Hydroxylation reactions phenylalanine

Peroxidases have also been utilized for preparative-scale oxidations of aromatic hydrocarbons. Procedures have been optimized for hydroxylation of l-tyrosine, D-(-)-p-hydroxyphenylglycine, and L-phenylalanine by oxygen, di-hydroxyfumaric acid, and horseradish peroxidase (89). Lactoperoxidase from bovine milk and yeast cytochrome c peroxidase will also catalyze such hydroxylation reactions (89). 

Figure 9-6. Synthesis of tyrosine from phenylalanine. Hydroxylation of phenylalanine to tyrosine is one of several reactions in the body that require tetrahydrobiopterin as a cofactor to provide electrons and hydrogen as reducing equivalents.

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. 

The metabolism of phenylalanine will now be considered in some detail, as two inborn errors of metabolism are known that affect this pathway. Phenylalanine is first hydroxylated by phenylalanine hydroxylase to form another aromatic amino acid tyrosine (Fig. 8). The coenzyme for this reaction is the reductant tetrahydrobiopterin which is oxidized to dihydrobiopterin. Phenylalanine hydroxylase is classified as a monooxygenase as one of the atoms of 02 appears in the product and the other in HzO. The tyrosine is then trans-aminated to p-hydroxyphenylpyruvate, which is in turn converted into homogentisate by p-hydroxyphenylpyruvate hydroxylase. This hydroxylase is an example of a dioxygenase, as both atoms of 02 become incorporated into the product (Fig. 8). The homogentisate is then cleaved by homogentisate oxidase, another dioxygenase, before fumarate and acetoacetate are produced... 

A schematic representation of the phenylalanine hydroxylation reaction is shown in Figure 19-2. PAH reduces and cleaves molecular oxygen into two hydroxyl groups. One of the hydroxyl functional groups is found in the reaction product tyrosine, while the other oxygen atom is used to modify the reaction cofactor BII4, creating the pterin 4a-carbinolamine. 

The reductant here is tetrahydrohiopterin, an electron carrier that has not been previously discussed and is derived from the cofactor biopterin. Because biopterin is synthesized in the body, it is not a vitamin. Tetrahydrohiopterin is initially formed by the reduction of dihydrobiopterin by NADPH in a reaction catalyzed by dihydrofolate reductase (Figure 23.28). NADH reduces the quinonoid form of dihydrobiopterin produced in the hydroxylation of phenylalanine back to tetrahydrohiopterin in a reaction catalyzed by dihydropteridine reductase. The sum of the reactions catalyzed by phenylalanine hydroxylase and dihydropteridine reductase is... 

Essential amino acids are indispensable in that the body is incapable of replacing them simply by using its own synthesizing facilities they must either be supplied in adequate quantities from outside or generated by degradation of body proteins. Non-essential amino acids are synthesized in the liver, muscles, kidneys and intestine. Only the synthesis of arginine from ornithine and the hydroxylation of phenylalanine to tyrosine are liver-specific reactions. 

Tryptophan is hydroxylated in a tetrahydrobiopterin-requiring reaction similar to the hydroxylation of phenylalanine. The product, 5-hydroxy-tryptophan, is decarboxylated to form serotonin. 

Phenylalanine can be hydroxylated to form tyrosine, which subsequently can be hydroxylated to form dopa (3,4-dihydroxyphenylalanine). Both hydroxylation reactions are catalyzed by mixed-function oxidases, which require tetrahydrobiopterin as a cofactor. 

C. Tetrahydrobiopterin is involved in hydroxylation reactions that occur in phenylalanine to tyrosine, tyrosine to dopamine, and tryptophan to serotonin, but not in the conversion of dopa to melanin. 

Both tyrosine and tryptophan hydroxylases belong to a small family of monooxygenases, that also includes phenylalanine hydroxylase all three enzymes require tetrahydro-biopterin as a substrate to drive the hydroxylation reaction." Deficiencies in the enzymes responsible for formation and recycling of tetrahydrobiopterin result in variant forms of phenylketonuria and hyperphenylalaninemia characterized by low levels of monoamine neurotransmitters and severe neurological abnormalities. "... 

