Monooxygenation reactions, cytochrome

The most numerous and most complex monooxygenation reactions are those employing a type of heme protein called cytochrome P-450. This cytochrome is usually present in the smooth ER rather than the mitochondria. Like mitochondrial cytochrome oxidase, cytochrome P-450 can react with 02 and bind carbon monoxide, but it can be differentiated from cytochrome oxidase because the carbon monoxide complex of its reduced form absorbs light strongly at 450 ran—thus the name P-450. 

Though the investigation of photocatalytic oxygenations performed of the laboratory scale are often motivated by attempts to understand and mimic the catalytic cycle of cytochrome P450 (a natural catalyst of monooxygenation reactions), the results obtained [159, 253, 266] could be applied to industrial processes as well. 

Monooxygenation reactions involve the reduction of one atom of molecular oxygen to water and the incorporation of the other oxygen atom into the substrate [Eq. (9.1)]. The electrons involved in the reduction of CYP are transferred from NADPH by another enzyme called NADPH-cytochrome P450 oxidoreductase (CYPOR). 

Monooxygenations are those oxidations in which one atom of molecular oxygen is reduced to water while the other is incorporated into the substrate. Microsomal monooxygenation reactions are catalyzed by nonspecific enzymes such as the flavin-containing monooxygenases (FMOs) or the multienzyme system that has cytochrome P450s (CYPs) as the terminal oxidases. 

Figure 8.2 Generalized scheme showing the catalytic cycle of cytochrome P450 enzymes in monooxygenation reactions. Fe = iron atom in P450 heme. RH = substrate. ROH = product. b5 = cytochrome bs. ox and red indicate the reduced and (1 electron) oxidized states of the reductase involved in the electron transfer. See text for details. (From Guengerich, F.P., Client. Res. Toxicol. 14, 611, 2001. With permission.)...

Cytochromes P450. Monooxygenation reactions are of major significance in drug metabolism and are mediated by various enzymes that differ markedly in their structure and... 

Estabrook RW (2005) Steroid hydroxylations a paradigm for cytochrome P450 catalyzed mammalian monooxygenation reactions. Biochem Biophys Res Commun 338 290-298... 

For example, mitochondrial cytochrome CYPllAl (also referred to as P450scc) cleaves the side chain of cholesterol to form pregnenolone, the precursor of all steroid hormones [30]. This process consists of three sequential monooxygenation reactions and proceeds via 22(P)-hydroxycholesterol and 20a,22(R)-dihydroxycholesterol, followed by the cleavage of the C20-C22 bond, ultimately yielding pregnenolone (Scheme 5.4). Earlier studies had revealed that... 

Several diverse metal centres are involved in the catalysis of monooxygenation or hydroxylation reactions. The most important of these is cytochrome P-450, a hemoprotein with a cysteine residue as an axial ligand. Tyrosinase involves a coupled binuclear copper site, while dopamine jS-hydroxylase is also a copper protein but probably involves four binuclear copper sites, which are different from the tyrosinase sites. Putidamonooxin involves an iron-sulfur protein and a non-heme iron. In all cases a peroxo complex appears to be the active species. 

Scheme 1. Monooxygenation and dismutation reactions catalyzed by cytochrome P450 and catalase, respectively.

Progress in decoding the mechanism of cytochrome P-450, which catalyzes epoxidation and hydroxylation of various hydrocarbons, has stimulated the search for comparatively simple and effective iron porphyrin systems [20-24], The reaction mechanism of monooxygenation can be illustrated by the following diagram ... 

Biological systems overcome the inherent unreactive character of 02 by means of metalloproteins (enzymes) that activate dioxygen for selective reaction with organic substrates. For example, the cytochrome P-450 proteins (thiolated protoporphyrin IX catalytic centers) facihtate the epoxidation of alkenes, the demethylation of Al-methylamines (via formation of formaldehyde), the oxidative cleavage of a-diols to aldehydes and ketones, and the monooxygenation of aliphatic and aromatic hydrocarbons (RH) (equation 104). The methane monooxygenase proteins (MMO, dinuclear nonheme iron centers) catalyze similar oxygenation of saturated hydrocarbons (equation 105). ... 

The use of a synthetic model system has provided valuable mechanistic insights into the molecular catalytic mechanism of P-450. Groves et al. [34]. were the first to report cytochrome P-450-type activity in a model system comprising iron meso-tetraphenylporphyrin chloride [(TPP)FeCl] and iodosylbenzene (PhIO) as an oxidant which can oxidize the Fe porphyrin directly to [(TPP)Fe =0] + in a shunt pathway. Thus, (TPP)FeCl and other metalloporphyrins can catalyze the monooxygenation of a variety of substrates by PhIO [35-40], hypochlorite salts [41, 42], p-cyano-A, A -dimethylanihne A -oxide [43-46], percarboxylic acids [47-50] and hydroperoxides [51, 52]. Catalytic activity was, however, rapidly reduced because of the destruction of the metalloporphyrin during the catalytic cycle [34-52]. When (TPP)FeCl was immobilized on the surface of silica or silica-alumina, catalytic reactivity and catalytic lifetime both increased significantly [53]. There have been several reports of supported catalysts based on such metalloporphyrins adsorbed or covalently bound to polymers [54-56]. Catalyst lifetime was also significantly improved by use of iron porphyrins such as mew-tetramesitylporphyrin chloride [(TMP)FeCl] and iron mcA o-tetrakis(2,3,4,5,6-pentafluorophenyl)por-phyrin chloride [(TPFPP)FeCl], which resist oxidative destruction, because of steric and electronic effects and thereby act as efficient catalysts of P-450 type reactions [57-65]. 

The hydroxylation of small alkanes, such as propane and ethane, is particularly attractive to the chemical industry. However, monooxygenation of carbon centers is a reaction that is difficult to achieve chemically as it requires extreme temperatures. An attractive alternative is the use of cytochrome P-450 enzymes (P-450s) that can catalyze such reactions with a high degree of regio- and stereoselectivity. Thus, there has been much work directed toward evolving P-450s with altered substrate specificities that can catalyze hydroxylation of Cl-ClO alkanes. 

The transformation of a few polycyclic aromatic hydrocarbons has also been investigated in yeasts. The metabolism of naphthalene, biphenyl, and benzo [a] pyrene has been examined in a strain of Debaryomyces hansenii and in number of strains of Candida sp. The results using C. lipolytica showed that the transformations were similar to those carried out by fungi the primary reaction was formation of the epoxides that were then rearranged to phenols (Cerniglia and Crow 1981). Benzo[a]pyrene is transformed by Saccharomyces cerevisiae to the 3- and 9-hydroxy compounds and the 9,10-dihydrodiol, and the cytochrome P-448 that mediates the monooxygenation has been purified and characterized (King et al. 1984). 

This reaction is known as a monooxygenation and is supported by two main groups of monooxygenases cytochromes P450 and flavine monooxygenases. 

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