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

Abstract

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.

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