Benzene monooxygenation

Toluene/o-xylene monooxygenase in P. stutzeri strain 0X1 carried ont snccessive monooxygenation of o-xylene (Bertoni et al. 1998), and the tolnene-4-monooxygenase of Pseudomonas mendocina KRl and tolnene-3-monooxygenase of Ralstonia pickettii PKOl can hydroxylate benzene, tolnene, and o-xylene (Tao et al. 2004 Vardar and Wood 2004). 

Zhou N-Y, A Jenkins, CKN Chan, KW Chion, DJ Leak (1999) The alkene monooxygenase from Xanthobacter strain Py2 is closely related to aromatic monooxygenases and catalyzes aromatic monooxygenation of benzene, toluene, and phenol. Appl Environ Microbiol 65 1589-1595. 

Xanthobacter sp. strain Py2 may be grown with propene or propene oxide. On the basis of amino acid sequences, the monooxygenase that produces the epoxide was related to those that catalyzes the monooxygenation of benzene and toluene (Zhou et al. 1999). The metabolism of the epoxide is initiated by nucleophilic reaction with coenzyme M followed by dehydrogenation (Eigure 7.13a). There are alternative reactions, both of which are dependent on a pyridine nucleotide-disulfide oxidoreductase (Swaving et al. 1996 Nocek et al. 2002) ... 

As an alternative, successive monooxygenation of benzene to phenol, catechol, and 1,2,3-trihydroxybenzene may be accomplished by the toluene 4-monooxygenase of Pseudomonas mendocina strain JKRl and the 3-monooxygenase of Ralstonia Pseudomonas) pickettii strain PKOl (Taoetal. 2004). 

Mutations at the active site of CYPlOl (cytochrome P450j,j jj) from a strain of Pseudomonas putida made possible the monooxygenation of chlorinated benzenes with less than three substituents to chlorophenols, with concomitant NIH shifts for 1,3-dichlorobenzene (Jones et al. 2001). Further mutations made it possible to oxidize even pentachlorobenzene and hexachlorobenzene to pentachlorophenol (Chen et al. 2002). Integration of the genes encoding cytochrome PTSO. into Sphingobium chlorophenolicum enabled this strain to partially transform hexachlorobenzene to pentachlorophenol (Yan et al. 2006). 

From a biocatalytic point of view the most interesting question to be answered is what enables these enzymes to catalyze the monooxygenation of even relatively inert chemical substrates, such as benzene derivatives, imder mild conditions. To answer this question several examples of monooxygenases, with different cofactors used to perform the job, will be discussed. 

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... 

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