Phenylalanine hydroxylase mechanism

Qf-receptor blocking agent, 1, 176 Phenylalanine hydroxylase in tyrosine synthesis from phenylalanine, 1, 261 L-Phenylalanine hydroxylase mechanism, 1, 261 Phenyl azide formation... 

Tire tetrahydrobiopterin formed in this reaction is similar in structure to a reduced flavin. The mechanism of its interaction with 02 could reasonably be the same as that of 4-hydroxybenzoate hydroxylase. However, phenylalanine hydroxylase, which catalyzes the formation of tyrosine (Eq. 18-45), a dimer of 451-residue subunits, contains one Fe per subunit,113 313i whereas flavin monooxygenases are devoid of iron. Tyrosine hydroxylase416 193 and tryptophan hydroxylase420 have very similar properties. All three enzymes contain regulatory, catalytic, and tetramerization domains as well as a common Fe-binding motif in their active sites.413 421 4213... 

Such an attack could lead in step a either to an epoxide (arene oxide) or directly to a carbocation as shown in Eq. 18-47. Arene oxides can be converted, via the carbocation step b, to end products in which the NIH shift has occurred.438 The fact that phenylalanine hydroxylase also catalyzes the conversion of the special substrate shown in Eq. 18-48 to a stable epoxide, which cannot readily undergo ring opening, also supports this mechanism. 

Figure 19-2. Aromatic amino acid hydroxylase reaction. Aromatic amino acids are hy-droxylated by a common mechanism catalyzed by a family of hydroxylases.The enzyme family consists of phenylalanine hydroxylase, tyrosine hydroxylase, and tryptophan hydroxylase. In addition to substrate, all three enzymes require molecular oxygen and the cofactor tetrahydrobiopterin.Tetrahydrobiopterin is consumed in this reaction and converted into pterin 4cx-carbinolamine. DOPA, dihydroxyphenylalanine.

Andersen OA, Flatmark T, Hough E Crystal structure of the ternary complex of the catalytic domain of human phenylalanine hydroxylase with tetrahydrobiopterin and 3-(2-thienyl)-L-alanine, and its implications for the mechanism of catal-... 

Teigen K, Frpystein NA, Martinez A The structural basis of the recognition of phenylalanine and pterin cofactors by phenylalanine hydroxylase implications for the catalytic mechanism. / Mol Biol 294 807-823,1999. 

Phenylketonuria, caused by a deficiency of phenylalanine hydroxylase, is one of the most common genetic diseases associated with amino acid metabolism. If this condition is not identified and treated immediately after birth, mental retardation and other forms of irreversible brain damage occur. This damage results mostly from the accumulation of phenylalanine. (The actual mechanism of the damage is not understood.) When it is present in excess, phenylalanine undergoes transamination to form phenylpyruvate, which is also converted to phenyllactate and phenyl-acetate. Large amounts of these molecules are excreted in the urine. Phenylacetate gives the urine its characteristic musty odor. Phenylketonuria is treated with a low-phenylalanine diet. 

Tetrahydrobiopterin (BPH4) is the natural cofactor required for the mammalian aromatic amino acid monooxygenases phenylalanine, tyrosine and tryptophan hydroxylase [4,89]. During the course of the reaction catalyzed by these enzymes, a molecule of oxygen is cleaved in order to hydroxylate the respective amino acid substrate. The remaining atom of oxygen is reduced to water at the expense of the cofactor, which is oxidized to the quinonoid form. Despite the many studies on the pterin-dependent hydroxylases, their precise mechanism of action is not well understood. This discussion will focus on mammalian phenylalanine hydroxylase (PAH), which has been favored for investigation due to its relative stability and ease of... 

The pathway for the synthesis of serotonin from tryptophan is very similar to the pathway for the synthesis of norepinephrine from tyrosine (Fig. 48.7). The first enzyme of the pathway, tryptophan hydroxylase, uses an enzymic mechanism similar to that of tyrosine and phenylalanine hydroxylase and requires BH4 to hydroxylate the ring structure of tryptophan. The second step of the pathway is a decarboxylation reaction... 

