Aromatic amino acid hydroxylases, and

Affinities between NOSs and BH4 are stronger than those between aromatic amino acid hydroxylases and BH4, so the purified NOS from animal tissues still contain 0.2-0.5 BH4 molecules per heme moiety [128]. BH4 tightly binds to endothelial and neural NOSs with dissociation constants in the nanomolar range, and this binding is reported to stabilize the dimeric structure of NOS [129-131], whereas aromatic amino acid hydroxylases do not have BH4 in the proteins. BH4 functions as a one electron donor to a heme-dioxy enzyme intermediate. The BH4 radical remains bound in NOS and is subsequently reduced back to BH4 by an electron provided by the NOS reductase domain [128]. 

PTPS (6-Pyruvoyl Tetmhydropterin Synthase). 6-Pyruvoyl tetrahy-dropterin synthase catalyzes formation of tetrahydrobiopterin biosynthesis. Tetrahydrobiopterin is a cofactor for several important enzymes, such as aromatic amino acid hydroxylases and nitric oxide synthase (57). H. pylori protein HPAG1 0913 shares homology with members of the protein domain family PTPS. H. pylori protein shares poor sequence identity of 14% with the PTPS profile at an E-value of 10 10 and covers about 95% of the length of the profile. Fold recognition results also confirm the relationship between H. pylori protein and the PTPS protein domain family. A fold recognition algorithm ensures fitness of the H. pylori protein sequence on the three-dimensional structure of PTPS from... 

Youdim, M.B.H. ed. (1979) Aromatic Amino Acid Hydroxylase and Mental Disease, John Wiley, New York, p. 340. 

The tetrahydropterin-dithioiene iigand generated considerabie excitement and speculation. It was unique in biochemistry, and is unusual in chemistry. While dithiolenes were well-known ligands for molybdenum and other metals, this was the first time a dithioiene was proposed to play a role in biochemistry. On the other hand, tetrahydropterins were already known molecules in biochemical processes, such as the tetrahydrobiop-terin cofactor used by aromatic amino acid hydroxylases and tetrahydro-folate in Cl transfer in methionine synthesis (Figure 2.3). Certainly, this was the first time a pterin was found to be in combination with a dithioiene an where in chemistry. 

Blau, K. (1979), Phenylalanine hydroxylase deficiency Biochemical, physiological, and clinical aspects of phenylketonuria and related phenylalaninaemias. In Aromatic Amino Acid Hydroxylases and Mental Disease (ed. M.B.H. Youdim), John Wiley Sons Ltd., London, Chapter 3, pp. 77-139. 

The first step of 5-HT biosynthesis is catalyzed by the rate-hmiting enzyme tryptophan hydroxylase (TPH). Two isoforms, TPHl and TPH2, have been identified in the periphery and in 5-HT neurons, respectively. Both isoforms are members of the aromatic amino acid hydroxylase gene family, together with phenylalanine (PAH) and tyrosine hydroxylases (TH). The human TPHl gene located on chromosome llplS.l, spans a region of 30 kb, contains at... 

One of the best characterized physiological functions of (6R)-tetrahydrobio-pterin (BH4, 43) is the action as a cofactor for aromatic amino acid hydroxylases (Scheme 28). There are three types of aromatic amino acid hydroxylases phenylalanine hydroxylase [PAH phenylalanine monooxygenase (EC 1.14.16.1)], tyrosine hydroxylase [TH tyrosine monooxygenase (EC 1.14.16.2)] and tryptophan hydroxylase [TPH tryptophan monooxygenase (EC 1.14.16.4)]. PAH converts L-phenylalanine (125) to L-tyrosine (126), a reaction important for the catabolism of excess phenylalanine taken from the diet. TH and TPH catalyze the first step in the biosyntheses of catecholamines and serotonin, respectively. Catecholamines, i.e., dopamine, noradrenaline and adrenaline, and serotonin, are important neurotransmitters and hormones. TH hydroxylates L-tyrosine (126) to form l-DOPA (3,4-dihydroxyphenylalanine, 127), and TPH catalyzes the hydroxylation of L-tryptophan (128) to 5-hydroxytryptophan (129). The hydroxylated products, 127 and 129, are decarboxylated by the action of aromatic amino acid decarboxylase to dopamine (130) and serotonin (131), respectively. 

