Alanine: oxoacid aminotransferase

Phosphoglycerate dehydrogenase 2 phosphoserine aminotransferase 3 phosphoserine phosphatase 4 serine hydroxymethyltransferase 5 glycine aminotransferase 6 phosphoglyceromutase, enolase, pyruvate kinase (Fig. 44) 7 alanine oxoacid aminotransferase... 

The soluble and particulate alanine 2-oxoglutarate aminotransferases of tomato fruits (which may be identical proteins) (Rech and Crouzet, 1974 Gazeu-Reyjal and Crouzet, 1976) are estimated to have molecular weights of 100,000 (gel filtration). Some plant transaminases are evidently much smaller. Ornithine 2-oxoacid aminotransferase has a molecular weight estimated at 48,000 (Lu and Mazelis, 1975), and a molecular weight of 56,200 has been determined for L-alanine 5-ketooctanal aminotransferase from Conium maculatum (Roberts, 1977). Although very few estimates are available, and not many alternative methods have been used, it seems clear that the molecular weights of plant transaminases vary considerably from one enzyme to another. They may also differ from comparable animal enzymes (Lu and Mazelis, 1975). 

Amination of aldehydes by an amino acid aldehyde aminotransferase from Mercurialis perennis was inhibited strongly by pyruvate, oxaloacetate, and oxoglutarate, and this inhibition was suspected to be competitive (Hartmann et al., 1972). Formation of coniceine by an alanine 5-keto-octanal aminotransferase was inhibited competitively by pyruvate and uncompetitively by glyoxylate (Roberts, 1978). With the L-ornithine 2-oxoacid aminotransferase from Cucurbita pepo, severe inhibition by valine, leucine, and isoleucine could be observed, but there was no inhibition by lysine or proline (Lu and Mazelis, 1975). Cucurbita maxima ornithine aminotransferase, however, was reported to be inhibited by proline (Splittstoesser and Fowden, 1973), as well as by canavanine and diaminobutyrate. 

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 ... 

Aspartate aminotransferase catalyses the transfer of the NH2 group between aspartic acid and 2-oxoacids via the intervention of a pyridoxal coenzyme. The kinetically significant step, a 1,3-proton transfer, is thought to involve lysine-258 in a proton relay system (Scheme 18) replacement of lysine-258 with an alanine residue by site-directed muta-tion Q yields a mutant enzyme which is only weakly active but which can have activity restored by added amines (Scheme 18). 

Amino acid catabohsm is particularly important dining starvation. Because of the mass of muscle, amino acid catabohsm is particularly important in this tissue which, in starvation, supplies the liver with most of its gluconeogenic precursors (see also Fig. 13-11). Amino acids resulting from proteolysis during starvation are interconverted in the muscle so that 60% of the amino acid mass that leaves the muscle is either glutamine or alanine. The branched-chain amino acids valine, leucine, and isoleucine, which are aU essential amino acids, are deaminated in muscle by a specific aminotransferase, and the corresponding 2-oxoacids are transported to the liver for further metabohsm via branched-chain 2-oxoacid dehydrogenase (BCOADH). The aminotransferase is inactive in the hver, and this ensures that the peripheral tissues are supphed with valine, leucine, and isoleucine. 

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