[Preliminary data on the transamination reactions of aromatic amino acids in the blood].
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Aromatic L-Amino acids are important chiral building blocks for the synthesis of many drugs, pesticides, fine chemicals and food additives. Due to the high activity and steroselectivity, enzymatic synthesis of chiral building blocks has become the main research direction in asymmetric synthesis field. Guided by the phylogenetic analysis of transaminases from different sources, two representative aromatic transaminases TyrB and Aro8 in type I subfamily, from the prokaryote Escherichia coli and eukaryote Saccharomyces cerevisia, respectively, were applied for the comparative study of asymmetric transamination reaction process and catalytic efficiency of reversely converting keto acids to the corresponding aromatic L-amino acid. Both TyrB and Aro8 could efficiently synthesize the natural aromatic amino acids phenylalanine and tyrosine as well as non-natural amino acid phenylglycine. The chiral HPLC analysis showed the produced amino acids were L-configuration and the e.e value was 100%. L-alanine was the optimal amino donor, and the transaminase TyrB and Aro8 could not use D-amino acids as amino donor. The optimal molar ratio of amino donor (L-alanine) and amino acceptor (aromatic alpha-keto acids) was 4:1. Both of the substituted group on the aromatic ring and the length of fatty acid carbon chain part in the molecular structure of aromatic substrate alpha-keto acid have the significant impact on the enzyme-catalyzed transamination efficiency. In the experiments of preparative-scale transamination synthesis of L-phenylglycine, L-phenylalanine and L-tyrosine, the specific production rate catalyzed by TryB were 0.28 g/(g x h), 0.31 g/(g x h) and 0.60 g/(g x h) and the specific production rate catalyzed by Aro8 were 0.61 g/(g x h), 0.48 g/(g x h) and 0.59 g/(g x h). The results obtained here were useful for applying the transaminases to asymmetric synthesis of L-amino acids by reversing the reaction balance in industry.
Transamination
Alanine
Transaminase
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ABSTRACT In lactococci, transamination is the first step of the enzymatic conversion of aromatic and branched-chain amino acids to aroma compounds. In previous work we purified and biochemically characterized the major aromatic aminotransferase (AraT) of a Lactococcus lactis subsp. cremoris strain. Here we characterized the corresponding gene and evaluated the role of AraT in the biosynthesis of amino acids and in the conversion of amino acids to aroma compounds. Amino acid sequence homologies with other aminotransferases showed that the enzyme belongs to a new subclass of the aminotransferase I subfamily γ; AraT is the best-characterized representative of this new aromatic-amino-acid-specific subclass. We demonstrated that AraT plays a major role in the conversion of aromatic amino acids to aroma compounds, since gene inactivation almost completely prevented the degradation of these amino acids. It is also highly involved in methionine and leucine conversion. AraT also has a major physiological role in the biosynthesis of phenylalanine and tyrosine, since gene inactivation weakly slowed down growth on medium without phenylalanine and highly affected growth on every medium without tyrosine. However, another biosynthesis aromatic aminotransferase is induced in the absence of phenylalanine in the culture medium.
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Abstract The degradation of l‐ and d‐aromatic amino acids has been studied in Candida maltosa . The following main metabolites were indentified: indolelactate was the degradation product of tryptophan, phenylpyruvate and phenyllactate were that of phenylalanine and p ‐hydroxyphenylpyruvate and p ‐hydroxyphenyllactate resulted from degradation of tyrosine. The first step of utilization of l‐aromatic amino acids was found to be a transamination. The cells contained increased aromatic aminotransferase activity if grown on medium supplemented with one of the three l‐aromatic amino acids. Three aromatic aminotransferases were separated by DEAE‐cellulose chromatography. The pathway for degradation of d‐aromatic amino acids involves deamination as the initial step. The enzyme was characterized to be a general d‐amino acid oxidase capable of utilizing various d‐amino acids. Synthesis of this enzyme was constitutive. Common products of both enzymatic reactions with the aromatic amino acids were the corresponding aromatic pyruvates. Formation of the aromatic lactate derivatives was catalyzed by an aromatic lactate dehydrogenase. This enzyme possessed a high substrate specificity for p ‐hydroxyphenylpyruvate, phenylpyruvate, and indolepyruvate and was synthesized constitutively.
