Catabolic Pathways and Enzymes Involved in the Anaerobic Degradation of Polycyclic Aromatic Hydrocarbons

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are hazardous environmental pollutants that can be degraded exclusively by microorganisms. Whereas the enzymes involved in aerobic PAH degradation have been studied intensively since decades, the degradation of PAH in anaerobic bacteria is sparsely characterized. Only a few anaerobic strains and enrichment cultures degrading PAHs have been described. Today only the anaerobic naphthalene and methylnaphthalene degradation pathways have been elucidated. A few key reactions of these pathways have been studied to some detail and are presented in this chapter. The initial activation of PAHs without oxygen represents a major challenge and is accomplished by direct carboxylation of non-substituted PAHs or by addition to fumarate in case of methylnaphathlene. In the case of naphthalene degradation, CoA-thioesterification yields 2-naphthoyl-CoA, which undergoes dearomatization by three consecutive two-electron transfer steps to the napthyl ring system. The non-substituted ring of 2-naphthoyl-CoA is dearomatized first by a member of the flavin-containing old yellow enzyme (OYE) family. After another reduction step by a second OYE-like enzyme, the dearomatization of the second ring is catalyzed by an ATP-dependent enzyme homologous to dearomatizing class I benzoyl-CoA reductases. The nonaromatic hexahydronaphthoyl-CoA ring system formed is then cleaved by modified β-oxidation reactions yielding cyclohexane ring containing intermediates. Though little is known about the anaerobic degradation of PAHs with more than two rings, similar principles as those identified for naphthalene degradation are proposed to be involved. The expected common enzymatic processes comprise: (i) initial carboxylation, (ii) activation to a CoA ester, (iii) the reductive dearomatization of the polycyclic ring systems by OYE-like enzymes and/or homologues of benzoyl-CoA reductases, and (iv) oxidation to acetyl-CoA by modified β-oxidation reactions via a cis-carboxycyclohexylacetyl-CoA intermediate.