Conversion of cis-2-carboxycyclohexylacetyl-CoA in the downstream pathway of anaerobic naphthalene degradation.

The cyclohexane derivative cis-2-(carboxymethyl)cyclohexane-1-carboxylic acid [(1R,2R)-/(1S,2S)-2-(carboxymethyl)cyclohexane-1-carboxylic acid] has previously been identified as metabolite in the pathway of anaerobic degradation of naphthalene by sulfate-reducing bacteria. We tested the corresponding CoA esters of isomers and analogues of this compound for conversion in cell free extracts of the anaerobic naphthalene degraders Desulfobacterium strain N47 and Deltaproteobacterium strain NaphS2. Conversion was only observed for the cis-isomer, verifying that this is a true intermediate and not a dead-end product. Mass-spectrometric analyses confirmed that conversion is performed by an acyl-CoA dehydrogenase and a subsequent hydratase yielding an intermediate with a tertiary hydroxyl-group. We propose that a novel kind of ring-opening lyase is involved in the further catabolic pathway proceeding via pimeloyl-CoA. In contrast to degradation pathways of monocyclic aromatic compounds where ring-cleavage is achieved via hydratases, this lyase might represent a new ring-opening strategy for the degradation of polycyclic compounds. Conversion of the potential downstream metabolites pimeloyl-CoA and glutaryl-CoA was proved in cell free extracts, yielding 2,3-dehydropimeloyl-CoA, 3-hydroxypimeloyl-CoA, 3-oxopimeloyl-CoA, glutaconyl-CoA, crotonyl-CoA, 3-hydroxybutyryl-CoA and acetyl-CoA as observable intermediates. This indicates a link to central metabolism via β-oxidation, a non-decarboxylating glutaryl-CoA dehydrogenase and a subsequent glutaconyl-CoA decarboxylase.