Transamination
Aromatic amine
Deamination
Mimosine
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Brevibacterium
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Abstract— The transamination between amino acids and aliphatic and aromatic keto acids has been investigated in homogenates of human and rat brain. Tryptophan, phenylalanine and 3,4‐dihydroxyphenylalanine (DOPA) at concentrations of 3.6 min and below trans‐aminated aromatic keto acids more rapidly than α‐ketoglutarate; lower K m values were found for tryptophan and phenylalanine in the presence of the aromatic keto acid. Rat brain and liver arninotransferases exhibited similar affinities for tryptophan in the presence of different keto acids. Branched chain keto acids were also acceptors of the amino groups of tryptophan and DOPA. In brain homogenates α‐ketoglutarate and p ‐hydroxyphenyl‐pyruvate were transaminated by tyrosine and 5‐hydroxytryptophan at about equal rates, whereas a‐ketoglutarate was transaminated more rapidly with aliphatic amino acids. At concentrations of 1.6 m DOPA and 0.8 mM p ‐hydroxyphenylpyruvate, transamination was 6‐fold greater than the rate of formation of dopamine. The dihydroxyphenylpyruvate formed during arninotransfer from DOPA by brain tissue was not readily decarboxylated, whereas 65–70 per cent of the indolepyruvate formed from tryptophan was decarboxylated. We suggest that an increased rate or degree of transamination between tryptophan and aromatic and branched chain keto acids may explain the increased excretion of non‐hydroxylated indolic acids in phenylketonuria and‘maple syrup urine’disease, respectively. Increased aminotransfers from tryptophan and DOPA may reduce the amounts of precursors available for the synthesis of serotonin and catecholamines, both of which are at low levels in the sera of untreated phenylketonurics.
Transamination
Alanine
Imino acid
Phenylpyruvic acid
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The transamination of aromatic l-amino acids (5-hydroxytryptophan, tryptophan, tyrosine, phenylalanine and kynurenine) was shown to be catalysed by enzyme preparations from rat small intestine. On the basis of the partial purification and characterization of these aromatic amino acid transaminases, it is suggested that rat small intestine contains several kinds of aromatic amino acid transaminases.
Transamination
Transaminase
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Although trypanosomatids are known to rapidly transaminate exogenous aromatic amino acids in vitro and in vivo, the physiological significance of this reaction is not understood. In postmitochondrial supernatants prepared from Trypanosoma brucei brucei and Crithidia fasciculata, we have found that aromatic amino acids were the preferred amino donors for the transamination of alpha-ketomethiobutyrate to methionine. Intact C. fasciculata grown in the presence of [15N]tyrosine were found to contain detectable [15N]methionine, demonstrating that this reaction occurs in situ in viable cells. This process is the final step in the recycling of methionine from methylthioadenosine, a product of decarboxylated S-adenosylmethionine from the polyamine synthetic pathway. Mammalian liver, in contrast, preferentially used glutamine for this reaction and utilized a narrower range of amino donors than seen with the trypanosomatids. Studies with methylthioadenosine showed that this compound was readily converted to methionine, demonstrating a fully functional methionine-recycling pathway in trypanosomatids.
Transamination
Crithidia fasciculata
Methionine synthase
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The enzymatic degradation of amino acids in cheese is believed to generate aroma compounds and therefore to be involved in the complex process of cheese flavor development. In lactococci, transamination is the first step in the degradation of aromatic and branched-chain amino acids which are precursors of aroma compounds. Here, the major aromatic amino acid aminotransferase of a Lactococcus lactis subsp. cremoris strain was purified and characterized. The enzyme transaminates the aromatic amino acids, leucine, and methionine. It uses the ketoacids corresponding to these amino acids and alpha-ketoglutarate as amino group acceptors. In contrast to most bacterial aromatic aminotransferases, it does not act on aspartate and does not use oxaloacetate as second substrate. It is essential for the transformation of aromatic amino acids to flavor compounds. It is a pyridoxal 5'-phosphate-dependent enzyme and is composed of two identical subunits of 43.5 kDa. The activity of the enzyme is optimal between pH 6.5 and 8 and between 35 and 45 degrees C, but it is still active under cheese-ripening conditions.
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Cheese ripening
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