) in two subsequent transitions. M* was stable between 0.9 and 1.1 M guanidine hydrochloride and had the hydrodynamic radius, circular dichroism, and fluorescence of a monomeric, compact "molten globule" state.

Intracellular predominance of the pyridoxal 5'-phosphate form of aspartate aminotransferase in Escherichia coli B and reversible transformation of this form by extracellular substances

The intracellular proportion of the pyridoxal 5'-phosphate form of aspartate aminotransferase to the total enzyme in E. coli B cells was determined by a newly devised method, dependent on selective inactivation of the intracellular pyridoxal 5'-phosphate form of the enzyme by extracellularly added sodium borohydride. A large portion (80-99%) of the intracellular aspartate aminotransferase was in pyridoxal 5'-phosphate form in both natural and synthetic medium-grown bacterial cells. The intracellular predominancy of pyridoxal 5'-phosphate did not vary during the growth of bacteria and during incubation of bacterial cells in various kinds of buffers with different pH values. In contrast, the saturation levels generally used to describe in vivo the proportions of the apo and holo vitamin B6-dependent enzymes did not reflect the intracellular amount of the pyridoxal 5'-phosphate (holo) form of aspartate aminotransferase probably because the intracellular pyridoxal 5'-phosphate form was changed to an apo form by the disruption of bacterial cells for preparing crude extract. Various extracellularly-added vitamin B6 antagonists decreased the intracellular amount of pyridoxal 5'-phosphate without decrease in the total intracellular activity of the enzyme. The modified forms were stable in E. coli B cells and reversed into pyridoxal 5'-phosphate form by incubation of the antagonist-treated cells in the buffer containing pyridoxal. The present results showed that the sodium borohydride reduction method can be used for further analysis of the in vivo interaction of pyridoxal 5'-phosphate and apoaspartate aminotransferase. The fact that about 50% of the intracellular pyridoxal 5'-phosphate form was changed to a modified form without impairment of cell growth in the presence of 4-deoxypyridoxine, and that about 50% of intracellular modified aspartate aminotransferase was reversed to pyridoxal 5'-phosphate by the removal of antagonist followed by incubation suggested that there exists characteristically 2 different fractions of pyridoxal 5'-phosphate forms of aspartate aminotransferase in E. coli cells.

[Arg292----Val] or [Arg292----Leu] mutation enhances the reactivity of Escherichia coli aspartate aminotransferase with aromatic amino acids

Arg292 of E. coli aspartate aminotransferase was substituted with valine or leucine by site-directed mutagenesis. In comparison with the wild-type enzyme, either of the mutant enzymes showed a decrease by over 5 orders of magnitude of kcat/km values for aspartate and glutamate. This supports the contention that Arg292 is important for determining the specificity of this enzyme for dicarboxylic substrates. In contrast, mutant enzymes displayed a 5- to 10-fold increase in kcat/Km values for aromatic amino acids as substrates. Thus, introduction of an uncharged, hydrophobic side chain into position 292 leads to a striking alteration in substrate specificity of this enzyme, thereby improving catalytic efficiency toward aromatic amino acids.

Asparagine catabolism in rat liver mitochondria

A large portion of mitochondrial asparagine (Asn) is degraded by asparagine amino-transferase to produce alpha-ketosuccinamate (alpha KSA), which is then hydrolized by omega-amidase to produce oxaloacetate (OAA) and ammonia. This is in contrast to the catabolism in the cytosol, where the main catabolic route for Asn occurs initially via asparaginase-catalyzed hydrolysis to form aspartate and ammonia. Mitochondrial production of OAA from Asn was followed by monitoring the decrease in the rate of succinate oxidation (which is inhibited by OAA) in both coupled and uncoupled mitochondria. Rapid OAA production was found to be dependent on the presence of both Asn and glyoxylate, and was eliminated by the aminotransferase inhibitor, aminooxyacetate (AOX). HPLC separation and quantitation of alpha-keto acids and amino acids allowed direct observation of the proposed mitochondrial pathway. Studies using L-[U-14C]Asn in mitochondria yielded labeled carbon in alpha KSA, OAA, and CO2 when either an alpha-keto acid or glyoxylate was provided. The extent of the labeled carbon in these products was greatly influenced by factors that affected the citric acid cycle and oxidative phosphorylation. Carbon dioxide production from Asn alone, even in the presence of AOX, suggested the existence of at least one additional Asn catabolic pathway in the rat liver mitochondria which does not involve alpha KSA as an intermediate.

