Tyrosine catabolic pathway

Nitisinone is a reversibile inhibitor of 4-hydroxy-phenylpyruvate oxidase, an enzyme that plays a crucial role in the tyrosine catabolic pathway. Nitisinone prevents the accumulation of the toxic metabolites fumaryl acetoacetate, succinyl acetoacetate and succinyl acetone. Nitisinone is used for the treatment of hereditary tyrosinemia type 1. After oral administration bioavailability is 90% and peak levels are reached at 2.5 hours after dosing. The drug is eliminated mainly in the urine but some CYP3A4-mediated metabolism seems to occur. The elimination half-life is 45 hours. Blood dyscrasias are frequently occurring side effects as are eye problems like conjunctivitis, corneal opacity and keratitis. Exfoliative dermatitis, erythematous rash and pruritus... 

Catabolism of tyrosine and tryptophan begins with oxygen-requiring steps. The tyrosine catabolic pathway, shown at the end of this chapter, results in the formation of fumaric acid and acetoaceticacid, Iryptophan catabolism commences with the reaction catalyzed by tryptophan-2,3-dioxygenase. This enzyme catalyzes conversion of the amino acid to N-formyl-kynurenine The enzyme requires iron and copper and thus is a metalloenzyme. The final products of the pathway are acetoacetyl-CoA, acetyl-Co A, formic add, four molecules of carbon dioxide, and two ammonium ions One of the intermediates of tryptophan catabolism, a-amino-P-carboxyrnuconic-6-semialdchydc, can be diverted from complete oxidation, and used for the synthesis of NAD (see Niacin in Chapter 9). 

Arias-Barrau E, ER Olivera, JM Lnengo, C Eemandez, B Galan, JL Garcia, E Dfaz, B Minambres (2004) The homogentisate pathway a central catabolic pathway involved in the degradation of L-phenylalanine, L-tyrosine, and 3-hydroxyphenylacetate in Pseudomonas putida J Bacterial 186 5062-5077. 

FIGURE 18-21 Catabolic pathways for tryptophan, lysine, phenylalanine, tyrosine, leucine, and isoleucine. These amino acids donate some of their carbons (red) to acetyl-CoA. Tryptophan, phenylalanine, tyrosine, and isoleucine also contribute carbons (blue) to pyruvate or... 

RGURE 18-23 Catabolic pathways for phenylalanine and tyrosine. In humans these amino acids are normally con-... 

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...

Tyrosine can spare the requirement for phenylalanine. In many species, about 50% of the phenylalanine requirement can be replaced by tyrosine. For this rea.son, the requirement for phenylalanine is sometimes expressed as the requirement for the sum of both amino adds. The biochemical relationship between phenylalanine and tyrosine is shown in Figure 8.28, Phenylalanine monooxygenase catalyzes the conversion of phenylalanine to tyrosine. This enzyme uses the cofactor tetrahydro-biopterin. Biopterin is not required in the diet. It is synthesized from GTR Folate (in plants and bacteria) and molybdopterin are also synthesized from GTP. Figure 8.28 also depicts the catabolic pathway for tyrosine. 

The concept of sparing of one nutrient by another was introduced earlier, where it was demonstrated that dietary carbohydrate can spare protein. Similarly, cysteine can spare methionine and tyrosine can spare phenylalanine. A certain proportion of dietary methionine is converted to cysteine. Mediionine normally supplies part of the body s needs for cysteine. With cysteine-free diets, methionine can supply all of the body s needs for cysteine. The methionine catabolic pathway that leads to cysteine production is shown in Figure 8.27. Only the sulfur atom of methionine appears in the molecule of cysteine serine supplies the carbon skeleton of cysteine. a-Ketobutyrate is a byproduct of the pathway. a-Ketobutyrate is further degraded to propionyl-CoA by BCKA dehydrogenase or pyruvate dehydrogenase. Propionyl-CoA is then converted to succinyl-CoA, an intermediate of the Krebs cycle. 

FIGURE 8.28 Phenylalanine and tyrosine catabolism. Phenylalanine is converted to tyrosine by phenylalanine monooxygenase. This enzyme requires tetrahydrobiopterin as a cofactor. This cofactor is synthesized in the body from GTP and must be in the fully reduced, tetrahydro form to be active. The cofactor is converted to the dihydro form in the course of the reaction. A separate enzyme, which uses NADPH as a reducing agent, catalyzes the reduction of dihydroprotein back to tetrahydrobiopterin. Oxygen is the cosubstrate of phenylalanine monooxygenase, as well as of two other enzymes, in the pathway shown. 

The Catabolic Pathways of Lysine, Tryptophan, Phenylalanine, "tyrosine, and Leucine. 

The major quantitative pathway of tyrosine catabolism produces acetoacetate and fumarate. If homogentisate oxidase is missing, the result is alcaptonuriadark urine. 

Fig. 20.1 Major catabolic pathway for phenylalanine and tyrosine. The loci of known enzymatic defects are indicated by dashed lines. Note that hereditary tyrosinemia is now believed to be due to fumarylacetoacetate hydrolase, the final step in the pathway. (Redrawn with modifications from Mazur A, Harrow B Textbook of Biochemistry. WB Saunders, Philadelphia, 1971)...

Sparnins VL, PJ Chapman (1976) Catabolism of L-tyrosine by the homoprotocatechuate pathway in... 

Phenylketonuria (PKU) is a group of inherited disorders caused by a deficiency of the enzyme phenylalanine hydroxylase (PAH) that catalyses the conversion of phenylalanine to tyrosine, the first step in the pathway for catabolism of this amino acid. As a result, the concentration of phenylalanine in the liver and the blood increases. This high concentration in the liver increases the rate of a side reaction in which phenylalanine is converted to phe-nylpyruvic acid and phenylethylamine, which accumulate in the blood and are excreted in the urine. 

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 second competitive pathway for the disposal of PA requires the initial conversion of PA into tyrosine. This reaction is catalyzed by the enzyme PAH (phenylalanine-4-monooxygenase EC 1.14.16.1). The resulting tyrosine molecule can then be catabolized into fumarate and ace-toacetate. Both products are nontoxic and can be further catabolized in the citric acid cycle. In Mrs. Urick and the majority of individuals suffering from HPA and PKU, there is a defect in the PAH enzyme system (NIH Consensus State-... 

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