HORT640 - Metabolic Plant Physiology
The aminotransferases or transaminases are enzymes catalyzing the transfer of an amino group, plus a proton and an electron pair, from an amino donor compound to the carbonyl position of an amino acceptor compound.
Usually, an amino acid acts as the amino donor and a 2-keto acid as the amino acceptor. However, in some cases aldehydes may serve as amino acceptor and amines as donors.
Most transaminations are freely reversible but cases of unidirectional (irreversible) transamination are known (Givan, 1980).
One of the best characterized plant aminotransferase is glutamate-oxaloacetate transaminase (GOT) (or aspartate aminotransferase (AAT) [EC 184.108.40.206]) which catalyzes the reversible interconversions of glutamate and aspartate, and their 2-keto analogs:
Glutamate + oxaloacetate (OAA) <---> 2-oxoglutarate + aspartate
A cDNA encoding the cytosolic form of this enzyme has been cloned from carrot (Turano et al, 1992). The enzyme is a dimer with 2 identical subunits (40 to 45 kDa). Consistent with this, in Zea mays variants have been found for electrophoretic mobility of subunits of GOT. Whereas the homozygous parental strains had only a single electrophoretic band each, differing from each other in electrophoretic mobility, in heterozygotes between electrophoretic variants, the enzyme shows 3 electrophoretic variants (the two homo-dimers plus the hetero-dimer) (Givan, 1980).
For a recent discussion of aspartate aminotransferase, alanine aminotransferase and asparagine aminotransferase see: Ireland RJ, Lea PJ 1999 The enzymes of glutamine, glutamate, asparagine, and aspartate metabolism. In (BK Singh ed) "Plant Amino Acids: Biochemistry and Biotechnology", Marcel Dekker, NY, pp. 49-109.
The sequences of aminotransferases have been studied extensively by using algorithms for sequence comparison, hydropathy patterns and secondary structure predictions. Based on these criteria, the aminotransferases have been divided into four subgroups on the basis of their mutual structural relatedness (Mehta et al, 1993). Subgroup I comprises aspartate, alanine, tyrosine, histidinol-phosphate, and phenylalanine aminotransferases; subgroup II acetylornithine, ornithine, omega-amino acid, 4-aminobutyrate and diaminopelargonate aminotransferases; subgroup III D-alanine and branched-chain amino acid aminotransferases, and subgroup IV serine and phosphoserine aminotransferases (Mehta et al, 1993).
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Givan CV 1980 Aminotransferases in higher plants. In (BJ Miflin ed) "The Biochemistry of Plants", Vol. 5, Academic Press, New York, pp. 329-357.
Ireland RJ, Lea PJ 1999 The enzymes of glutamine, glutamate, asparagine, and aspartate metabolism. In (BK Singh ed.) "Plant Amino Acids: Biochemistry and Biotechnology", Marcel Dekker, NY, pp. 49-109.
Mehta PK, Hale TI, Christen P 1993 Aminotransferases: demonstration of homology and division into evolutionary subgroups. Eur. J. Biochem. 214: 549-561.
Turano FJ, Weisemann JM, Matthews BF 1992 Identification and expression of a cDNA clone encoding aspartate aminotransferase in carrot. Plant Physiol. 100: 374-381.