A CoA-Transferase and Acyl-CoA Dehydrogenase Convert 2-(Carboxymethyl)cyclohexane-1-Carboxyl-CoA During Anaerobic Naphthalene Degradation

The CoA thioester of 2-(carboxymethyl)cyclohexane-1-carboxylic acid has been identified as a metabolite in anaerobic naphthalene degradation by the sulfate-reducing culture N47. This study identified and characterised two acyl-CoA dehydrogenases (ThnO/ThnT) and an intramolecular CoA-transferase (ThnP) encoded within the substrate-induced thn operon, which contains genes for anaerobic degradation of naphthalene. ThnP is a CoA transferase belonging to the family I (Cat 1 subgroup) that catalyses the intramolecular CoA transfer from the carboxyl group of 2-(carboxymethyl)cyclohexane-1-carboxyl-CoA to its carboxymethyl moiety, forming 2-carboxycyclohexylacetyl-CoA. Neither acetyl-CoA nor succinyl-CoA functions as an exogenous CoA donor for this reaction. The flavin-dependent homotetrameric dehydrogenase ThnO is specific for (1R,2R)-2-carboxycyclohexylacetyl-CoA with an apparent K_m value of 61.5 μM, whereas ThnT is a promiscuous enzyme catalysing the same reaction at lower rates. Identifying these three enzymes confirmed the involvement of the thn gene cluster in the anaerobic naphthalene degradation pathway. This study establishes a modified metabolic pathway for anaerobic naphthalene degradation upstream of 2-(carboxymethyl)cyclohexane-1-carboxyl-CoA and provides further insight into the subsequent second-ring cleavage reaction.