Aspartate phenylalanine transaminase

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. 

A new development is the industrial production of L-phenylalanine by converting phenylpyruvic add with pyridoxalphosphate-dependent phenylalanine transaminase (see Figure A8.16). The biotransformation step is complicated by an unfavourable equilibrium and the need for an amino-donor (aspartic add). For a complete conversion of phenylpyruvic add, oxaloacetic add (deamination product of aspartic add) is decarboxylated enzymatically or chemically to pyruvic add. The use of immobilised . coli (covalent attachment and entrapment of whole cells with polyazetidine) is preferred in this process (Figure A8.17). 

This enzyme [EC 2.6.1.1] (also known as transaminase A, glutamicioxaloacetic transaminase, and glutamic aspartic transaminase) catalyzes the reversible reaction of aspartate with a-ketoglutarate to produce oxaloace-tate and glutamate. Pyridoxal phosphate is a required cofactor. The enzyme has a relatively broad specificity, and tyrosine, phenylalanine, and tryptophan can all serve as substrates. 

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. 

S )-Enantiomcrs of fluorophcnylalanincs and 4-(trifluoromethyl)phenylalanine were successfully prepared from the corresponding 2-oxo acids by the transfer of an amino group from (S)-aspartic acid catalyzed by a specific transaminase of microbial origin20. The biomimetic reduction of other imines with NAD coenzymes has also been described21 28. 

Commercial preparations of pig heart glutamate-oxaloacetate transaminase have been screened for their ability to transaminate various a-keto acids with l-[ N]glutamate (32). In addition to l-[ N]aspartate, enzyme preparations were able to catalyze the formation of labeled tyrosine, phenylalanine, leucine, and dihydroxyphenylalanine, as demonstrated by HPLC (17). However, these amino acids have not yet been obtained in radiopure form by this method. The -keto acid analogs of valine and tryptophan were not transaminated by the enzyme preparations. Glutamate-oxaloacetate transaminases obtained from several commercial sources have varying abilities to transaminate the -keto acid... 

Depending on the substrate preference of the employed transaminase, the following couples of sacrificial amine donor/keto acceptors were used, which are often derived from the a-aminoacid pool (such as alanine/pyruvate, phenylalanine/ phenylpyruvate, glutamic acid/a-ketoglutarate, aspartic acid/a-ketosuccinate) or constitute simple amines/ketones, such as 2-propylamine/acetone and 2-butyl-amine/2-butanone. It should be kept in mind that the absolute configuration of a chiral amine-donor has to match the stereospecificity of the co-TA in order to be accepted. 

Aspartic acid, alanine, phenylalanine, and lysine were manufactured by enzymatic route. Immobilized E. coli cells expressing aspartate or the immobilized enzyme has been used in the commercial production of aspartic acid from ammonia and fumaric acid. Chibata and coworkers also produced alanine by microbial Pseudomonas dacunhae) L-aspartate P-decarboxylase with aspartate as the starting material. Phenyl alanine was manufactured from fw s-cinnamic acid and ammonia by the enzymatic route by phenyl alanine ammonia lyase as catalyst or from phenyl pyruvate and aspartic acid using transaminase. 

Another approach in directed evolution was the evolution of an aspartate transaminase to an enz3une possessing the properhes of the closely related tyrosine transaminase [128]. Eight roxmds of DNA shuffling led to mutants with 100- to 270-fold increase of for phenylalanine and a 40- to 150-fold increase for tyrosine. 

---