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Aromatic hydrocarbon metabolism in the dissimilatory iron-reducing bacterium, Geobacter metallireducens

Wen-Tyng Li, University of Massachusetts Amherst

Abstract

Geobacter metallireducens is the first microorganism in pure culture shown to be able to completely oxidize the aromatic hydrocarbon toluene to carbon dioxide coupled to the reduction of iron. Two parallel routes involving sequential oxidation in the upper pathway for toluene oxidation are proposed. One is hydroxylation of the aromatic ring to form p-cresol, with subsequent oxidation to produce p-hydroxybenzylalcohol, p-hydroxybenzaldehyde and p-hydroxybenzoate. The other is the sequential oxidation of the methyl group to form benzylalcohol, benzaldehyde and benzoate. G. metallireducens can utilize all of the intermediates in the putative upper pathway. The presence of toluene enhances the utilization of p-cresol, p-hydroxybenzylalcohol and benzaldehyde. Some of the intermediates are found to accumulate in the growth medium during the simultaneous utilization experiments. The activity of the catabolic enzyme representing each step in the upper pathway has been demonstrated. The data suggest that the expression of these enzymes is regulated dependent upon the growth substrates. The reactions of p-hydroxybenzylalcohol dehydrogenase and benzylalcohol dehydrogenase are coupled to NAD$\sp+$ but not NADP$\sp+$. The reactions of p-hydroxybenzaldehyde dehydrogenase and benzaldehyde dehydrogenase are coupled to NADP$\sp+$ but not NAD$\sp+$. All of the catabolic enzyme activities exist in the soluble protein fraction of the cells. The demonstration of benzaldehyde hydroxylase activity and several lines of evidence suggest that at least one connection is present between two parallel routes in the upper pathway. The results from the simultaneous utilization experiments suggest that the primary transformation product of toluene is benzylalcohol rather than p-cresol. The soluble protein fraction of the cells exhibits benzoyl-CoA ligase, p-hydroxybenzoyl-CoA ligase, cyclohexanecarboxyl-CoA ligase and pimelyl-CoA ligase activities. The reactions of these enzymes are ATP and Mg$\sp{++}$ dependent. Three activation mechanisms are proposed to be involved in the metabolism of aromatic compounds in G. metallireducens. G. metallireducens can utilize cyclohexanecarboxylate and pimelate as the growth substrates. Both were detected as the intermediates during the radioisotope dilution experiments. This suggests that G. metallireducens may carry out the reductive pathway for the cleavage of the aromatic ring.

Subject Area

Microbiology

Recommended Citation

Li, Wen-Tyng, "Aromatic hydrocarbon metabolism in the dissimilatory iron-reducing bacterium, Geobacter metallireducens" (1996). Doctoral Dissertations Available from Proquest. AAI9619406.
https://scholarworks.umass.edu/dissertations/AAI9619406

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