Aspartate: 2-oxoglutarate aminotransferase

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.

Aspartate aminotransferase (most active in liver) Aspartate + Oxoglutarate (a-kitoglutarate) Oxalacetate + Glutamate... 

The aminotransferases constitute a group of enzymes that catalyze the interconversion of amino acids to 2-oxo-acids by transfer of amino groups. Aspartate aminotransferase (EC 2.6.1.1 L-aspartate 2-oxoglutarate aminotransferase AST) and alanine aminotransferase (EC 2.6.1.2 L-alanine 2-oxoglutarate aminotransferase ALT) are examples of aminotransferases that are of clinical interest. 

Model studies (pyridoxal catalyzed conversion of a-amino acid to oxo-acid) indicates that the prototropic shift is in the aldimine <—> ketimine tautomerization, and this step can be greatly accelerated by general acid-base catalysis. Aspartate ( 2-oxoglutarate) aminotransferase (EC 2.6.1.1), which catalyzes transamination between Asp and 2-oxoglutarate (oxaoacetate and Glu), is the most extensively studied representative PLP enzyme. The enzyme is a homodimer containing one PLP molecule per subunit. Experimental observations pertaining to apartate aminotransferase are ... 

The metabolism of amino acids is complex and is described in standard text books. These are usually converted by aminotransferases to the corresponding 2-oxoacids which are partly oxidized in the matrix of muscle mitochondria and partly exported to the liver. Glutamate and aspartate yield 2-oxoglutarate and oxaloacetate, respectively, which enter the citrate cycle directly, and other 2-... 

As examples, two enzymes that will be discussed again later in this chapter are alanine transaminase (alanine aminotransferase) and aspartate transaminase (aspartate aminotransferase). In both cases, the amino group is transferred to 2-oxoglutarate (also known as a-ketoglutarate), which is oxoacid, above, forming glutamate as amino acid2. For example, the alanine transaminase (ALT) reaction is ... 

This pyridoxal-phosphate-dependent enzyme [EC 2.6.1.5], also known as tyrosine transaminase, catalyzes the reaction of L-tyrosine with a-ketoglutarate (or, 2-oxoglutarate) to produce 4-hydroxyphenylpyruvate and L-glutamate. L-Phenylalanine can act as the substrate instead of tyrosine. In some systems, the mitochondrial enzyme may be identical with aspartate aminotransferase. 

Oxoglutarate is the normal acceptor of the amino group. In the aminotransferase reaction, 2-oxoglutarate is transaminated to give glutamate. There are at least 13 different aminotransferases, but their specificities are not all known. The most important are (a) aspartate aminotransferase, which catalyzes the following reversible reaction ... 

Rakhmanova, T.I., Popova, T.N. (2006). Regulation of 2-oxoglutarate metabolism in rat liver by NADP-isocitrate dehydrogenase and aspartate aminotransferase. Biochemistry (Mosc.) 71(2) 211-17. 

H]pyridoxamine which was converted by aspartate aminotransferase apoca-zyme in the presence of a-oxoglutarate into pyridoxal with retention of H. A knowledge of the stereochemical course of the latter reaction which removes the C-4 Hgi atom allowed the conclusion that the [4 - H]pyridoxamine had the (R) configuration (Fig. 48). The reduction by NaB H4 must have involved [101] attack on the Re face of the w-electron system at C-4 of the species (Fig. 48, 1). The conclusion for aspartate aminotransferase has recently been confirmed by Zito and Martinez-Carrion [93] and the same approach when applied to tyrosine decarboxylase [103], also showed the hydride attack to occur on the Re face at C-4 of the coenzyme-enzyme imine bond. In both cases, therefore, the Re face at C-4 must be exposed to the solvent side in the binary complex. 

FIGURE 4.5 The aspartate aminotransferase reaction L-aspartate -I-2-oxoglutarate and its products oxalo-acetate + L-glutamate. 

Multispecificity was demonstrated for 600-fold purified aspartate aminotransferase from bush beans which utilized either oxaloacetate or 2-oxoglutarate as the amino acceptor substrate (but not pyruvate or glyoxy-late) and could transaminate aspartate, phenylalanine, tyrosine, and tryptophan. Phenylalanine was utilized at about one-tenth of the rate of aspartate the other aromatic amino acids were used even less rapidly (Forest and Wightman 1972a). On the assumption that these rates cannot be accounted for by undetected contaminants, a multispecific character has thus been established for the bush bean aspartate aminotransferase. This kind of multispecificity resembles that shown for mitochondrial aspartate aminotransferases from animal tissues (though in the cytosol of animal cells, aspartate and aromatic amino acids, e.g., tyrosine, do appear to be transaminated by separate enzymes (cf. Braunstein, 1973)). 

Deamination of amino acids in animal tissue is generally effected by transamination with an a-keto-acid. In the majority of cases, this is 2-oxoglutarate formed by the citric acid cycle. Aspartate aminotransferase and alanine aminotransferase are examples of this kind of reaction. In Figure 2.7, transamination involving these enzymes is depicted as it is known to occur in mammalian liver. Note that the scheme shown here requires participation of oxalacetate and pyruvate and thus is intimately connected with metabolic pathways considered earlier. Serine and glycine are readily interconvertible in animal tissue by the enzyme serine hydroxymethyltransferase. It is worth noting also that decarboxylation of serine to ethanolamine as mentioned above can be followed by A -methylation to yield choline. Choline is both an essential component of many... 

Aspartate 4-semialdehyde, seen, for example, in Scheme 12.13, which provided a pathway for the biosynthesis of the essential amino acid methionine (Met, M) and in Scheme 12.14, which holds a representation of the biosynthesis of threonine (Thr, T), is also a place to begin to describe a pathway to lysine (Lys, K). As shown in Scheme 12.19, aspartate 4-semialdehyde undergoes an aldol-type reaction with pyruvate (CHsCOCO ") in the presence of dihydropicoUnate synthase (EC 4.2.1.52) to produce a series of intermediates that, it is presumed, lead to (5)-23-dihydropyridine-2,6-dicarboxylate. Then, dihydrodipicolinate reductase (EC 1.3.1.26) working with NADPH produces the tetrahydropyridine, (S)-2,3,4,5-tetrahydropyridine-2,6-dicarboxylate.This heterocycle, in the presence of glutamate (Glu, E) and water, is capable of transamination directly to 2-oxoglutarate and (2S, 6S)-2,3-diaminopimelate in the presence of LL-diaminopimelate aminotransferase (EC 2.6.1.83), while the latter, in the presence of the pyridoxal dependent racemase... 

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