Reaction monooxygenase

H)2-D3 is a weak agonist and must be modified by hydroxylation at position Cj for full biologic activity. This is accomplished in mitochondria of the renal proximal convoluted tubule by a three-component monooxygenase reaction that requires NADPFl, Mg, molecular oxygen, and at least three enzymes (1) a flavoprotein, renal ferredoxin reductase (2) an iron sulfur protein, renal ferredoxin and (3) cytochrome P450. This system produces l,25(OH)2-D3, which is the most potent namrally occurring metabolite of vitamin D. 

Carotene cleavage enzymes — Two pathways have been described for P-carotene conversion to vitamin A (central and eccentric cleavage pathways) and reviewed recently. The major pathway is the central cleavage catalyzed by a cytosolic enzyme, p-carotene 15,15-oxygenase (BCO EC 1.13.1.21 or EC 1.14.99.36), which cleaves p-carotene at its central double bond (15,15 ) to form retinal. Two enzymatic mechanisms have been proposed (1) a dioxygenase reaction (EC 1.13.11.21) that requires O2 and yields a dioxetane as an intermediate and (2) a monooxygenase reaction (EC 1.14.99.36) that requires two oxygen atoms from two different sources (O2 and H2O) and yields an epoxide as an intermediate. ... 

Robertson JB, JC Spain, JD Haddock, DT Gibson (1992) Oxidation of nitrotoluenes by toluene dioxygenase evidence for a monooxygenase reaction. Appl Environ Microbiol 58 2643-2648. 

Marasco, E. and C. Schmidt-Dannert (2008). Identification of bacterial carotenoid cleavage dioxygenase homologs that cleave the interphenyl u,(J double bond of stilbene derivatives via a monooxygenase reaction. Chembiochem. 9(9) 1450-1461. 

Several successful examples of coupling this regeneration system to synthesis reactions with different electrochemical reactors have been reported, including ADH and monooxygenase reactions [39, 42, 43]. 

While the cytochrome P-450 monooxygenase reaction described in Eq. (1) often involves hydroxylation of carbon, many other reactions are catalyzed by these enzyme systems. These reactions include oxidation of nitrogen and sulfur, epoxidation, dehalogenation, oxidative deamination and desulfuration, oxidative N-, O-, and S-dealkylation, and peroxidative reactions (56). Under anaerobic conditions, the enzyme system will also catalyze reduction of azo, nitro, N-oxide, and epoxide functional groups, and these reductive reactions have been recently reviewed (56, 57). Furthermore, the NADPH-cytochrome P-450 reductase is capable of catalyzing reduction of quinones, quinonimines, nitro-aromatics, azoaromatics, bipyridyliums, and tetrazoliums (58). 

Gorsky LD, Koop DR, Coon MJ. On the stoichiometry of the oxidase and monooxygenase reactions catalyzed by liver microsomal cytochrome P-450. Products of oxygen reduction. J Biol Chem 1984 259(11) 6812-6817. 

Karuzina, I.I. and Archakov, A.I. (1994) Hydrogen peroxide-mediated inactivation of microsomal cytochrome P450 during monooxygenase reactions. Free Radical Biology and Medicine, 17 (6), 557-567. 

Miller, S.M. and Khnman, J.P. (1985). Secondary isotope effects and structure-reactivity correlations in the dopamine heta-monooxygenase reaction evidence for a chemical mechanism. Biochemistry 24, 2114-2127... 

The following is review on the molecular and physical properties of this class of monooxygenases, which are also known as hydroxylases. A typical monooxygenase reaction is the hydroxylation of an alkane to an alcohol which involves a reduced cosubstrate that reduces a second atom within the O2 molecule to form water. Flavin-containing monooxygenases include lysine oxygenase and 4-hydroxybenzoate hydroxylase. Reduced pteri-dines are involved in the phenylalanine hydroxylase and tryptophan hydroxylase reactions. See also Cytochrome P-450... 

The first of two conversions catalyzed by the metalloenzyme tyrosinase is the selective ortho-hydroxylation of a phenol moiety, yielding catechol. Few synthetic systems exist that are capable of selectively performing the same monooxygenase reaction [172-178]. The group of Casella has studied a series of dicopper complexes based on 2,6-disubstituted benzene hgands providing two N3 compartments [179-181] (26a,b) as well as the asymmetrically substituted tripodal amine-based ligand (27) [182]. 

