Cresol monooxygenases

Whited GM, DT Gibson (1991b) Toluene-4-monooxygenase, a three-component enzyme system that catalyzes the oxidation of toluene to p-cresol in Pseudomonas mendocina KRl. J Bacteriol 173 3010-3016. 

This FAD-dependent enzyme [EC 1.14.13.7], also known as phenol 2-monooxygenase, catalyzes the reaction of phenol with NADPH and dioxygen to produce catechol, NADP+, and water. Other substrates include resorcinol and (9-cresol. 

In a case analogous to the combination of the two plasmid encoded ortho- and mta-fission pathways in B13 for specific halo-aromatic utilization, another bacterium was modified with a hybrid pathway for this purpose. B. cepacia G4 degrades toluene, phenol, benzene, and ortho-cresol via a unique toluene ottfw-monooxygenase (Tom) pathway (Shields et al., 1989) encoded by the toluene catabolic plasmid TOM (Shields et al., 1995). 

The application of direct electrochemistry of small redox proteins is not restricted to cytochrome c. For example, the hydroxylation of aromatic compounds was possible by promoted electron transfer from p-cresol methylhydroxylase (a monooxygenase from Pseudomonas putida) to a modified gold electrode [87] via the blue copper protein azurin. All these results prove that well-oriented non-covalent binding of redox proteins on appropriate electrode surfaces increases the probability of fast electron transfer, a prerequisite for unmediated biosensors. Although direct electron-transfer reactions based on small redox proteins and modified electrode surfaces are not extensively used in amperometric biosensors, the understanding of possible electron-transfer mechanisms is important for systems with proteins bearing catalytic activity. 

TOM pathway occurs in Burkholderia cepacia G4 (formerly known as Pseudomonas cepacia G4), which uses toluene orf/jo-monooxygenase in the initial attack to form o-cresol (Shields Montgomery, 1989), which is converted to 3-methylcatechol (Fig. 5.5). 

TBU pathway occurs in Burkholderiapickettii PKO1 (formerly known as Pseudomonaspick-ettii PKOl), which hydroxylates the aromatic ring using toluene meto-monooxygenase to produce m-cresol (Olson et al, 1994), which is transformed to 3-methylcatechol (Fig. 5.5). 

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