Tyrosine, degradation oxidation

Tyrosine Oxidation (p-Hydroxyphenylpyruvate Oxidase) (Eq. 17) The interruption of tyrosine degradation at the stage of p-hydroxy-phenylpyruvate (pHPP) in scurvy has long been considered Ae clearest evidence of a biochemical role for ascorbic acid. The intmmediate accumulates and is excreted when doses of tyrosine are given to scorbutic man, other primates, and guinea pigs, and to premature infants (K8). 

HPPD catalyzes an early step in a tyrosine degradation pathway [12] that is widely distributed in nature [13] and thus, as in animals, treatment of plants with inhibitors causes significant accumulation of tyrosine [8, 14]. HPP derived from transamination of tyrosine, is converted into HGA via HPPD, HGA is oxidized via HGA oxidase to 4-maleylacetoacetate, which is further degraded via 4-maleylacetoacetate isomerase and 4-fumarylacetoacetate lyase to fumarate and acetoacetate. In microbes the pathway provides assimilable carbon from tyrosine... 

Gruen (757) found that the presence of most of the amino acids tested during hydrolysis does not affect the tryptophan recovery. Of the hydroxyamino acids, threonine had no effect at all, while tyrosine has a small effect. On the other hand serine was found to reduce the recovery of tryptophan by about 10% due to a deamination reaction of serine. This produces pyruvic acid which, as a ketoacid, interacts with the indole nucleus of tryptophan (287). Whereas cysteine had no effect, cystine caused a substantial loss of tryptophan (about 40%), and was itself recovered on the analyzer partly in the reduced cysteine form. These results would indicate that a major factor in the loss of tryptophan during acid hydrolysis of a protein is degradative oxidation by cystine. 

Laccase is one of the main oxidizing enzymes responsible for polyphenol degradation. It is a copper-containing polyphenoloxidase (p-diphenoloxidase, EC 1.10.3.2) that catalyzes the oxidation of several compounds such as polyphenols, methoxy-substituted phenols, diamines, and other compounds, but that does not oxidize tyrosine (Thurston, 1994). In a classical laccase reaction, a phenol undergoes a one-electron oxidation to form a free radical. In this typical reaction the active oxygen species can be transformed in a second oxidation step into a quinone that, as the free radical product, can undergo polymerization. 

Thyroxine is actually a simple derivative of the aromatic amino acid tyrosine (see Section 13.1), but is believed to be derived by degradation of a larger protein molecule containing tyrosine residues. One hypothesis for their formation invokes suitably placed tyrosine residues in the protein thyroglobulin being iodinated to di-iodotyrosine. These residues then react together by phenolic oxidative coupling. 

Figure 25-5 shows the principal catabolic pathways, as well as a few biosynthetic reactions, of phenylalanine and tyrosine in animals. Transamination to phenylpyruvate (reaction a) occurs readily, and the product may be oxidatively decarboxylated to phen-ylacetate. The latter may be excreted after conjugation with glycine (as in Knoop s experiments in which phenylacetate was excreted by dogs after conjugation with glycine, Box 10-A). Although it does exist, this degradative pathway for phenylalanine must be of limited importance in humans, for an excess of phenylalanine is toxic unless it can be oxidized to tyrosine (reaction b, Fig. 25-5). Formation of phenylpyruvate may have some function in animals. The enzyme phenylpyruvate tautomerase, which catalyzes interconversion of enol and oxo isomers of its substrate, is also an important immunoregulatory cytokine known as macrophage migration inhibitory factor.863...

The chemical reactions used to degrade these aromatic compounds are numerous and complex. As was mentioned in Chapter 16, some fungi initiate the attack on lignin with peroxidases and produce soluble compounds that can be attacked by bacteria. In other cases elimination reactions may be used to initiate degradation. For example, some bacteria release phenol from tyrosine by P elimination (Fig. 14-5). However, more often hydroxylation and oxidative degradation of side chains lead to derivatives of benzoic acid or of the various hydroxybenzoic... 

Effect of Fiber Degradation on the Corrosion Solution. Hydrolysis and oxidation of protein and cellulose have been described in the literature primarily with the focus on degradation in industrial processing conditions. In alkaline conditions, amino acids are released from silk in a chain unzipping mechanism in acidic conditions, the scissions are random (8,9). As the polymer deteriorates, free carboxyl and amine end groups are formed. Tyrosine oxidizes to a quinone this reaction gives aged silk its yellow coloration. Amorphous areas of the fiber are attacked first. 

Gow AJ, Duran D, Malcolm S, Ischiropoulos H (1996) Effects of peroxynitrite-induced protein modifications on tyrosine phosphorylation and degradation. FEES Lett 385 63-66 Grune T, Merker K, Sandig G, Davies KJ (2003) Selective degradation of oxidatively modified protein substrates by the proteasome. Biochem Biophys Res Commun 305 709-718 Halliwell B (2002) Hypothesis proteasomal dysfunction a primary event in neurogeneration that leads to nitrative and oxidative stress and subsequent cell death. Ann N Y Acad Sci 962 182-194... 

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