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Query: UNIPROT:P17174 (aspartate aminotransferase)
 14,872 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Arabidopsis contains five isoenzymes of aspartate aminotransferase (AspAT) localized to the cytosol, chloroplast, mitochondria, or peroxisomes. To define the in vivo function of individual isoenzymes, we screened for Arabidopsis mutants deficient in either of the two major isoenzymes, cytosolic AAT2 or chloroplastic AAT3, using a native gel activity assay. In a screen of 8,000 M2 seedlings, three independent mutants deficient in cytosolic AAT2 (aat2) and two independent mutants deficient in chloroplastic AAT3 (aat3) were isolated. Mapping of aat2 and aat3 mutations and the five AspAT genes (ASP1-ASP5) established associations as follows: the mutation affecting aat2 maps with and cosegregates with ASP2, one of two expressed genes for cytosolic AspAT; the mutation affecting aat3 maps to the same location as the ASP5 gene encoding chloroplastic AspAT. Phenotypic analysis of the aat2 and aat3 mutants revealed a dramatic aspartate-related phenotype in one of the mutants deficient in cytosolic AAT2. The aat2-2 mutant displays an 80% reduction in levels of aspartate transported in the phloem of light-grown plants, and a 50% reduction in levels of asparagine transported in dark-adapted plants. These results indicate that cytosolic AAT2 is the major isoenzyme controlling aspartate synthesized for nitrogen transport in the light, and that this aspartate pool is converted to asparagine when plants are dark adapted.
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PMID:Arabidopsis mutants define an in vivo role for isoenzymes of aspartate aminotransferase in plant nitrogen assimilation. 961 Nov 68

Aminotransferase reversibly catalyzes the transamination reaction by a ping-pong bi-bi mechanism with pyridoxal 5'-phosphate (PLP) as a cofactor. Various kinds of aminotransferases developing into catalysts for particular substrates have been reported. Among the aminotransferases, aromatic amino acid aminotransferase (EC 2.6.1. 57) catalyzes the transamination reaction with both acidic substrates and aromatic substrates. To elucidate the multiple substrate recognition mechanism, we determined the crystal structures of aromatic amino acid aminotransferase from Paracoccus denitrificans (pdAroAT): unliganded pdAroAT, pdAroAT in a complex with maleate as an acidic substrate analog, and pdAroAT in a complex with 3-phenylpropionate as an aromatic substrate analog at 2.33 A, 2. 50 A and 2.30 A resolution, respectively. The pdAroAT molecule is a homo-dimer. Each subunit has 394 amino acids and one PLP and is divided into small and large domains. The overall structure of pdAroAT is essentially identical to that of aspartate aminotransferase (AspAT) which catalyzes the transamination reaction with only an acidic amino acid. On binding the acidic substrate analog, arginine 292 and 386 form end-on salt bridges with carboxylates of the analog. Furthermore, binding of the substrate induces the domain movement to close the active site. The recognition mechanism for the acidic substrate analog in pdAroAT is identical to that observed in AspAT. Binding of the aromatic substrate analog causes reorientation of the side-chain of the residues, lysine 16, asparagine 142, arginine 292* and serine 296*, and changes in the position of water molecules in the active site to form a new hydrogen bond network in contrast to the active site structure of pdAroAT in the complex with an acidic substrate analog. Consequently, the rearrangement of the hydrogen bond network can form recognition sites for both acidic and aromatic side-chains of the substrate without a conformational change in the backbone structure in pdAroAT.
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PMID:Crystal structures of Paracoccus denitrificans aromatic amino acid aminotransferase: a substrate recognition site constructed by rearrangement of hydrogen bond network. 966 48

