Catabolism hydroxy amino acids

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... 

Three of the amino acids, alanine, aspartic acid, and glutamic acid are readily formed by transamination from products of the citric acid cycle. This has been explained in the chapter. Carbon Catabolism of Amino Acids. Glutamic acid is the probable precursor of a considerable number of the other nonessential amino acids, namely, proline, hydroxy-proline, ornithine, and from it arginine. 

From the observation of Dakin (p. 83) that phenylserine is oxidized in the body to benzoic acid (equation 3), Knoop had the insight to propose that the catabolism of /3-hydroxy-a-amino acids leads to the forma-... 

The aromatic amino acid L-phenylalanine (primary metabolite) is directed into the phe-nylpropanoid pathway leading to hydroxy-cinnamic acids, lignin and flavonoids by the activity of L-phenylalanine ammonia-lyase (PAL), which brings about its nonoxidative deamination yielding ammonia and tvans-cinnamic acid (Fig. 1). PAL is one of the most studied plant enzymes, and its crystal structure has recently been solved [2]. PAL is related to the histidine and tyrosine ammonia-lyases of amino acid catabolism. A class of bifunctional PALs found in monocotyle-donous plants and yeast can also deaminate tyrosine [3]. A single His residue is responsible for this switch in substrate preference [3, 4]. All three enzymes share a unique MIO (4-methylidene-imidazole-5-one) prosthetic group at the active site. This is formed auto-catalytically from the tripeptide Ala-Ser-Gly by cyclization and dehydration during a late... 

Besides the mineralcorticoid effect on the electrolyte metabolism, the adrenocortical hormones also influence the metabolism of glucose by promoting glycogen formation in the liver, especially from protein. This effect is termed the glucocorticoid effect and is manifested especially by the 11/3-hydroxy compounds. Emphasizing the breakdown of protein, this effect is also called the catabolic effect. The mechanism of this hormone action seems to involve the de novo formation of more enzymes of amino acid metabolism, e.g., tyrosine a-ketoglutarate transaminase, tr5rptophan pyrrolase, etc. (cf. Enzyme Induction, Chapt. VII-7). 

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