Monoxygenase reactions

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... 

Tetraethyl and tetramethyl lead under oxidative dealkylation metabolize to the highly neurotoxic metabolites, triethyl and trimethyl lead, respectively. In the liver, the reaction is catalyzed by a cytochrome P-450 dependent monoxygenase system (Kimmel et al. 1977). Complete oxidation of alkyl lead to inorganic lead also occurs (Bolanowska 1968). 

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.

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...

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]. 

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. 

HYDROGEN-ATOM TRANSFER REACTIONS Peptidyl- a-hydroxylating monoxygenase... 

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. 

L-lactate and oxalate were also tested with lactate-2-monoxygenase and oxalate decarboxylase and excellent results were obtained. CPG columns were employed in both instances. Good linearity was obtained between 0.005 -1 mM for L-lac-tate [3] and between 0.1-3 mM for oxalate [24]. Similarly, urea was measured with a precision better than 1 % in the linearity range 0.01-200 mM using Jack bean urease. The reaction of urea with ethanol to produce ethylcarbamate is of interest in fermentation monitoring. 

Electrophilic C-H activations can also be effected in water. At first glance, water would appear to be particularly unpromising as a solvent for such reactions. Because of their extremely poor coordinating ability (no fully characterized alkane complex is known [33]) alkanes should not be able to compete with water for coordination sites. Moreover, the intermediate metal-alkyl species would be prone to hydrolytic decomposition. In one respect, however, water is almost an ideal medium for C-H functionalization the O-H bond energy exceeds the corresponding C-H bond energy of even methane. Indeed, the selective oxidation of methane to methanol is carried out by methane monoxygenase in aqueous medium [17]. 

This example demonstrates the ability of zeolites to reproduce the control (typical of enzyme proteins) of both the interactions between active sites and the thermodynamics of reactions at those active sites. The next example demonstrates the zeolite s ability to kinetically control product selectivity in an oxidation reaction typical of the monoxygenase enzymes. 

Tyrosine is not an essential amino acid in animals because it is synthesized from phenylalanine in a hydroxylation reaction. The enzyme involved, phenylala-nine-4-monoxygenase, requires the coenzyme tetrahydrobiopterin (Section 14.3), a folic acid-like molecule derived from GTP. Because this reaction also is a first step in phenylalanine catabolism, it is discussed further in Chapter 15. 

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