Mitochondrial processing peptidase is a heterodimer consisting of alpha-mitochondrial processing peptidase (alpha-MPP) and beta-MPP. We investigated the role of alpha-MPP in substrate recognition using a recombinant yeast MPP. Disruption of amino acid residues between 10 and 129 of the alpha-MPP did not essentially impair binding activity with beta-MPP and processing activity, whereas truncation of the C-terminal 41 amino acids led to a significant loss of binding and processing activity. Several acidic amino acids in the region conserved among the enzymes from various species were mutated to asparagine or glutamine, and effects on processing of the precursors were analyzed. Glu353 is required for processing of malate dehydrogenase, aspartate aminotransferase, and adrenodoxin precursors. Glu377 and Asp378 are needed only for the processing of aspartate aminotransferase and adrenodoxin precursors, both of which have a longer extension peptide than the others studied. However, processing of the yeast alpha-MPP precursor, which has a short extension peptide of nine amino acids, was not affected by these mutations. Thus, effects of substitution of acidic amino acids on the processing differed with the precursor protein and depended on length of the extension peptides. alpha-MPP may function as a substrate-recognizing subunit by interacting mainly with basic amino acids at a region distal to the cleavage site in precursors with a longer extension peptide.
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PMID:Role of alpha-subunit of mitochondrial processing peptidase in substrate recognition. 973 75

The protective effects of various kinds of dietary amino acids against the hepatotoxic action of D-galactosamine (GalN) were examined. Male Wistar rats fed with 20% casein diets containing 10% or 5% amino acid for one week were injected with GalN (800 mg/kg body weight), and the serum aspartate aminotransferase (AST), alanine aminotransferase (ALT), and lactate dehydrogenase (LDH) activities, the hepatic glycogen concentration, and the serum glucose-level were examined 20 hours after the injection. In the groups with the 10% amino acid diets, activities of AST, ALT, and LDH in serum of 10% L-glutamine (Gln), 10% L-asparagine (Asn), and 10% L-serine (Ser) groups were significantly lower than those of the control group, and in the groups with the 5% amino acid diets, those activities of 5% L-histidine (His), 5% L-tyrosine (Tyr), 5% L-lysine (Lys), and 5% L-glycine (Gly) groups were also lower than those of the control group. The concentration of liver glycogen of 10% Gln-, 10% Asn-, and 10% Ser- groups and those levels of 5% His-, 5% Tyr-, 5% Lys-, and 5% Gly-groups were also significantly higher than that of the control group. As a result, it was found that some kinds of dietary amino acid such as L-Ser, L-Asn, L-His, L-Lys, L-Tyr, and L-Gly, in addition to L-Gln were effective to protect the rats from GalN-induced injury.
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PMID:Effects of various kinds of dietary amino acids on the hepatotoxic action of D-galactosamine in rats. 1019 13

Nitrogen assimilation is a vital process controlling plant growth and development. Inorganic nitrogen is assimilated into the amino acids glutamine, glutamate, asparagine, and aspartate, which serve as important nitrogen carriers in plants. The enzymes glutamine synthetase (GS), glutamate synthase (GOGAT), glutamate dehydrogenase (GDH), aspartate aminotransferase (AspAT), and asparagine synthetase (AS) are responsible for the biosynthesis of these nitrogen-carrying amino acids. Biochemical studies have revealed the existence of multiple isoenzymes for each of these enzymes. Recent molecular analyses demonstrate that each enzyme is encoded by a gene family wherein individual members encode distinct isoenzymes that are differentially regulated by environmental stimuli, metabolic control, developmental control, and tissue/cell-type specificity. We review the recent progress in using molecular-genetic approaches to delineate the regulatory mechanisms controlling nitrogen assimilation into amino acids and to define the physiological role of each isoenzyme involved in this metabolic pathway.
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PMID:THE MOLECULAR-GENETICS OF NITROGEN ASSIMILATION INTO AMINO ACIDS IN HIGHER PLANTS. 1501 1

