Enzyme monoxygenase

Tyrosine hydroxylase (TH) is an enzyme that catalyzes the hydroxylation of tyrosine to 3,4-dihydroxypheny-lalanine in the brain and adrenal glands. TH is the rate-limiting enzyme in the biosynthesis of dopamine. This non-heme iron-dependent monoxygenase requires the presence of the cofactor tetrahydrobiopterin to maintain the metal in its ferrous state. 

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. 

ZHAO, Y., CHRISTENSEN, S.K, FANKHAUSER, C., CASHMAN, J.R., COHEN, J.D., WEIGEL, D., CHORY, J., A role for flavin monoxygenase-like enzymes in auxin biosynthesis, Science, 2001, 291, 306-309. 

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.

Cashman JR. Flavin-containing monoxygenase. In Ioannides C, ed. Enzyme Systems that Metabolise Drugs and Other Xenobiotics. Chichester John Wiley and Sons, 2002 67-93. 

Tyrosinase is both an oxidase and a hydroxylase. Some other copper enzymes have only a hydroxylase function. One of the best understood of these is the peptidylglycine a-hydroxylating monoxygenase, which catalyzes the first step of the reaction of Eq. 10-11. The enzyme is a colorless two-copper protein but the copper atoms are 1.1 nm apart and do not form a binuclear center.570 Ascorbate is an essential cosubstrate, with two molecules being oxidized to the semidehydro-ascorbate radical as both coppers are reduced to Cu(I). A ternary complex of reduced enzyme, peptide, and 02 is formed and reacts to give the hydroxylated product.570 A related two-copper enzyme is dopamine (J-monooxygenase, which utilizes 02 and ascorbate to hydroxylate dopamine to noradrenaline (Chapter 25).571/572 These and other types of hydroxylases are compared in Chapter 18. 

Figure 18-19 The ammonia oxidation system of the bacterium Nitrosomonas. Oxidation of ammonium ion (as free NH3) according to Eq. 18-17 is catalyzed hy two enzymes. The location of ammonia monooxygenase (step a) is uncertain but hydroxylamine oxidoreductase (step b) is periplas-mic. The membrane components resemble complexes I, III, and IV of the mitochondrial respiratory chain (Fig. 18-5) and are assumed to have similar proton pumps. Solid green lines trace the flow of electrons in the energy-producing reactions. This includes flow of electrons to the ammonia monoxygenase. Complexes HI and IV pump protons out but complex I catalyzes reverse electron transport for a fraction of the electrons from hydroxylamine oxidoreductase to NAD+. Modified from Blaut and Gottschalk.315...

Cell-free extracts of N. europaea will only oxidize hydroxylamine, probably because the enzyme responsible for the initial oxidation of ammonia is associated with the membrane and so is lost on disruption of the cell. This enzyme, ammonia monooxygenase, also catalyzes the oxidation of methane.1540 While the methane has a higher Km value for oxidation by ammonia monooxygenase than for oxidation by a methane monooxygenase, it is still possible that both processes would be catalyzed by ammonia monoxygenase in the natural environment. 

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]. 

The nasal route of drug delivery avoids the liver first-pass effect, but the pseudo-first-pass effect owing to nasal metabolism of drugs is still a concern. Many enzymes such as carboxylesterase, aldehyde dehydrogenase, glutathione transferases, UDP-glucoronyl transferase, epoxide hydrolases, CYP-dependent monoxygenases, exo- and endopeptidases and proteases are present in the nasal mucosa.106 108,110,116 CYP enzymes are present abundantly in the olfactory epithelium.107,110... 

Oxidation is by far the most important Phase I metabolic reaction. One of the main enzyme systems involved in the oxidation of xenobiotics appears to be the so called mixed function oxidases or monooxygenases, which are found mainly in the smooth endoplasmic reticulum of the liver but also occur, to a lesser extent, in other tissues. These enzymes tend to be nonspecific, catalysing the metabolism of a wide variety of compounds (Table 9.2). Two common mixed function oxidase systems are the cytochrome P-450 (CYP-450) and the flavin monoxygenase (FMO) systems (Appendix 12). The overall oxidations of these systems take place in a series of oxidative and reductive steps, each step being catalysed by a specific enzyme. Many of these steps require the presence of molecular oxygen and either NADH or NADPH as co-enzymes. 

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. 

Type II copper(II) sites are present in mononuclear copper enzymes such as dioxygenases, monoxygenases, nitrite reductases, and nonblue oxidases. The high molecular weight of most of these enzymes and the unfavorable electron relaxation time of copper ion in the oxidized form have up to now precluded the application of NMR spectroscopy. 

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)... 

Flavin-containing monoxygenase oxidizes a variety of xenobiotics that contain nucleophilic nitrogen, sulfur, and phosphorus atoms. An early assay for this enzyme was published by Cashman and Proudfoot (1988). A more recent assay, developed by Kawaji et al. (1993) uses benzydamine, a nonsteroidal anti-inflammatory drug, as a substrate. The benzydamine N-oxide formed is fluorescent. 

The mechanism proposed for the 4-electron reduction of dioxygen to water by cytochrome c oxidase may be similar to that proposed for the monoxygenase enzymes, the cytochromes P450, as well as the peroxidases and catalase, all of which are discussed iu Sectiou 9 below. DFT calculations of the mechanism of the bond cleavage step, specifically addressing CcO, have been reported and an excellent overview provided. ... 

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