Monoxygenase activity

The transformation of A-nitrosodimethylamine by Pseudomonas mendocina KRl that has tolu-ene-4-monoxygenase activity was initiated by monooxygenation to the A-nitro compound, which produced A-nitromethylamine and formaldehyde, presumably by hydroxylation of the methyl group (Fournier et al. 2006). 

Pinto, A., Abraham, N. G., Mullane, K. M., Cytochrome P450-dependent monoxygenase activity and endothelial-dependent relaxations induced by arachidonic acid. J. Pharmucol. Exp. Ther. 236 (1986),... 

Dahlstrom-King L, Couture J, Plaa GL. 1992. Influence of agents affecting monoxygenase activity on taurolithocholic acid-induced cholestasis. Toxicol Lett 63 243-252. 

The selective catalytic epoxidation of alkenes has become the most important reaction catalyzed by heme proteins in organic synthesis. As described above, the monoxygenase activity of a heme peroxidase is restricted to CPO due to the open substrate access of the ferryl subunit for this enzyme. HRP catalyzes epoxidations only after mutagenetic variations, as shown for the substrate trans-P-methylstyrene [234]. An exception of this rule is the regioselective epoxidation of (T.TJ-piperylpiperidide, which is successfully catalyzed by native HRP [265]. 

Figure 9.1 (A) Chromatogram of 384 pmol of Dopa (1.03 min) and 1.92 nmol of l-tyrosine (1.55 min). (B) and (C) Chromatograms of an acidic ethanol extract of an incubation mixture in the assay of tyrosine 3-monoxygenase activity of bovine adrenal medulla microsomes. (B) Zero-time control with a single peak of L-tyrosine (1.55 min). (C) The formation of Dopa (1.03 min) following a reaction period of 20 minutes. Volumes of 20 fiL of the diluted (twice) incubation mixture were injected into the liquid chromatograph. Detection wavelengths for excitation and emission were 281 and 314 nm, respectively. (From Haavik and Flatmark, 1980.)...

Testosterone has been used as a model substrate for different cytochrome P450 monoxygenase activities. As a result of these oxidation reactions, multiple chemically related products are formed. In the assay developed for this activity, seven products are distinguished. 

Nebert, D.W., J.R. Robinson, and H. Kon (1973). Genetically mediated differences in monoxygenase activities and spin state of cytochrome P450 iron from rabbit, rat, and mouse liver. J. Biol. Chem. 248, 7637-7647. 

Efforts to solubilize and isolate the P-450 monoxygenases from the microsomal membranes have been hampered by the facile conversion to a catalytically inactive form, cytochrome P-420. In 1968, however, a successful solubilization of catalytically active P-450 from hepatic microsomes was reported.30, 31 he solubilized system, which effected w-hydroxylation of fatty acids, required three coiq>onents for catalytic activity cytochrome P-450, NADPH-cytochrome c reductase, and a heat-stable, chloroform-soluble factor, the active component of which was identified as the microsomal lipid phosphatidylcholine.32 Further studies with this solubilized and reconstituted system have Indicated that the cytochrome P-450 and P-448 fractions have different catalytic activities 33-35 and that the terminal oxidase activity resides in the b-type cytochrome (P-450 or P-448) fraction rather than the cytochrome c reductase or lipid fractions.33, 36, 37 xhe cytochrome P-450 and P-448 were found to coiq>ete for reductase when present together.38 Cytochrome bs did not appear to be an obligatory component of the reconstituted systera.39 Cytochrome P-450 and P-448 fractions from rat liver were found to contain high levels of an epoxide hydrase, which can convert intermediate oxides to vicinal diols. Further purification has afforded fractions relatively free of cross-contamination of monoxygenase and hydrase enzymes. Recently, differential solubilization of monoxygenase activity towards a Type I substrate (naphthalene) and a Type II substrate (aniline) was... 

The development and reports of methods for colorless chlorophyll derivative (RCCs, FCCs, and NCCs) analysis are relatively recent and the structures of the compounds are being elucidated by deduction from their chromatographic behaviors, spectral characteristics (UV-Vis absorbance spectra), mass spectrometry, and nuclear magnetic resonance analysis. The main obstacle is that these compounds do not accumulate in appreciable quantities in situ and, moreover, there are no standards for them. The determination of the enzymatic activities of red chlorophyll catabolite reductase (RCCR) and pheophorbide a monoxygenase (PAO) also helps to monitor the appearance of colorless derivatives since they are the key enzymes responsible for the loss of green color. ... 

Dioxygen reduction (oxidase activity) and activation for incorporation into organic substrates are catalysed by a number of mononuclear non-haem iron enzymes. We will first consider the intramolecular dioxygenases, in which both atoms of oxygen are introduced into the substrate, then the monoxygenases (in which we choose to include the pterin-dependent hydroxylases), the large family of a-hetoacid-dependent enzymes, and finally isopenicillin N-synthase. 

Evans, J.P., Ahn, K. and Klinman, J.P. (2003). Evidence that dioxygen and substrate activation are tightly coupled in dopamine (3-monoxygenase. J. Biol. Chem. 278, 49691-49698... 

Chen, P. and Solomon, E.I. (2004). Oxygen activation by the noncoupled binuclear copper site in peptidylglycine-a-hydroxylating monoxygenase. Reaction mechanism and role of the noncoupled nature of the active site. J. Am. Chem. Soc. 126, 4991-5000... 

Fig. 2. Schematic representation of paclitaxel biosynthesis. Dimethylallyl-diphosphate and isopentenyl-diphosphate are condensed through geranylgeranyl diphosphate synthase activity to render geranylgeranyl-diphosphate (GGPP). GGPP is converted into taxa-4(5), 11 (12)-diene in a reaction catalyzed by the taxane synthase (TS). A series of reactions catalyzed by cytochrome P450 monoxygenases lead to the production of a taxane intermediate that is further converted to baccatin III through enzymes-driven oxidation and oxetane ring formation. The side chain moiety of paclitaxel is derived from L-phenylalanine. Three consecutive arrows mean multiple steps. Ac, acetyl Bz, benzoyl.

Figure 2.6 Schematic representation of the heme group orientation and different substrate accessibility in the heme active sites in monoxygenases and peroxidases.

One particular feature of this mechanism is that the two metals in the active site can provide three electrons, which means that a fourth electron has to come from a different source, in this case a redox active tyrosine residue. A different solution to the same problem has been found in the enzyme methane monoxygenase, which involves a binuclear iron center and will be discussed subsequently. 

Dopamine /3-hydroxylase is a monoxygenase that catalyzes the hydroxylation of dopamine to form norepinephrine. This enzyme is localized in the chromaffin granules of the adrenal medulla and in the storage vesicles of central and peripheral catecholaminergic neurons. Since these compounds are unstable, this activity is often assayed by following the formation of octopamine from tyramine. For example, in the assay developed by Feilchenfeld et al. (1982), the reactant tyramine was separated from the product octopamine by reversed-phase, ion-paired HPLC (/uBondapak C18 using a mobile phase of 17% (v/v)... 

Several biologically active peptides have a C-terminal a-amide structure that is formed by the action of a copper- and ascorbate-dependent monoxygenase on a C-terminal glycine. 

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