Nitrogen (N) is an essential requirement for kernel growth in maize (Zea mays); however, little is known about how N assimilates are metabolized in young earshoots during seed development. The objective of this study was to assess amino acid metabolism in cob and spikelet tissues during the critical 2 weeks following silking. Two maize hybrids were grown in the field for 2 years at two levels of supplemental N fertilizer (0 and 168 kg N/ha). The effects of the reproductive sink on cob N metabolism were examined by comparing pollinated to unpollinated earshoots. Earshoots were sampled at 2, 8, 14, and 18 d after silking; dissected into cob, spikelet, and/or pedicel and kernel fractions; then analyzed for amino acid profiles and key enzyme activities associated with amino acid metabolism. Major amino acids in the cob were glutamine (Gln), aspartic acid (Asp), asparagine (Asn), glutamate, and alanine. Gln concentrations dropped dramatically from 2 to 14 d after silking in both pollinated and unpollinated cobs, whereas all other measured amino acids accumulated over time in unpollinated spikelets and cobs, especially Asn. N supply had a variable effect on individual amino acid levels in young cobs and spikelets, with Asn being the most notably enhanced. We found that the cob performs significant enzymatic interconversions among Gln, alanine, Asp, and Asn during early reproductive development, which may precondition the N assimilate supply for sustained kernel growth. The measured amino acid profiles and enzymatic activities suggest that the Asn to Gln ratio in cobs may be part of a signal transduction pathway involving aspartate aminotransferase, Gln synthetase, and Asn synthetase to indicate plant N status for kernel development.
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PMID:Amino acid metabolism in maize earshoots. Implications for assimilate preconditioning and nitrogen signaling. 1553 10

We have increased the contents of several amino acids in the seeds of Arabidopsis thaliana by introduction of aspartate aminotransferase (AAT), an enzyme of the aspartate biosynthetic pathway. mRNA was prepared from one-week-old seedlings of Glycine max cv. enrei and the cDNA encoding AAT5 was isolated and linked to the CaMV35S promoter in the plant vector pBI121. The AAT5 gene encodes a protein of 462 amino acid residues that shows 51% amino acid sequence similarity to A. thaliana chloroplast Asp3. The soybean AAT5 also contains a chloroplast transit peptide and is able to functionally complement a Saccharomyces cerevisiae mutant lacking the Asp5 gene. A. thaliana was transformed with the AAT5 gene from Agrobacterium tumefaciens by the vacuum infiltration method. The AAT5 gene was detected in the transcript and genomic DNA from the transgenic T2 plants. The T3 progeny showed a 3:1 segregation ratio indicating the presence of a single integration. Expression of G. max AAT5 in A. thaliana transformants caused 3-, 4-, 23-, and 50-fold increases in the contents of free glycine, alanine, asparagine, and glutamine, respectively, in the T3 seeds. A decrease in the contents of valine, tyrosine, isoleucine, leucine, and phenylalanine by several folds was also observed. Thus, it is of interest that a key gene expression resulted in marked changes of metabolites in plant seeds.
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PMID:Variation of the amino acid content of Arabidopsis seeds by expressing soybean aspartate aminotransferase gene. 1623 95

The free amino acid concentrations in cotyledons and axes of soybean (Glycine max [L.] Merr. cv. Wells) seedlings were determined by automated single column analysis after germination at 10 and 23 C. After 5 days germination at 10 C, glutamate and aspartate were in high concentration in both cotyledons and axes (38 and 24% of total free amino acids recovered, respectively), whereas the concentrations of their amide derivatives, asparagine and glutamine, were low in cotyledons (4.4%) and high in axes (21%). In contrast, after 5 days germination at 23 C, asparagine and glutamine accounted for 22 and 45% of total free amino acids in cotyledons and axes respectively, and aspartate and glutamate concentrations were low. The activities of glutamine synthetase and asparagine synthetase were considerably lower in tissues from the 10 C treatment than those from the 23 C treatment.Aspartate and glutamate concentrations were nearly equal in all but one sample. Both glutamate oxaloacetate transaminase and glutamate dehydrogenase activities were much higher in axis tissues at 23 C as compared to 10 C. Arrhenius plots of axis glutamate oxaloacetate transaminase and glutamate dehydrogenase activities were biphasic and triphasic, respectively, with energies of activation for both increasing with low temperature. Energies of activation were identical for glutamate oxaloacetate transaminase from 10 and 23 C treatments but much higher for glutamate dehydrogenase from 23 C-treated axes. This indicates a difference in enzyme complement for glutamate dehydrogenase with the two treatments.Hydrolysis of free amino acid sample (basic fraction) aliquots showed large quantities of peptides in 23 C-treated axes at 2 days, while few or no peptides were found in the 10 C treatment. Amino acid residues most prevalent in peptides were aspartate, threonine, serine, glutamate, and glycine.
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PMID:Low Temperature Effects on Soybean (Glycine max [L.] Merr. cv. Wells) Free Amino Acid Pools during Germination. 1666 May 75