PAH, TH and TPH are highly homologous enzymes. These enzymes catalyze a hydroxylation reaction of aromatic amino acids that requires reduced pterin cofactor 43, molecular oxygen, and iron (Scheme 28). Iron is present at the active sites of the enzymes. Ferrous iron (Fe(II)) is essential for the catalysis, although, the iron was found to be in the ferric form (Fe(III)) when the enzymes were purified from tissues or cells. The ferric iron at the active site of the enzymes was found to be reduced to the ferrous form by BH4 [125]. Thus, BH4 serves a bi-functional role for aromatic amino acid hydroxylases one is the reduction of iron at the active sites from the ferric form to the ferrous form and the other is an electron donor for the hydroxylation reaction. 

Although regulation is unique for each member of the aromatic amino acid hydroxylase family, the catalytic mechanism and cofactor requirements for members of the family are identical. During the reactions of all three enzymes, the dioxygen molecule is cleaved and incorporated as a hydroxyl group into both the aromatic amino acid and BH4. Each enzyme in the family displays its own unique substrate specificity profile. Two interesting questions about this enzyme family relate to the actual hydroxylation mechanism and how enzyme activity is altered by changes in BH4 levels. Problems in any one of these hydroxylation systems can arise from either an inadequate supply of the BH4 cofactor or a defect in the enzyme or its expression. 

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.

Figure 10.10. Role of tetrahydrobiopterin in aromatic amino acid hydroxylases. Phenylalanine hydroxylase, EC 1.14.16.1 tyrosine hydroxylase, EC 1.14.16.2 tryptophan hydroxylase, EC 1.14.16.4 and dihydrobiopterin reductase (dihydropteridine reductase), EC 1.6.99.7.

Fitzpatrick PF (1998) The aromatic amino acid hydroxylases. Advances in Enzymology and Related Areas of Molecular Biology 74B, 235-94. 

The Aromatic Amino Acid Hydroxylase Mechanism A Perspective from Computational Chemistry Elaine Olsson, Knut Teigen, Aurora Martinez and Vidar R. Jensen... 

An increase in the aromatic amino acids phenylalanine and tyrosine occurs due to the lowered hepatic uptake of enterally released amino acids and their restricted catabolism in the liver. They are also released to a greater extent from the muscles in cases of cirrhosis with catabolism. Furthermore, phenylalanine hydroxylase... 

The fourth class, the pterin-dependent hydroxylases, includes the aromatic amino acid hydroxylases, which use tetrahydrobiopterin as cofactor for the hydroxylation of Phe, Tyr, and Trp. The latter two hydroxylases catalyse the rate-limiting steps in the biosynthesis of the neurotransmitters/hormones dopamine/noradreanalme/ adrenaline and serotonin, respectively. 

Epinephrine (adrenaline) (Figure 32-7) is synthesized from tyrosine by conversion of tyrosine to 3,4-dihydro-xyphenylalanine (dopa) by tyrosine-3-monooxygenase (tyrosine hydroxylase) in the cytosol. The mixed-function oxidase requires molecular oxygen and tetrahydro-biopterin, which is produced from dihydrobiopterin by NADPH-dependent dihydrofolate reductase. In the reaction, tetrahydrobiopterin is oxidized to dihydrobiopterin, which is reduced to the tetrahydro form by NADH-dependent dihydropteridine reductase. These reactions are similar to the hydroxylations of aromatic amino acids (phenylalanine and tryptophan), in which an obligatory biopterin electron donor system is used (Chapter 17). 

Phenylalanine hydroxylase (EC 1.14.16.1) is a member of the tetrahydrobi-opterin-dependent aromatic-amino acid hydroxylases, which also include tryptophane and tyrosine hydroxylase [183-186]. Phenylalanine hydroxylase from Chromobakterium violaceum is a monomeric enzyme of approximately 35 kD molecular mass and 296 amino acid residues. It contains a single copper ion (Cu2+) in its active center [187]. 

Regeneration of BH4 is an essential part of the phenylalanine hydroxylating system (see also Cofactor functions ). During the catalytic event of aromatic amino acid hydroxylases, molecular oxygen is transferred to the corresponding amino acid and BH4 is oxidized to BH4-4a-carbinolamine (Figure 15). " °° Two enzymes are... 

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