The 5,6,7,8-Tetrahydro-2-Naphthoyl-Coenzyme A Reductase Reaction in the Anaerobic Degradation of Naphthalene and Identification of Downstream Metabolites

Abstract

Anaerobic degradation of polycyclic aromatic hydrocarbons is poorly investigated despite its significance in anoxic sediments. Using alternative electron donors for the 5,6,7,8-tetrahydro-2-naphthoyl-CoA reductase reaction, we observed intermediary metabolites of anaerobic naphthalene degradation via in vitro enzyme assays with cell extracts of anaerobic naphthalene degraders. The identified metabolites provide evidence that ring reduction terminates at the stage of hexahydro-2-naphthoyl-CoA and a sequence of β-oxidation-like degradation reactions starts with a hydratase acting on this intermediate. The final product of this reaction sequence was identified as cis -2-carboxycyclohexylacetyl-CoA, a compound for which a further downstream degradation pathway has recently been published (P. Weyrauch, A. V. Zaytsev, S. Stephan, L. Kocks, et al., Environ Microbiol 19:2819–2830, 2017, https://doi.org/10.1111/1462-2920.13806 ). Our study reveals the first ring-cleaving reaction in the anaerobic naphthalene degradation pathway. It closes the gap between the reduction of the first ring of 2-naphthoyl-CoA by 2-napthoyl-CoA reductase and the lower degradation pathway starting from cis -2-carboxycyclohexylacetyl-CoA, where the second ring cleavage takes place.