Anaerobic Metabolism of Catechol by the Denitrifying Bacterium Thauera aromatica —a Result of Promiscuous Enzymes and Regulators?

Abstract

The anaerobic metabolism of catechol (1,2-dihydroxybenzene) was studied in the betaproteobacterium Thauera aromatica that was grown with CO 2 as a cosubstrate and nitrate as an electron acceptor. Based on different lines of evidence and on our knowledge of enzymes and genes involved in the anaerobic metabolism of other aromatic substrates, the following pathway is proposed. Catechol is converted to catechylphosphate by phenylphosphate synthase, which is followed by carboxylation by phenylphosphate carboxylase at the para position to the phosphorylated phenolic hydroxyl group. The product, protocatechuate (3,4-dihydroxybenzoate), is converted to its coenzyme A (CoA) thioester by 3-hydroxybenzoate-CoA ligase. Protocatechuyl-CoA is reductively dehydroxylated to 3-hydroxybenzoyl-CoA, possibly by 4-hydroxybenzoyl-CoA reductase. 3-Hydroxybenzoyl-CoA is further metabolized by reduction of the aromatic ring catalyzed by an ATP-driven benzoyl-CoA reductase. Hence, the promiscuity of several enzymes and regulatory proteins may be sufficient to create the catechol pathway that is made up of elements of phenol, 3-hydroxybenzoate, 4-hydroxybenzoate, and benzoate metabolism.

Anaerobic Catabolism of Aromatic Compounds: a Genetic and Genomic View

“…The further degradation of protocatechuate may yield 3-hydroxybenzoyl-CoA (282). Recently, anaerobic catechol catabolism in T. aromatica was investigated (90). Interestingly, it appears that no enzyme of the pathway was specific for catechol.…”

“…The hbcL gene from T. aromatica has been cloned and expressed in Escherichia coli cells, and the purified HbcL protein was shown to act not only on 3-hydroxybenzoate but also on protocatechuate, 4-HBA, benzoate, and gallate (3,4,5-trihydroxybenzoate). Thus, HbcL appears to be a promiscuous aromatic acid-CoA ligase (90).…”

“…Such reductive dehalogenation in cell extracts has not yet been demonstrated, and the R. palustris strain CGA009 genome does not contain any genes that resemble known reductive dehalogenase genes (101). Fur- 90 CARMONA ET AL. MICROBIOL.…”

“…Rather, the promiscuity of several enzymes and regulators involved in phenol, 3-hydroxybenzoate, 4-HBA, and benzoate metabolism may be sufficient to reconstruct the catechol pathway. Accordingly, the gene products that were induced when T. aromatica cells were grown on catechol are those that were postulated to be involved in these pathways (90).…”

“…7) (293). This gene (here named hcrL) has been cloned and expressed in E. coli cells, and the purified 4-HBA-CoA ligase was shown to act not only on 4-HBA but also on protocatechuate, 3-hydroxybenzoate, and benzoate (90). Although a proteomic study of Azoarcus sp.…”