Structure of folic acid showing its components. The numbered part participates in one-carbon transfer reactions. In nature, folate occurs largely as polyglutamyl derivatives in which the glutamate residues are attached by isopeptide linkages via the y-carboxyl group. The pteridine ring structure is also present in tetrahydrobiopterin, a coenzyme in the hydroxylation of phenylalanine, tyrosine, and tryptophan (Chapter 17). 

The answer is e. (Murray, pp 307-346. Scriver, pp 1667—1724. Sack, pp 121-138. Wilson, pp 287—3177) In humans, tyrosine can be formed by the hydroxylation of phenylalanine. This reaction is catalyzed by the enzyme phenylalanine hydroxylase. A deficiency of phenylalanine hydroxylase results in the disease called phenylketonuria [PKU(261600)]. In this disease it is usually the accumulation of phenylalanine and its metabolites rather than the lack of tyrosine that is the cause of the severe mental retardation ultimately seen. Once formed, tyrosine is the precursor of many important signal molecules. Catalyzed by tyrosine hydroxylase, tyrosine is hydroxylated to form L-dihydroxyphenylalanine (dopa), which in turn is decarboxylated to form dopamine in the presence of dopa decarboxylase. Then, norepinephrine and finally epinephrine are formed from dopamine. All of these are signal molecules to some degree. Dopa and inhibitors of dopa decarboxylase are used in the treatment of Parkinson s disease, a neurologic disorder. Norepinephrine is a transmitter at smooth-muscle junctions innervated by sympathetic nerve libers. Epinephrine and dopamine are catecholamine transmitters synthesized in sympathetic nerve terminals and in the adrenal gland. Tyrosine is also the precursor of thyroxine, the major thyroid hormone, and melanin, a skin pigment. 

Hydroxylation of phenylalanine is a typical reaction of hydroxyl radicals, in which the resultant tyrosine is hydroxylated at the ortho- meta- or paraposition, and produces three isomeric tyrosines, u-tyrosine (2-hydroxyphenylalanine), m-tyrosine (3-hydroxyphenylalanine), and p-tyrosine (4-hydroxyphenylala-nine) [108], shown in Scheme 15. This reaction has been used extensively to trap hydroxyl radicals in vivo, and thereby facilitate their measurement [108]. 

Nitryl chloride, formed from peroxynitrite and HOCl is also capable of nitrating, chlorinating and hydroxylating tyrosine [ 116]. However, the reduction of nitrotyrosine by hypohalous acid has also been demonstrated. Several recent studies indicate that peroxynitrite can lead to generation of both nitrated and hydroxylated phenylalanine residues. The formation of 3-nitrotyrosine and 4-nitrophenylalanine was favoured over the hydroxylation reaction when phenylalanine was treated with peroxy nitrite in vitro [117]. 

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. 

The reaction catalyzed by phenylalanine-4-monooxygenase is irreversible. The electrons required for the hydroxylation of phenylalanine are carried to O, from NADPH by tetrahydrobiopterin. 

Oxidative cleavage of the O-alkyl linkage in glycerolipids is catalyzed by a microsomal tetrahydropteridine (Pte-H4)-dependent alkyl monooxygenase (Fig. 12) (T.-C. Lee, 1981). The required cofactor, Pte H4, is regenerated from Pte-Hj by an NADPH-linked pteridine reductase, a cytosolic enzyme. Oxidative attack on the ether bond in lipids is similar to the enzymatic mechanism described for the hydroxylation of phenylalanine. Fatty aldehydes produced via the cleavage reaction can be either oxidized to the corresponding acid or reduced to the alcohol by appropriate enzymes. 

The biosynthesis of heterocyclic and/or aromatic rings from noncyclic precursors requires complex chemical reactions. Most of these reactions (except the hydroxylation of phenylalanine to tyrosine) have been lost through evolution in animal metabolism. 

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