Asparagine aminotransferase

ASPARAGINE AMINOTRANSFERASE ASPARAGINE SYNTHETASE 1 -Aspartamido-j8-N-acetylglucosamine ami-dohydrolase,... 

Lloyd and Joy (1978) have confirmed the transmination of asparagine as a major pathway of the breakdown of the amide in leaves. The product 2-oxosuccinamate may be deaminated, but the majority was reduced to 2-hydroxysuccinamate which tended to accumulate. Very low levels of activity of the asparagine aminotransferase were detected in maturing pea seeds compared to the K+-activated asparaginase (K. W. Joy, personal communication). 

In 1980 (Lea and Miflin, 1980) the only well-documented report of the enzyme activity was in soybean leaves (Streeter, 1977), although Uoyd and Joy (1978) had detected the conversion of [ ]asparagine to [ ]hydroxysucdna-mic acid in pea leaves, probably using 2-oxosuccinamic acid as an intermediate (see steps 1 and 2 in Fig. 6). Asparagine aminotransferase activity has been detected in high levels in leaves (where it is present in the peroxisomes see Ireland and Joy, 1983a), but the enzyme is either absent or present only in very... 

This enzyme [EC 2.6.1.21], also known as D-aspartate aminotransferase, D-amino acid aminotransferase, and D-amino acid transaminase, catalyzes the reversible pyridoxal-phosphate-dependent reaction of D-alanine with a-ketoglutarate to yield pyruvate and D-glutamate. The enzyme will also utilize as substrates the D-stereoisomers of leucine, aspartate, glutamate, aminobutyrate, norva-hne, and asparagine. See o-Amino Acid Aminotransferase... 

This vitamin Be-dependent enzyme [EC 2.6.1.14], also referred to as asparagine-oxo-acid aminotransferase, catalyzes the reversible reaction of asparagine and a 2-0X0 acid to yield 2-oxosuccinamate and an amino acid. 

Fig. 1.3 Reactions showing synthesis of glutamate in brain. Aspartate aminotransferase (1) glu-taminase (2) glutamate dehydrogenase (3) GABA aminotransferase (4) alanine aminotransferase (5) ornithine aminotransferase (6) Al-pyrroline 5-carboxylic acid dehydrogenase (7) and asparagine synthetase (8)...

Fig. I. Metabolic map for synthesis and metabolism of glutamate and aspartate. AAT = aspartate aminotransferase AS = asparagine synthetase GAD = glutamic acid decarboxylase GDH = glutamate dehydrogenase GS = glutamine synthetase OAT = ornithine D-aminotransferase P5CDH = l-pyrroline-5-carboxylate dehydrogena.se PAG = phosphate-activated glutaminase PO = proline oxidase TCA = tricarboxylic acid.

The synthesis of asparagine from oxaloacetate using glutamate and glutamine as nitrogen donors involves two enzymes aspartate aminotransferase, E.C. 2.6.1.1 [Eq. (4)] and asparagine synthetase E.C. 6.3.5.4 [Eq. (5)]. 

The specific activity of these enzymes in the plant fraction of lupin nodules increases over the same time period as the increase in Ne-fixing activity in the bacteroids (Scott et al., 1976 Bolandet ai, 1979 Reynolds and Farnden, 1979). Asparagine synthetase has only been reported to date in nodules of lupin (Scott et ai, 1976 Radyukina et ai, 1977 Boland et al., 1979), however, numerous reports of aspartate aminotransferase activity in the plant fraction of several legume species have appeared (Table I). 

This is the normal aminotransferase reaction of 2-amino acids and yields with asparagine the 2-oxo amide I ... 

The properties of plant aminotransferases have recently been reviewed by Wightman and Forest (1978) and by Givan (this volume, Chapter 8). Aminotransferases are known to have multiple specificity, and it is possible that asparagine may be acting as a substrate for the ubiquitous aspartate aminotransferases. 

Fig. 5. Enzymes responsible for the synthesis of asparagine in plants, (a) Asparagine-oxoacid aminotransferase, (b) 9-cyanoaIanine synthase, (c) aqtaragine synthetase. (From Siedechowicz et al.. 1988a.)...

Fig. 6. Pathways of asparagine degradation in leaves of Pisum sativum. (1) Asparagine-oxoacid aminotransferase, (2) reduction, (3) asparaginase, (4) co-amidase, (S) aspartate-oxoglutarate aminotransferase, (6) malate dehydrogenase. (From Sieciechowicz et al., 1988a.) GLYOX, Glyox-ylate HOBA, 4-hydroxy 2-oxobutyrate HSE, homoserine PYR, pyruvate OSA, 2-oxosuccinamic acid HSA, hydroxysucdnamic add OAA, oxaloacetate MAL, malate OG, oxyglutarate.

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