The relationship between reductive dehalogenation and other aryl substituent removal reactions catalyzed by anaerobes

Anaerobic bacteria are known to catalyze the removal of a variety of aromatic substituents, including -COOH, -OH, -OCH₃, -CH₃, and halogens. We investigated whether reductive dehalogenation was related to other types of aryl substituent removal reactions. A dehalogenating bacterial consortium was tested for its ability to use benzoic acids substituted in the 3 position with the functional groups listed above. In addition to dehalogenation, the enrichment (as well as the dehalogenating pure culture) was able to transform 3-methoxybenzoic acid to 3-hydroxybenzoic acid without a lag. This reaction exhibited Michaelis-Menten kinetics with an apparent K_m of 5 μM. To test the hypothesis that the two reactions were related, we developed a mathematical model incorporating a competitive inhibition term to account for the influence of one substrate on the degradation of the other. However, experimental evidence showed no significant difference in the rates of 3-chlorobenzoic acid or 3-methoxybenzoic acid degradation in either the presence or absence of the other substrate. In addition, an anaerobe known to degrade methoxylated aromatic substrates was not able to transform chlorinated analogues. Finally, the isolated dechlorinating organism, strain DCB-1 was able to transform 3-methoxybenzoic acid in the presence of 1 mM thiosulfate, but the dehalogenation of 3-chlorobenzoic acid under these conditions was completely inhibited. Therefore, it is unlikely that a relationship exists between dehalogenation and other anaerobic aromatic substituent removal mechanisms.