Aromatic hydroxylase

Tu, S.-C. (1991). Oxygenated flavin intermediates of bacterial luciferase and flavoprotein aromatic hydroxylases enzymology and chemical models. Adv. Oxygenated Processes 3 115-140. 

Prieto MA, A Perez-Aranda, JL Garcia (1993) Characterization of an Escherichia coli aromatic hydroxylase with a broad substrate range. J Bacteriol 175 2162-2167. 

Suske, W. A., M. Held, A. Schmidt, T. Fleischmann, M. G. Wubbolts, and H.-P. E. Kohler, Purification and characterization of 2-hydroxybiphenyl 3-monooxygenase, a novel NADH-dependent, FAD-containing aromatic hydroxylase from Pseudomonas azelaica HBP1 , J. Biol. Chem., 272, 24257-24265 (1997). 

Most aromatic hydroxylases are either cytochrome- or flavin-dependent enzymes the three enzymes that catalyze hydroxylation of the aromatic amino acids phenylalanine, tyrosine, and tryptophan are apparently unique in... 

PHBH is the protype of the flavoprotein aromatic hydroxylases. Each subunit of this dimeric enzyme contains two active sites which, during catalysis, are alternately visited by the isoalloxazine ring of the FAD cofactor (31). Catalysis is iiutiated by reduction of the flavin in the exterior active site. The reduced flavin then moves to the interior active site where the reactions with oxygen occur. A similar conformational flexibility of the FAD cofactor has been observed in the crystal structures of phenol hydroxylase (EC 1.14.13.7) and 3-hydroxybenzoate 4-hydroxylase (EC 1.14.13.23). PHBH obeys the following kinetic mechanism ... 

Ballou DP, Entsch B, Cole LJ. Dynamics involved in catalysis by single-component and two-component flavin-dependent aromatic hydroxylases. Biochem. Biophys. Res. Comm. 2005 338 590-598. 

PHBH in its native structure is a homodimer that contains one FAD per monomer, and it has a monomer molecular weight of 45,000. The active site of each monomer is constructed from the flavin and amino acid residues in that monomer. There is no clear evidence that a dimer is necessary for activity. PHBH is one member of a large family of similar flavoproteins that probably have a common evolutionary descent, based on structural and sequence similarities. This family is often referred to as the one-component aromatic hydroxylases. This aspect distinguishes them from another large and diverse group of flavoprotein hydroxylases, which is the two-component flavin-dependent hydroxylases that use two different proteins to carry out the catalysis of hydroxylation. The latter group, which is described in a separate section, has no common ancestry with the one-component enzymes. 

Some flavin-dependent aromatic hydroxylases and the monooxygenase reactions catalyzed... 

While the majority of aromatic hydroxylases are bacterial catabolic enzymes, several have been discovered in mammals, playing a role in ubiquinone biosynthesis, neurotransmitter chemistry, and cell differentiation. ... 

FMOs are not homologues of the aromatic hydroxylases - they apparently evolved independently to harness... 

The aromatic hydroxylases or mixed-function oxidases are no exception to the above generalization. For maximum activity they require a transition metal ion and an electron donor such as one of the pyridine or flavin nucleotide coenzymes further, they probably utilize molecular oxygen as the source of the hydroxylic oxygen an example is the liver microsomal hydroxylating system (27). As yet there is no comprehensive explanation to cover the mode of action of these enzymes, for on the one hand there are the specific hydroxylases which catalyze such conversions as L-phenylalanine to L-tyrosine (26) or tryptophan to 5-hydroxytrypto-... 

Since the formation of 5-hydroxyindoles is markedly increased in patients and animals with nonhepatic tumors, it would appear that tryptophan hydroxylase must occur in the tumor. Thus it may be that, unlike other aromatic hydroxylases, tryptophan hydroxylase is present in all tissues where serotonin is formed. In this respect it may resemble tyrosine hydroxylase, which converts tyrosine to 3,4-dihydroxyphenylalanine. This reaction occurs in adrenal medulla (Rosenfeld el al., 1958) and in other tissues which form noradrenaline, such as sympathetic nerves and ganglia (Goodall and Kirshner, 1958). 

Metabolism of the Aromatic Amino Adds 4.4.1 AROMATIC HYDROXYLASES... 

An important group of mixed function oxidases which occur in all types of organism and which are critical in the metabolism of the aromatic amino adds and other aromatic substrates are the aromatic hydroxylases. Their mode of action, the overall stoichiometry of which is represented in the sequence below, results in the introduction of a phenolic hydroxyl group in an aromatic ring system and has been the subject of intensive study. Pyridine and flavin nucleotides, cytochromes, metals (Fe, Cu), ascorbate and pteridine derivatives (H2X) may serve as electron donors. Most, but not all, of these en2yme reactions require transition metal ions for full activity. 

Phenylalanine hydroxylase occurs only in mammalian liver (that is, in the rat, guinea-pig, rabbit, d<, chicken, and human) (see also 259). No activity has been observed in (rat) lung, kidney, brain, or muscle. The system is quite speciOc for L-phenylalanine. Tjrro-sine is not formed from n-phenylalanine, nor are the corresponding p-phenols formed from N-acetyl- or N-chloroacetyl-L-phenylalanine, L-phenylalanine ethyl ester, DL-phenylglycine, phenylserine, phenylpyruvic acid, phenylethylamine, benzoic acid, hippuric acid, cinnamic acid, or mandelic acid (768), or from aniline, acetanilide, tryptophan, kynurenine, anthranilic acid, or phenylacetate (557). This specificity is a distinguishing character of the enzyme, which occurs in the same tissue as the nonspecific aromatic hydroxylase described above. 

The structure of PHBH is shown in Figure 8.12. It is an enzyme of 394 amino acids, with a three domain structure composed of an N-terminal "Rossman fold" domain that binds the ADP of the FAD coenzyme, a second domain that binds the para-hydroxybenzoate substrate and, a third, largely helical domain that is involved in dimerization wdth another subunit. The tricyclic isoalloxa-zine ring of the flavin is bound at the interface between the first two major domains, and there is known to be movement of flavin associated with the accommodation of substrate [46]. The structure of PHBH in the presence of NADPH has not been determined, but it is thought that there is only a transient association of the two cofactors to effect flavin reduction after which the NADP+ dissociates to permit substrate binding [46]. Although PHBH itself has not been the focus of studies in applications in synthesis, other Class A FPMO aromatic hydroxylases, such as hydroxybiphenyl monooxygenase from Pseudomonas azelaica have been used for the preparation of hydroxylated compounds. 

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