The discovery of the NIH Shift has provided new insights into the mechanisms of aromatic hydroxylation and a new criterion for studying model hydroxylating systems. It is also important in studies on drug metabolism and has led to the development of new enzyme assays for at least two important hydroxylases, phenylalanine hydroxylase (JO) and tryptophan hydroxylase (23). 

The rapid-quench method [78] was used in Ref. 83 to analyze the mechanism of a bacterial phenylalanine hydroxylase, a mononuclear nonheme iron protein that uses tetrahydropterin as the source of the two electrons needed to activate O2 for the hydroxylation of phenylalanine to tyrosine. Mossbauer spectra of samples prepared by freeze-quenching the reaction of the complex enzyme— Fe(ll)-phenylalanine-6-methyltetrahydropterin with O2 revealed the accumulation of an intermediate at short reaction times (20-100 ms). The Mossbauer parameters of the intermediate (3 = 0.28mms, A q= l.26mms ) suggested it to be a high-spin Fe(IV) complex similar to those that have previously been detected in the reactions of other mononuclear Fe(ll) hydroxylases. 

Sidney Velick to amino acid metabolism. By 19561 had become heavily involved in hydroxyl-ation mechanisms, having discovered tryptophan hydroxylase, phenylalanine hydroxylase, and tyrosine hydroxylase and having purified and studied the last two enzymes. 

Phenylalanine hydroxylase is typical of this class of enzymes and has perhaps been most widely studied. The conversion of L-phenylalanine to L-tyrosine, which this enzyme catalyses, serves a dual role in the metabolism of higher organisms. It appears not only to be an obligatory step in the catabolism of L-phenylalanine to carbon dioxide but it also provides an endogenous source of the amino acid L-tyrosine and its metabolites. Some insight into the mechanism of this and other enzyme catalysed hydroxylations and the nature of the oxygen species involved has been derived from studies of model reactions and from studies with substrate analogues. 

There is a possibility that a couple of nonheme monooxygenases other than methane monooxygenase possess paired iron centers, but most of the iron proteins are suggested to contain a monomeric iron site. These include tyrosine hydroxylase [13-15] phenylalanine hydroxylase [16, 17], and isopenicillin N synthase [18,19]. Unfortunately, the active site structures of this class of enzymes have not been elucidated to date. Neither have the reaction mechanisms of these understood (recently, the crystal structure of isopenicillin N synthase has been reported) [20]. The function of this enzyme is not hydroxylation reaction but catalyzes the cyclization of L-5-(a-aminoadipoyl)-L-cysteinyl-D-valine to afford isopenicillin, while the catalytic reaction of this enzyme is assumed to include the reductive activation of dioxygen which affords water and high valent 0x0 iron species as... 

Reaction 1, the substitution of a hydroxyl group for hydrogen in the para position, involves an enzyme, phenylalanine hydroxylase (EC 1.14.3.1) acting with tetrahydrobiopterin and molecular oxygen to yield tyrosine, quinonoid dihydrobiopterin and water [44]. Catalase and another, unidentified, protein are necessary for full enzymic activity—the reaction mechanism and the structure of phenylalanine hydroxylase are not yet fully understood. A second enzyme, dihydropteridine reductase, catalyses the reduction by NADH of the quinonoid dihydrobiopterin to tetrahydrobiopterin. 

The process occurring here is reminiscent of the N.I.H. shift, which is well known to occur in iron hydroxylases such as cytochrome P-450 and mammalian PAH [1,167], For example, action of PAH on [4-3H]phenylalanine produces >90% [3-3H]tyrosine. Here, a presumed electrophilic iron-oxy species produces a carbonium ion intermediate from which a 1,2-shift occurs, giving a resonance stabilized cation rearomatization through loss of H+ (or 3H+) gives the observed product as a result of a heavy atom isotope effect. Thus, it appears that the N.I.H. shift mechanism for copper has been discovered for a chemical model system prior to its observation in proteins. 

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