Phenylalanine catabolism

Figure 30-13. Alternative pathways of phenylalanine catabolism in phenylketonuria. The reactions also occur in normal liver tissue but are of minor significance.

Metabolic disorders of phenylalanine catabolism include phenylketonuria (PKU) and several hyper-phenylalaninemias. 

Phenylalanine Catabolism Is Genetically Defective in Some People... 

Another inheritable disease of phenylalanine catabolism is alkaptonuria, in which the defective enzyme is homogentisate dioxygenase (Fig. 18-23). Less serious than PKU, this condition produces few ill effects, although large amounts of homogentisate are excreted and its oxidation turns the urine black. Individuals with alkaptonuria are also prone to develop a form of arthri-... 

Another subgroup of the 2His-lcarboxylate family is dependent on a reduced pterin cofactor (5). They catalyze hydrox-ylations at the aromatic positions of amino acids in phenylalanine catabolism and hormone biosynthesis (Fig. 2). Unlike the a-KG-dependent enzymes, the pterin co-substrate does not ligate to the iron directly. In the reaction cycle, the pterin cosubstrate supplies two electrons for the heteiolysis of O2 to give a yet to be characterized iron-oxygen hydroxylating species. 

Phenylalanine catabolism, 468 chemical structure, 19 plasma conoentralion, 46o sparing by tyrosine, 467,469 Phenylketonuria, 467,469 Phenylpyruvic acid, 469 FhIP,889,B90 Phlebotomy, 759 Phlorizin hydrolase, 109-110 Phorbol esters, cancer and, 916 Phosphatases, 54, 66 Phosphate, 694 in biologLcal fluids, 696 in bore, 697... 

L-Phenylalanine,which is derived via the shikimic acid pathway,is an important precursor for aromatic aroma components. This amino acid can be transformed into phe-nylpyruvate by transamination and by subsequent decarboxylation to 2-phenylacetyl-CoA in an analogous reaction as discussed for leucine and valine. 2-Phenylacetyl-CoA is converted into esters of a variety of alcohols or reduced to 2-phenylethanol and transformed into 2-phenyl-ethyl esters. The end products of phenylalanine catabolism are fumaric acid and acetoacetate which are further metabolized by the TCA-cycle. Phenylalanine ammonia lyase converts the amino acid into cinnamic acid, the key intermediate of phenylpropanoid metabolism. By a series of enzymes (cinnamate-4-hydroxylase, p-coumarate 3-hydroxylase, catechol O-methyltransferase and ferulate 5-hydroxylase) cinnamic acid is transformed into p-couma-ric-, caffeic-, ferulic-, 5-hydroxyferulic- and sinapic acids,which act as precursors for flavor components and are important intermediates in the biosynthesis of fla-vonoides, lignins, etc. Reduction of cinnamic acids to aldehydes and alcohols by cinnamoyl-CoA NADPH-oxido-reductase and cinnamoyl-alcohol-dehydrogenase form important flavor compounds such as cinnamic aldehyde, cin-namyl alcohol and esters. Further reduction of cinnamyl alcohols lead to propenyl- and allylphenols such as... 

Tyrosine is not an essential amino acid in animals because it is synthesized from phenylalanine in a hydroxylation reaction. The enzyme involved, phenylala-nine-4-monoxygenase, requires the coenzyme tetrahydrobiopterin (Section 14.3), a folic acid-like molecule derived from GTP. Because this reaction also is a first step in phenylalanine catabolism, it is discussed further in Chapter 15. 

Phenylalanine catabolism occurs primarily in the liver, where the first step is a hydroxylation in the para position to file ///C /...20and%20Settings/Bariski/Desktop/26134 9780471022053 9780585329482 0471022055/files/page 260.html[12/10/2009 10 14 55]... 

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. 

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. 

PAH is mainly found in the liver and in the kidneys and converts L-phenylalanine ( -Phe) to L-tyrosine (L-Tyr) (Scheme 1), which is the rate-limiting step in the catabolism of L-Phe to carbon dioxide and water. TH is present in the central nervous system (CNS),... 

Tetrahydrobiopterin, another cofactor of amino acid catabolism, is similar to the pterin moiety of tetrahydrofolate, but it is not involved in one-carbon transfers instead it participates in oxidation reactions. We consider its mode of action when we discuss phenylalanine degradation (see Fig. 18-24). 

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... 

T Given that many amino acids are either neurotransmitters or precursors or antagonists of neutrotransmitters, genetic defects of amino acid metabolism can cause defective neural development and mental retardation. In most such diseases specific intermediates accumulate. For example, a genetic defect in phenylalanine hydroxylase, the first enzyme in the catabolic pathway for phenylalanine (Fig. 18-23), is responsible for the disease phenylketonuria (PKU), the most common cause of elevated levels of phenylalanine (hyperphenylalaninemia). 

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

FIGURE 18-25 Alternative pathways for catabolism of phenylalanine in phenylketonuria. In PKU, phenylpyruvate accumulates in the tissues, blood, and urine. The urine may also contain phenylacetate and phenyllactate. 

Fumarate is hydrated to malate in a freely reversible reaction cat alyzed by fumarase (also called fumarate hydratase, see Figure 9.6). [Note- Fumarate is also produced by the urea cycle (see p. 251), in purine synthesis (see p. 293), and during catabolism of the amino acids, phenylalanine and tyrosine (see p. 261).]... 

Phenylalanine and tyrosine Hydroxylation of phenylalanine leads to the formation of tyrosine (Figure 20.7). This reaction, catalyzed by phenylalanine hydroxylase, is the first reaction in the catabolism of phenylalanine. Thus, the metabolism of phenyl alanine and tyrosine merge, leading ultimately to the formation of fumarate and acetoacetate. Phenylalanine and tyrosine are, therefore, both glucogenic and ketogenic. 

Leucine, isoleucine, lysine, and tryptophan form acetyl CoA or ace toacetyl CoA directly, without pyruvate serving as an intermediate (through the pyruvate dehydrogenase reaction, see p. 107). As men tioned previously, phenylalanine and tyrosine also give rise to acetoacetate during their catabolism (see Figure 20.7). Therefore, there are a total of six ketogenic amino acids. 

Amino acids whose catabolism yields either acetoacetate or one of its precursors, acetyl CoA or acetoacetyl CoA, are termed ketogenic. Tyrosine, phenylalanine, tryptophan, and isoleucine are both ketogenic and glucogenic. Leucine and lysine are solely ketogenic. 

Catabolism of histidine in most organisms proceeds via an initial elimination of NH3 to form urocanic acid (Eq. 14-44). The absence of the enzyme L-histidine ammonia-lyase (histidase) causes the genetic disease histidinemia 284/285 A similar reaction is catalyzed by the important plant enzyme L-phenylalanine ammonia-lyase. It eliminates -NH3+ along with the pro-S hydrogen in the (3 position of phenylalanine to form frans-cinnamate (Eq. 14-45). Tyrosine is converted to p-coumarate by the same enzyme. Cinnamate and coumarate are formed in higher plants and are converted into a vast array of derivatives (Box 21-E,... 

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...

Microbial Catabolism of Phenylalanine, Tyrosine, and Other Aromatic Compounds... 

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