Determination of human aspartate aminotransferase isoenzymes by their differential sensitivity to proteases

The effect of various proteases (trypsin, chymotrypsin, subtilisin, protease 401, and thermolysin) on the mitochondrial isoenzyme (m-AST) and cytoplasmic isoenzyme (c-AST) of human and swine aspartate aminotransferase (AST;EC 2.6.1.1) was evaluated. All procedures including the reaction with proteases and the subsequent determination of the AST activity were carried out in an automatic analyzer. The mammalian c-AST was efficiently inactivated by chymotrypsin, subtilisin and protease 401 while m-AST activity decreased very slowly with these proteases. Thermolysin and trypsin showed much less effect on c-AST activity. Especially, chymotrypsin at concentrations of 0.5-1.0 g/L inactivated human c-AST almost completely but showed no detectable inactivating effect on m-AST. Thus chymotrypsin appears to be the most suitable protease for the differential determination of AST isoenzymes in human serum. Further studies on the effects of proteases with AST from other species showed that Escherichia coli AST resembled mammalian m-AST while Pseudomonas AST resembled c-AST.

Intramitochondrial location and some characteristics of chicken liver aspartate aminotransferase

Chicken liver mitochondrial aspartate aminotransferase was found to be located in the intermembrane space and bound to the inner mitochondrial membrane. Purification of two mitochondrial fractions containing aspartate aminotransferase activity was performed. Both fractions showed similar chromatographic behaviour and identical isoelectric point and molecular weight values. There were no significant differences in the general kinetic mechanism, Km values, substrates inhibition and effect of various anions on the activity of mitochondrial aspartate aminotransferase purified from both fractions.

An enzymatic method for the synthesis of [14C]pyridoxal 5'-phosphate from [14C]pyridoxine

A new enzymatic method for the synthesis of [14C]pyridoxal 5'-phosphate is presented. [14C]Pyridoxal 5'-phosphate was synthesized from [14C]pyridoxine through the successive actions of pyridoxal kinase and pyridoxamine 5'-phosphate oxidase in a reaction mixture containing ATP, [14C]pyridoxine, and both enzymes. [14C]Pyridoxal 5'-phosphate was isolated by omega-aminohexyl-Sepharose 6B column chromatography. The overall yield of the product was more than 60%, starting from 550 nmol of [14C]pyridoxine. The radiochemical purity of the products, as determined by thin-layer and ion-exchange chromatography, was greater than 98%.

Preliminary X-ray data for aspartate aminotransferase from Escherichia coli

Crystals of the aspartate aminotransferase from Escherichia coli (aspC gene product) have been examined by X-ray analysis. The crystals grow as elongated rectangular prisms, with the symmetry of space group C2221. Unit cell dimensions are a = 156 A, b = 87.6 A, c = 80.6 A and alpha = beta = gamma = 90 degrees. There is one protein subunit of molecular weight 43,600 per asymmetric unit.

Effect of phosphate and other inorganic anions on the activity of chicken liver cytosolic aspartate aminotransferase

Chicken liver aspartate aminotransferase was inhibited by several inorganic anions. The inhibitory effect of the anions was related to their chaotropic character. Apparent Km (2-oxoglutarate) and Km (L-aspartate) values depended on the molarity of the buffer. The profile of the curves obtained did not depend on the nature of the enzyme sample assayed. Phosphate slightly inhibited the holoaspartate aminotransferase and was a strong inhibitor of apoaspartate aminotransferase with respect to pyridoxal phosphate.

The complete amino acid sequence of aspartate aminotransferase from Escherichia coli: sequence comparison with pig isoenzymes

The amino acid sequence of aspartate aminotransferase from E. coli B was determined by the alignment of seven cyanogen bromide peptides. The established sequence of the subunit was composed of 396 amino acid residues, and the molecular weight was calculated to be 43,573. The sequence was compared with those of the pig cytoplasmic and mitochondrial isoenzymes, showing that nearly 30% of all residues were invariant and that the E. coli enzyme exhibited the same degree of homology (about 40%) with either of them. Although majority of the residues were substituted, the functional residues constituting the active site structure were conserved.

Subcellular distribution of aspartate aminotransferase isoenzymes in chicken heart: quantitative study

1. The content of the two aspartate aminotransferase isoenzymes in isolated mitochondria and in the cytosolic fraction from chicken heart was determined by radioimmunoassays. 2. The cationic isoenzyme was found to be associated with the mitochondrial fraction; its content measured in the cytosolic fraction was within the range of that of contaminating mitochondrial marker enzymes. 3. The anionic isoenzyme was found exclusively in the cytosolic fraction, in mitochondria a content of less than 0.05% of the total was measured. 4. Thus, in birds, the anionic and the cationic isoenzyme of aspartate aminotransferase show the same strict intracellular heterotopism as found previously in mammals.