Methionine degradation, role

Lipid radical transfer has been demonstrated for trp, arg, his, and lys (99, 383, 384), all of which have reactive N groups on their side chains, and radical decomposition products from these amino acids have been identified (381, 382, 390). Tyrosine and methionine degradation by oxidizing lipids has also been demonstrated (390), but the intermediate radicals in the reaction may be too unstable for detection. Lipid radical adducts to amino acids are important flavor precursors (340) and also may play critical roles in pathological processes in vivo (186, 388). 

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... 

A heterocyclic sulfur-containing compound, 2-methyl-thiophene, was identified in boiled crayfish tail meat and pasteurized crabmeat. Thiazole and 3-methylthiopropanal were identified in the crayfish hepatopancreas. Heterocyclic sulfur-containing compounds play important roles in generating meaty aromas in a variety of meat products and are considered important volatile aroma components of marine crustaceans (12— 14). The 2-methylthiophene could be an important flavor cemponent in boiled crayfish tail meat. Both thiazole find 3-methylthiopropanal were important contributors to the desirable meaty aroma associated with crayfish hepatopancreas. The 3-methyl-thiopropanal, identified in boiled crayfish hepatopancreas, is derived from Strecker degradation of methionine (15), and has been considered to be an important cemponent in basic meat flavor (16). Pyridine was detected in the headspace of the hepatopancreas from freshly boiled crayfish. Pyridine and 2-ethylpyridine have been previously reported as components in the atmospheric distillate from a sample of crayfish hepatopancreas frozen for three months (2). 

Degradation of the sulfur amino acids cysteine and methionine to H2S and other volatile sulfur compounds has been observed under laboratory conditions but their roles under wine fermentation conditions are less clear (Eschenbruch 1974 Jiranek et al. 1995a Moreira et al. 2002 Perpete et al. 2006 Rankine 1963 Vos and... 

The enzymes are widely distributed in microorganisms, plants, and animals. " Three Mo-MPT enzymes have been found in mammals (1) xanthine dehydrogenase see Dehydrogenase) has many, varied roles in purine catabolism, drug metabolism, and oxidative stress response, (2) aldehyde oxidase is important in drug metabolism and the synthesis of retinoic acid from retinal, and (3) sulfite oxidase plays a cmcial role in the detoxification of sulfite produced in the degradation of cysteine and methionine. Genetic Mo-MPT deficiency in... 

This enzyme s role in humans is to assist the detoxification of propionate derived from the degradation of the amino acids methionine, threonine, valine, and isoleucine. Propionyl-CoA is carboxylated to (5 )-methylmalonyl-CoA, which is epimerized to the (i )-isomer. Coenzyme Bi2-dependent methylmalonyl-CoA mutase isomerizes the latter to succinyl-CoA (Fig. 2), which enters the Krebs cycle. Methylmalonyl-CoA mutase was the first coenzyme B -dependent enzyme to be characterized crystallographically (by Philip Evans and Peter Leadlay). A mechanism for the catalytic reaction based on ab initio molecular orbital calculations invoked a partial protonation of the oxygen atom of the substrate thioester carbonyl group that facilitated formation of an oxycyclopropyl intermediate, which connects the substrate-derived and product-related radicals (14). The partial protonation was supposed to be provided by the hydrogen bonding of this carbonyl to His 244, which was inferred from the crystal structure of the protein. The ability of the substrate and product radicals to interconvert even in the absence of the enzyme was demonstrated by model studies (15). 

Collective name for polyprenylated 2,3-dimethylben-zoquinone derivatives, [e.g., PQ-9 (n=9), C5jH ,02, Mr 749.22, bright yellow platelets, mp. 48-49 C, u max 314 nm (petroleum ether)], isolated from chloro-plasts. The P. play a role as redox substrates in photosynthesis for cyclic and non-cyclic electron transport where they are converted reversibly into the corresponding hydroquinones (plastoquinols). The biosynthesis of P. proceeds from homogentisic acid, a product of L-tyrosine degradation, through prenylation and methylation (methyl group from L-methionine) to the plastoquinols which are dehydrated to the P.. ... 

The degradation of methionine in its role of ethylene production leads to CO2, formate, and ethylene from the Ci, C2, and Ca,4, respectively, of the a-aminobutyryl moiety. The fate of the methylthio fragment is unclear but it is suggested that it is utilized to resynthesize methionine by transfer to some four-carbon acceptor (Adams and Yang, 1977). 

In animals homocysteine arises from methionine through its role as a methyl donor, as will be discussed in a subsequent section. It may either be reutilized for methionine production or degraded. In animal tissues the degradative pathway plays a major role in sulphur nutrition. Much of the cysteine sulphur, and through it sulphate, can be derived from dietary methionine. The transsulphuration from homocysteine to produce cysteine is very much like that in the other direction. It also involves cystathionine but is not a reversal of the synthetic pathway. Quite diflFerent reactions are involved. Homocysteine reacts with serine to produce the... 

The thermal generation of flavor is a very essential process for the "taste" of many different foodstuffs, e.g. cocoa, coffee, bread, meat. The resulting aromas are formed through non-enzymatic reactions mainly with carbohydrates, lipids, amino acids (proteins), and vitamins under the influence of heat. Thiamin (vitamin B ) and the amino acids, cysteine and methionine, belong to those food constituents which act as flavor precursors in thermal reactions. The role of thiamin as a potent flavor precursor is related to its chemical structure which consists of a thiazole as well as a pyrimidine moiety. The thermal degradation of this heterocyclic constituent leads to very reactive intermediates which are able to react directly to highly odoriferous flavor compounds or with degradation products of amino acids or carbohydrates. 

The sulfur-containing amino acids, cysteine and methionine, are essential components of proteins and also participate in special reactions. In animals methionine is of particular importance because of its role in the transfer of methyl groups. Cysteine is a component of glutathione, 7-glutamyl-cysteinyl-glycine. The synthesis of these important amino acids occurs in plants and microorganisms by as yet undefined pathways. Certain interconversions at the amino acid level have been studied in animals, and some of the enzymes concerned have been identified. More is known about the degradation of the amino acids. 

Taurine is the common name for P-aminoethylsulfonic acid, a C2 compound that is a degradation product from the amino adds cysteine and methionine. It is present in the bile as the so-called taurodiohc add, a cholic acid amide from which it can be released by add hydrolysis. It recdved its common name from its discovery in bovine bile juices in 1827. As soon as it became evident that taurine has been detected in hydrolysates from bovine testes instead of bile, the compound has experienced constant interest as food additive. On the other hand, its real physiological roles are dearly documented [50]. 

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