Hydrocarbons, monooxygenation

The active site similarities listed above belie a remarkable functional diversity, which includes phosphate ester hydrolysis, dioxygen and NO reduction, reversible O2 binding, and O2 activation, the last of which includes enzymes involved in ribonucleotide reduction, hydrocarbon monooxygenation, and fatty acyl desaturation. At the overall protein level, the purple acid phosphatases (PAPs) seem to be completely unrelated, both structurally and functionally, to any of the others in this class. Similarly, the flavo-diiron enzymes form a structurally and probably functionally distinct family of proteins, catalyzing both dioxygen and NO reduction. These last two examples illustrate that attempts to shoehorn all of these enzymes into a single class can sometimes provide a simplistic and misleading view of their chemistry and biochemistry. 

M. Moghadam, S. Tangestaninejad, M. H. Habibi, V. Mirkhani, A convenient preparation of polymer-supported manganese porphyrin and its use as hydrocarbon monooxygenation catalyst, /. Mol. Catal. A 217 (2004) 9. 

Arene hydrocarbon dioxygenases are capable of carrying out a number of reactions other than the introduction of both atoms of oxygen into the substrate. Illustrative examples of monooxygenation carried out by dioxygenases include the following ... 

Whereas the metabolism of aromatic hydrocarbons takes place by dioxygenation, their biotransformation by yeasts and fungi is normally initiated by monooxygenation to the epoxide followed by hydrolysis to the trani-dihydrodiols. Phenols may subsequently be formed either by elimination or by nonenzymatic rearrangement of the epoxide ... 

In many mammals induction of monooxygenation by polycyclic aromatic hydrocarbons is accompanied by the formation of a hemoprotein not seen to any appreciable extent in non-induced animals. This leads to an alteration in the microsomal hemoprotein populations, a change in the metabolic activity of the microsomes and, hence, possible alterations in the toxicity of other chemicals (27, 8). 

Initial studies designed to obtain a valid subcellular fractionation scheme for rainbow trout liver illustrated the aryl-hydrocarbon (benzo[a]pyrene] hydroxylase activity separated with glucose-6-phosphatase (35). This observation indicated that the trout hemoprotein P-450-mediated monooxygenation system was located within the endoplasmic reticulum (microsomal fraction). 

This initial study demonstrating induction of monooxygenation and hemoprotein P-450 in the rainbow trout by polycyclic aromatic hydrocarbons, but not by phenobarbital, was extended further. 

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 ... 

Table IV. Oxidation of Hydrocarbons with Fe20(0Ac)2Cl2(bipy)2, 5, [Fe402(0Ac)7(bipy)2]+, 6, and Fe20(0Ac)(tmima)2(Cl04)3, 7, Using TBHP or H202 as Monooxygen Transfer Reagents...

In the past 25 years, Fe =0 and Mn =0 have also emerged as reactive intermediates in the oxidation of hydrocarbons they are not formed by interaction with dioxygen but rather by monooxygen donors (see below). 

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 activation of dioxygen for the monooxygenation of saturated hydrocarbons by the methane monooxygenase enzyme systems (MMO hydroxylase/reductase) represents an almost unique biochemical oxygenase, especially for the transformation of methane to methanol. The basic process involves the insertion of an oxygen atom into the C-H bond of the hydrocarbon via the concerted reduction of O2 by the reductase cofactor (equation 120). 

The ability of the Fe (DPAH)2/02/PhNHNHPh system (where PhNHNHPh is a mimic for flavin rednctases) to monooxygenate saturated hydrocarbons closely parallels the chemistry of the methane monooxygenase proteins. However, the enzyme oxygenates 2-Me-bntane with an isomer distribution of 82% primary alcohol, 10% secondary, and 8% tertiary. The present model gives a distribution of 21% primary, 29% secondary, and 50% tertiary. Clearly the protein affords a cavity that is selective for -CH3 groups. 

Cytochromes P-450, the CO complexes of which have a characteristic absorption maximum at 450 nm, use molecular oxygen to catalyze monooxygenation of versatile organic compounds such as hydrocarbons, sulfides, and amines with reducing agents (Eq. 1) [27-33],... 

Cytochrome P450 enzymes use molecular oxygen to create ROS that monooxygenate many natural and xenobiotic species. With natural species and some foreign chemicals, the oxygenation is helpful, producing metabolites that are eliminated from the body. With others, however, such as aromatic hydrocarbons, the ROS metabolites are far more toxic than the parent compounds and create OS. Aerobic life is dependent upon the formation and deactivation of ROS. OS arises when ROS are formed at a rate that exceeds the rate of deactivation. 

Scheme 6-6 Proposed mechanism for the activation of O2 by methane monooxygenase (M.MO) for the monooxygenation of hydrocarbons (RH)...

Table I shows our results with C1-C3 and cycloCg hydrocarbons and manganese porphyrin catalysts 1 and 2 (Figure 1), with iodosylbenzene as the monooxygen transfer reagent, at room temperature in methylene chloride. It is evident that the supramolecule and open-faced porphyrin catalysts have similar reactivities with the hydrocarbons studied. Also, it is unfortunate that methane is not activated to methanol however, ethane, propane, and cyclohexane are converted to their respective alcohols. Hence, we did not see any special reactivity with the supramolecule catalyst, 1, and rationalize that too much flexibility in the "basket handles" does not provide the shape selectivity that we hoped for to gain a kinetic advantage with the difficult to react methane gas.

Table I. Carbon-Hydrogen Activation of Hydrocarbons Using Compounds 1 and 2 as Catalysts and Iodosylbenzene as the Monooxygen Transfer Agent4...

Table n. Comparison of the C-H Bond Reactivity of C2, C3, and CycloC Hydrocarbons with Mn3-40i-2LxLy Catalysts, 1-4, Using t-Butyl Hydroperoxide as the Monooxygen Transfer Reagenta... 

Table m. Carbon-Hydrogen Activation of C1-C3 Hydrocarbons with a Manganese-Substituted Keggin Ion Catalyst Using t-Butyl Hydroperoxide as the Monooxygen Transfer Reagent in Benzene a... 

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). 

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