Aminotransferases, aromatic

AROMATIC AMINO ACID-GLYOXYLATE AMINOTRANSFERASE Aromatic amino acid hydroxylases, PHENYLALANINE HYDROXYLASE TYROSINE HYDROXYLASE TRYPTOPHAN HYDROXYLASE ARRHENIUS CONSTANT... 

PHENYLALANINE AMINOTRANSFERASE PHENYLALANINE AMMONIA-LYASE PHENYLALANINE DECARBOXYLASE PHENYLALANINE DEHYDROGENASE PHENYLALANINE MONOOXYGENASE PHENYLALANINE RACEMASE PHENYLALANINE AMINOTRANSFERASE AROMATIC AMINO ACID AMINOTRANSFERASE... 

Free amino acids are further catabolized into several volatile flavor compounds. However, the pathways involved are not fully known. A detailed summary of the various studies on the role of the catabolism of amino acids in cheese flavor development was published by Curtin and McSweeney (2004). Two major pathways have been suggested (1) aminotransferase or lyase activity and (2) deamination or decarboxylation. Aminotransferase activity results in the formation of a-ketoacids and glutamic acid. The a-ketoacids are further degraded to flavor compounds such as hydroxy acids, aldehydes, and carboxylic acids. a-Ketoacids from methionine, branched-chain amino acids (leucine, isoleucine, and valine), or aromatic amino acids (phenylalanine, tyrosine, and tryptophan) serve as the precursors to volatile flavor compounds (Yvon and Rijnen, 2001). Volatile sulfur compounds are primarily formed from methionine. Methanethiol, which at low concentrations, contributes to the characteristic flavor of Cheddar cheese, is formed from the catabolism of methionine (Curtin and McSweeney, 2004 Weimer et al., 1999). Furthermore, bacterial lyases also metabolize methionine to a-ketobutyrate, methanethiol, and ammonia (Tanaka et al., 1985). On catabolism by aminotransferase, aromatic amino acids yield volatile flavor compounds such as benzalde-hyde, phenylacetate, phenylethanol, phenyllactate, etc. Deamination reactions also result in a-ketoacids and ammonia, which add to the flavor of... 

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

Historically, the amine was an aromatic amine but is now generalized to any amine. A Schiff base, also called an aldimine, is formed in the pyridoxal 5-phosphate-dependent aminotransferase reactions. 

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). 

Previously, AAT had been transformed into an L-tyrosine aminotransferase (TAT) by site-specific mutation of up to six amino acid residues lining the active site of wild-type AAT. The hextuple AAT-mutant achieved kinetic data towards the transamination of aromatic substrates such as i-phenylalanine within an order of magnitude of wild-type TAT (Onuffer, 1995). 

Aminotransferases are potentially applicable to the production of a wide range of amino acids, because enzymes are available for d- and L-amino acids. In addition, a wide range of aminotransferases with side-chain specifidty are known, including enzymes for the production of amino acids with aromatic side chains, acidic side chains, branched alkyl side chains, etc. 

On the other hand, the pathways to both aromatic amino acids, Phe (1)/Tyr (2), in vascular plants are only beginning to be clarified, and could not readily be deduced from bacterial/fungal sequence/comparisons, for example, in terms of substrate(s). Thus the key to fully understanding the pathways to these two aromatic amino acids in plants was to identify the enzymes, as well as to ultimately establish their substrate specificities/ feedback properties that is, of the actual dehydratases, dehydrogenases, and aromatic aminotransferases involved, including how transcriptional regulation is attained. 

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