Aromatic amino acid hydroxylases

AROMATIC AMINO ACID-GLYOXYLATE AMINOTRANSFERASE Aromatic amino acid hydroxylases, PHENYLALANINE HYDROXYLASE TYROSINE HYDROXYLASE TRYPTOPHAN HYDROXYLASE ARRHENIUS CONSTANT... 

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... 

Studies with the macrophage NOS were the first to show that NO synthesis was partially dependent on added Hbiopterin (Kwon et ai, 1989 Stuehr et al., 1990), which is a redox active cofactor utilized by the aromatic amino acid hydroxylases (Nichol et al., 1985). The requirement for H4biopterin has since been expanded to include all NOS isoforms studied to date. Mayer et al. (1990)... 

THE AROMATIC AMINO ACID HYDROXYLASE MECHANISM A PERSPECTIVE FROM COMPUTATIONAL CHEMISTRY... 

Of the three aromatic amino acid hydroxylases, the reaction catalyzed by L-phenylalanine hydroxylase has been subjected to mechanistic scrutiny most often (B-71MI11003, B-74MH1005, B-76MI11006). Of a number of isomeric dihydrobiopterins that are possible, it is the p-quinonoid dihydrobiopterin (20) that is the coenzyme-derived product in the reaction catalyzed by this enzyme (Scheme 7) (B-71MIH003). (20) is reduced back to (19) by an... 

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. 

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]. 

BH4 is converted to 4a-hydroxytetrahydrobiopterin (95) with incorporating one atom of dioxygen in the C(4a) position of pterin by the hydroxylation reaction catalyzed by aromatic amino acid hydroxylases (Scheme 32). The formation of 95 was observed in the reaction catalyzed by all three hydroxylases using UV spectroscopy [151]. Dehydration of 95 was carried out by the action... 

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