Amide and ureide biogenic enzymes were measured in the plant fraction of soybean (Glycine max) nodules during the period 11 to 23 days after inoculation with Rhizobium japonicum (USDA 3I1b142). Enzymes involved in the initial assimilation of ammonia, i.e. glutamine synthetase, glutamate synthase, and aspartate aminotransferase, showed substantial increases in their specific activities over the time course. These increases paralleled the induction of nitrogenase activity in the bacteroid and leghemoglobin synthesis in the plant fraction. The specific activity of asparagine synthetase, however, showed a rapid decline after an initial increase in specific activity. Following the initial increases in the ammonia assimilatory enzymes, there was an increase in the activity of 5-phosphoribosylpyrophosphate amidotransferase, the enzyme which catalyzes the first committed step of de novo purine biosynthesis. This was followed by a dramatic increase in the purine oxidative enzymes, xanthine dehydrogenase and uricase. Smaller increases were observed in the activities of enzymes associated with the supply of metabolites to the purine biosynthetic pathway: phosphoglycerate dehydrogenase, serine hydroxymethylase, and methylene tetrahydrofolate dehydrogenase.The concentration of asparagine in the plant fraction decreased at the same time as the observed decrease in asparagine synthetase activity. This was followed by a recovery in plant fraction levels of asparagine in the presence of a continuing fall in the glutamine concentration and continued low asparagine synthetase activity.The data presented are consistent with initial assimilation of ammonia into glutamine and aspartate, which are metabolized by an elevation of endogenous purine biosynthetic enzymes, and then, by the induction of a specific group of purine oxidative enzymes, directed to allantoic acid production.
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PMID:Enzymes of amide and ureide biogenesis in developing soybean nodules. 1666 97

Glutamine-free culture of Vero cells has previously been shown to cause higher cell yield and lower ammonia accumulation than that in glutamine-containing culture. Nitrogen metabolism of asparagine and glutamate as glutamine replacer was studied here using nuclear magnetic resonance (NMR) spectroscopy. (15)N-labelled glutamate or asparagine was added and their incorporation into nitrogenous metabolites was monitored by heteronuclear multiple bond coherence (HMBC) NMR spectroscopy. In cells incubated with L: -[(15)N]glutamate, the (15)N label was subsequently found in a number of metabolites including alanine, aspartate, proline, and an unidentified compound. No detectable (15)NH(+)(4) signal occurred, indicating that glutamate was utilized by transamination rather than by oxidative deamination. In cells incubated with L: -[2-(15)N]asparagine, the (15)N label was subsequently found in aspartate, the amine group of glutamate/glutamine, and in two unidentified compounds. Incubation of cells with L: -[4-(15)N]asparagine showed that the amide nitrogen of asparagine was predominantly transferred to glutamine amide. There was no detectable production of (15)NH(+)(4), showing that most of the asparagine amide was transaminated by asparagine synthetase rather than deaminated by asparaginase. Comparing with a glutamine-containing culture, the activities of phosphate-activated glutaminase (PAG), glutamate dehydrogenase (GDH) and alanine aminotransferase (ALT) decreased significantly and the activity of aspartate aminotransferase (AST) decreased slightly.
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PMID:Nitrogen metabolism of asparagine and glutamate in Vero cells studied by (1)H/ (15)N NMR spectroscopy. 1795 33


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