Hydrogen (or electron) donors, coenzymes and electron carriers involved in the monooxygenase reactions are listed in Table 1. In some monooxygenase reactions, the substrate itself serves as an electron donor as well as an oxygen acceptor [Eq.(6)],... 

All the internal monooxygenases that have so far been purified and characterized contain flavin coenzymes. The external hydrogen donors include reduced NAD, reduced NADP, ascorbic acid and sulfhydryl compounds. Cofactors required for the external monooxygenases are flavin, pteridine, copper, nonheme iron and heme as cytochrome P-450. In some monooxygenase reactions, enzymes and/or electron carrier systems other than monooxygenase itself are involved in the transfer of an electron or hydrogen from the external hydrogen donor to the cofactor involved. 

Table 1. Electron donors, electron transport systems and cofactors involved in monooxygenase reaction ...

NO synthases are oxygenases that carry out a two-step oxidation of L-arginine to L-citrulline with production of NO. In the first step, a normal monooxygenase reaction, i -N -hydroxyarginine is formed (Eq. 18-65, step a). In the second step (Eq. 18-65, step b) NO is formed in a three-electron oxidation. In this equation the symbols and + indicate positions of incorporation of labeled 02 atoms in the intermediate and final products. 

The transformation of squalene into lanosterol. The squalene monooxygenase reaction requires 02, NADPH, FAD, phospholipid, and a cytosolic protein. The cyclase reaction has no known cofactor requirements. The reaction proceeds by means of a protonated intermediate that undergoes a concerted series of trans-1,2 shifts of methyl groups and hydride ions to produce lanosterol. 

Thyroid Hormone. Treatment of rats with thyroxin increases hepatic microsomal NADPH oxidation in both male and female rats, with the increase being greater in females. Cytochrome P450 content decreases in the male but not in the female. Hyperthyroidism causes a decrease in gender-dependent monooxygenase reactions and appears to interfere with the ability of androgens to increase the activity of the enzymes responsible. Gender differences are not seen in the response of mice and rabbits to... 

Noncompetitive inhibitors can bind to both the enzyme and enzyme-substrate complex to form either an enzyme-inhibitor complex or an enzyme-inhibitor-substrate complex. The net result is a decrease in Vmax but no change in Km. Metyrapone (Figure 9.6), a well-known inhibitor of monooxygenase reactions, can also, under some circumstances, stimulate metabolism in vitro. In either case the effect is noncompetitive, in that the Km does not change, whereas Vm. does, decreasing in the case of inhibition and increasing in the case of stimulation. 

NOS catalyzes two sequential monooxygenase reactions. First, NOS hy-droxylates L-arginine (132) to generate an enzyme-bound intermediate, N-hydroxy-L-argininc (133). Then, 133 is further oxidized to generate nitric oxide ( NO) and citrulline (134) [128] (Scheme 29). 

Consistent with this monooxygenase reaction stoichiometry, using 1802 in the reaction resulted in labeled phenol product. Also in line with methyl group migration is the observation that placement of four deuterium atoms in the two benzylic positions of the ligand in [Cu2(XYL—Me)]2+ (35b) resulted in retention of deuterium in the phenol product, whereas formation of -formaldehyde [167] indicates that this product originated from the other benzylic carbon in 35b. 

Single Species at Specified pH ill 9.3 Methane Monooxygenase Reaction... 

Table 9.3 Standard Transformed Gibbs Energies (in kJ moE ) of Reactions and Standard Apparent Reduction Potentials (in volts) at 289.15 K, 1 bar, pH 7, and Ionic Strength 0.25 M for Reactions Involved in the Methane Monooxygenase Reaction...

If these reactions were carried out in two galvanic cells in series, the electromotive force would be 0.773 + 1.166 = 1.939 V for a two electron change, and the standard transformed Gibbs energy of the overall monooxygenase reaction would be — 2 ( 1.939) = — 374.13 kJ mol-1, as expected. 

The methane monooxygenase reaction can, in principle, be carried out in two other ways by the enzyme complex that catalyzes it It can be carried out in three half-reactions at three catalytic sites as follows ... 

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