Evidence That Anaerobic Oxidation of Toluene in the Denitrifying Bacterium Thauera aromatica is Initiated by Formation of Benzylsuccinate from Toluene and Fumarate

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Abstract
Toluene is degraded anoxically to CO2 by the denitrifying bacterium Thauera aromatica. Toluene first becomes oxidized to benzoyl-CoA by O2-independent reactions. Benzoyl-CoA is then reduced to non-aromatic products by benzoyl-CoA reductase. We set out to study the reactions employed for the initial activation of toluene and its oxidation to the level of benzoate. Evidence is provided for a novel way of toluene degradation based on experiments with cell-free extracts and with whole toluene-grown cells: Cell-free extracts oxidized [14C]toluene to [14C]benzoyl-CoA via several radioactive intermediates. This reaction was strictly dependent on the presence of fumarate, coenzyme A and nitrate as electron acceptor; acetyl-CoA and ATP were not necessary for the reaction. The first product formed in vitro was benzylsuccinate; (2H8)toluene was converted to (2H7)benzylsuccinate. Formation of benzylsuccinate from toluene was independent of coenzyme A and nitrate, but it required the presence of fumarate. Other tricarboxylic acid cycle intermediates were converted to fumarate in cell extracts and therefore could partially substitute for fumarate. [14C]Benzylsuccinate was oxidized further to [14C]benzoyl-CoA and [14C]benzoate in cell extracts if coenzyme A and nitrate were present. No benzyl alcohol and benzaldehyde and no phenylpropionate could be detected as intermediates. In isotope trapping experiments with cell suspensions, two intermediates from [14C]toluene were detected, benzoate and benzylsuccinate. This corroborates the sequence of reactions deduced from in vitro experiments. A hypothetical degradation pathway for the anaerobic oxidation of toluene to benzoyl-CoA via an initial addition of fumarate to the methyl group of toluene and following β-oxidation of the benzylsuccinate formed is suggested.

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