Aromatic amino acid transaminase

There is a further possibility for conversion of methionine to MT. Transaminase enzymes can convert methionine to 4-methylthio-2-oxobutanoate (also, a-keto-y-methylthiobutyrate). One such enzyme is aromatic-amino-acid transaminase, EC 2.6.1.57, for which L-methionine is, albeit, less efficiently, a substrate (Equation 5) ... 

Thus, for phenylalanine (Phe, F), decarboxylation and dehydration (prephenate dehydratase, EC 4.2.1.51) to phenylpyruvate is followed by transamination with either of the enzymes tyrosine transaminase (EC 2.6.1.5) or aromatic amino acid transaminase (EC 2.6.1.57). Both of these use pyridoxal as cofactor and derive the nitrogen for the amino function from glutamate (Glu, E). 

A possible explanation for the superiority of the amino donor, L-aspartic add, has come from studies carried out on mutants of E. coli, in which only one of the three transaminases that are found in E. coli are present. It is believed that a branched chain transaminase, an aromatic amino add transaminase and an aspartate phenylalanine aspartase can be present in E. coli. The reaction of each of these mutants with different amino donors gave results which indicated that branched chain transminase and aromatic amino add transminase containing mutants were not able to proceed to high levels of conversion of phenylpyruvic add to L-phenylalanine. However, aspartate phenylalanine transaminase containing mutants were able to yield 98% conversion on 100 mmol l 1 phenylpyruvic acid. The explanation for this is probably that both branched chain transaminase and aromatic amino acid transminase are feedback inhibited by L-phenylalanine, whereas aspartate phenylalanine transaminase is not inhibited by L-phenylalanine. In addition, since oxaloacetate, which is produced when aspartic add is used as the amino donor, is readily converted to pyruvic add, no feedback inhibition involving oxaloacetate occurs. The reason for low conversion yield of some E. coli strains might be that these E. cdi strains are defident in the aspartate phenylalanine transaminase. 

EC2.6.1.57 aromatic L-amino acid transaminase (aminotransferase (transaminase))... 

The liver is also the principal metabolic center for hydrophobic amino acids, and hence changes in plasma concentrations or metabolism of these molecules is a good measure of the functional capacity of the liver. Two of the commonly used aromatic amino acids are phenylalanine and tyrosine, which are primarily metabolized by cytosolic enzymes in the liver [1,114-117]. Hydroxylation of phenylalanine to tyrosine by phenylalanine hydroxylase is very efficient by the liver first pass effect. In normal functioning liver, conversion of tyrosine to 4-hy-droxyphenylpyruvate by tyrosine transaminase and subsequent biotransformation to homogentisic acidby 4-hydroxyphenylpyruvic acid dioxygenase liberates CO2 from the C-1 position of the parent amino acid (Fig. 5) [1,118]. Thus, the C-1 position of phenylalanine or tyrosine is typically labeled with and the expired C02 is proportional to the metabolic activity of liver cytosolic enzymes, which corresponds to functional hepatic reserve. Oral or intravenous administration of the amino acids is possible [115]. This method is amenable to the continuous hepatic function measurement approach by monitoring changes in the spectral properties of tyrosine pre- and post-administration of the marker. 

Among the numerous enzymes that utilize pyridoxal phosphate (PLP) as cofactor, the amino acid racemases, amino acid decarboxylases (e.g., aromatic amino acids, ornithine, glutamic acid), aminotransferases (y-aminobutyrate transaminase), and a-oxamine synthases, have been the main targets in the search for fluorinated mechanism-based inhibitors. Pharmaceutical companies have played a very active role in this promising research (control of the metabolism of amino acids and neuroamines is very important at the physiological level). 

L-Amino acid transaminases are ubiquitous in nature and are involved, be it directly or indirectly, in the biosynthesis of most natural amino acids. All three common types of the enzyme, aspartate, aromatic, and branched chain transaminases require pyridoxal 5 -phosphate as cofactor, covalently bound to the enzyme through the formation of a Schiff base with the e-amino group of a lysine side chain. The reaction mechanism is well understood, with the enzyme shuttling between pyridoxal and pyridoxamine forms [39]. With broad substrate specificity and no requirement for external cofactor regeneration, transaminases have appropriate characteristics to function as commercial biocatalysts. The overall transformation is comprised of the transfer of an amino group from a donor, usually aspartic or glutamic acids, to an a-keto acid (Scheme 15). In most cases, the equilibrium constant is approximately 1. 

Enzymes with pyridoxine coenzymes in human metabolism (examples, several hundred enzymes). (1) Transaminases (aspartate-aminotransferase (EC 2.6.1.1), alanine-aminotransferase (EC 2.6.1.2) aromatic-amino acid aminotransferase (EC 2.6.1.57)) (2) decarboxylas (histidine-decarboxylase (EC 4.1.1.22) aromatic-amino acid decarboxylase (EC 4.1.1.28)) (3) a,jS-elimination (t-serine-... 

The biosynthesis of other volatile phenyl-propanoid-related compoimds such as phenyla-cetaldehyde and 2-phenylethanol, does not occur via trans-cinnamic acid and competes with PAL for Phe utilization [90, 96, 97]. Phenylacetaldehyde biosynthesis from Phe requires the removal of both the carboxyl and amino groups. A classical sequential two-step removal is believed to occur in tomato where Phe was shown to be first converted to phe-nylethylamine by aromatic amino acid decarboxylase (AADC) and further required the action of a hypothesized amine oxidase, dehydrogenase, or transaminase for phenylacetaldehyde formation [97]. On the other hand, in petunia, one bifunctional enzyme, phenylacetaldehyde synthase (PAAS) catalyzes the unprecedented efficient coupling of Phe decarboxylation to oxidation resulting in... 

Transaminases possess many features appropriate for effident biocatalysts, such as high turnover numbers and no requirement for external recycling of the co-factor. Because of the wide substrate tolerance of many amino transferases such as tyrosine amino transferase and branched-chain amino transferases from E. coU, these enzymes have been largely employed in the enantiospecific preparation of non-proteinogenic amino acids. These include straight-chain alkyl, diadd, branched-chain, aromatic, and bifunctional amino adds [65]. 

Tryptophan 331 is converted to tryptamine 332 by both aromatic L-amino acid decarboxylase (EC 4.1.1.28) and tyrosine decarboxylase (EC 4.1.1.25), and in both instances (334, 335) it was shown, either by use of the pro-R specific monoamine oxidase (335) or by degradation of the labeled tryptamines to glycine and use of the pro-S specific D-amino acid oxidase and pro-R specific glutamate pyruvate transaminase (334), that decarboxylation involved retention of configuration. Hydroxylation that leads to sporidesmin 333 has been shown to involve specific loss of the 3-pro-R hydrogen, and so again hydroxylation involves retention of configuration (102). 

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