Hydroxylases Monooxygenases

Salicylate is an intermediate in the metabolism of PAHs including naphthalene and phen-anthrene, and its degradation involves oxidation to catechol. The hydroxylase (monooxygenase) has been extensively studied (references in White-Stevens and Kamin 1972) and in the presence of an analog that does not serve as a substrate, NADH is oxidized with the production of H2O2 (White-Stevens and Kamin 1972). This uncoupling is characteristic of flavoenzymes and is exemplified also by the chlorophenol hydroxylase from an Azotobacter sp. that is noted later. 

In the metabolism of the aromatic amino acid tryptophan in mammals, two pathways for the formation of pyridine nucleotide coenzyme and in-doleamines are initiated by two well-known oxygenases, tryptophan 2,3-dioxygenase and tryptophan-5-hydroxylase (monooxygenase) (Fig. 2). Kotake and Ito (5) found in 1937 that rabbits fed D-tryptophan excreted D-kynurenine in the urine. Although the tryptophan-cleaving activity in... 

Phenylalanine hydroxylase [39] catalyzes the first step of phenylalanine degradation in mammals. Phenylalanine is converted into tyrosine (Scheme XI.9). The enzyme has one tightly bound, non-heme iron atom per subunit. Tyrosine hydroxylase catalyzes [40] the hydroxylation of tyrosine to produce dihydroxy-phenylalanine (DOPA), the first step in the biosynthesis of catechol-amine neurotransmitters (Scheme XI.9). This enzyme also contains one ferrous iron atom per subunit. These two enzymes, together with tryptophane hydroxylase (Scheme XI.9) [41], constitute a family of tetrahydropterin-dependent aromatic acid hydroxylases (monooxygenases) [42], Other dioxygenases catalyze the hydroxylation ofarenes [43],... 

Microbial hydroxylation is often the first step in degradation of organic compounds. The addition of a hydroxyl group into the toxic organic molecule makes the compound more polar and hence more soluble in water. This in turn makes the compound more biologically reactive. Enzymes catalyzing this reaction are hydroxylases, monooxygenases, or mixed-function oxidases. 

The Regulation of Glycolysis and the Pentose Phosphate Pathway John F. Turner and Donella H. Turner Hydroxylases, Monooxygenases, and Cytochrome P-450 Charles A. West One-Carhon Metabolism Edwin A. Cossins... 

The ECM is influenced by L-ascorbic acid in many different manners (Table 8.2), most importantly by acting as an essential cofactor in the oxidoreduction function by regenerating prosthetic metal ions in several hydroxylases, monooxygenases, and dioxygenases, enzymes that are involved in the synthesis of collagen, carnitine, and neurotransmitters (Arrigoni and DeTullio 2002, Levine 1986, Padh 1991). 

Some of the major enzyme groups that facilitate this transformation are heme-containing MOs of the cytochrome P450 type [111], alkane hydroxylases, xylene monooxygenases, styrene monooxygenases [105], and haloperoxidases [112],... 

Since endosulfan is a cytochrome P450-dependent monooxygenase inducer, the quantification of specific enzyme activities (e.g., aminopyrine-A -demethylase, aniline hydroxylase) may indicate that exposure to endosulfan has occurred (Agarwal et al. 1978). Because numerous chemicals and drugs found at hazardous waste sites and elsewhere also induce hepatic enzymes, these measurements are nonspecific and are not necessarily an indicator solely of endosulfan exposure. However, these enzyme levels can be useful indicators of exposure, together with the detection of endosulfan isomers or the sulfate metabolite in the tissues or excreta. 

Monooxygenases (Mixed-Function Oxidases, Hydroxylases) Incorporate Only One Atom of Molecular Oxygen Into the Substrate... 

Eppink MHM, SA Boeren, J Vervoort, WJH van Berkel (1997) Purification and properties of 4-hydroxyben-zoate 1-hydroxylase (decarboxylating), a novel flavin adenein dinucleotide-dependent monooxygenase from Candida parapilosis CBS604. J Bacteriol 179 6680-6687. 

Subramanian V, CS Vaidyanathan (1984) Anthranilate hydroxylase from Aspergillus niger new type of NADPH-linked nonheme iron monooxygenase. J Bacteriol 160 651-655. 

Hydrocarbon hydroxylases introducing oxygen into nonoxygenated rings Toluene, phenanthrene, pyrene monooxygenases Hydrocarbon epoxidases... 

Zhou N-Y, J Al-Dulayymi, MS Baird, PA Williams (2002) Salicylate 5-hydroxylase from Ralstonia sp. strain U2 a monooxygenase with close relationships to and shared electron transport proteins with naphthalene dioxygenase. / Bacfen o/ 184 1547-1555. 

The alkane hydroxylase belongs to a family of nonheme iron oxygenases. There is some structural similarity between the nucleotide sequence of the integral-membrane alkane hydroxylase and the subunits of the monooxygenase encoded by xylA and xylM in the TOL plasmid that are involved in hydrox-ylation of the methyl groups in toluene and xylene in Pseudomonas putida PaWl (Suzuki et al. 1991). 

Xun L, ER Sandvik (2000) Characterization of 4-hydroxyphenylactate 3-hydroxylase (HpaB) of Escherichia coli as a reduced flavin adenine dinucleotide-utilizing monooxygenase. Appl Environ Microbiol 66 481-486. Zaar A, J Gescher, W Eisenreich, A Bacher, G Fuchs (2004) New enzymes involved in aerobic benzoate metabolism m Azoarcus evansii. Mol Microbiol 54 223-238. 

The degradation of phenol is initiated by hydroxylation using a single-component NAD(P)H-depen-dent monooxygenase encoded by pheA (Nurk et al. 1991). In Bacillus thermoglucosidasius, however, the hydroxylase is a two-component system encoded by pheAl and pheA2, and hydroxylation requires supplementation by the second component with FAD(NADH) (Kirchner et al. 2003). 

Kirchner U, AH Westphal, R Muller, WJH van Berkel (2003) Phenol hydroxylase from Bacillus thermoglucosi-dasius A7, a two-component monooxygenase with a dual role for PAD. J Biol Chem 278 47545-47553. 

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