Degradation of Aromatic Hydrocarbons

Aromatic hydrocarbons are widely distributed in nature. They are found as lignin components, aromatic amino acids and xenobiotic compounds, among others. Microorganisms, mostly bacteria, degrade an impressive variety of such chemical structures. A broad range of peripheral reactions results in a restricted range of central intermediates, which are subject to ring-cleavage and funneling into the Krebs cycle. Key enzymes in aerobic aromatic hydrocarbon degradation are oxygenases, preparing aromatics for ring-cleavage by the introduction of hydroxyl functions. The diverse monooxygenases and dioxygenases involved, a significant proportion of them possessing relaxed substrate specificity, are discussed as well as the broad diversity of peripheral reactions that include several different side chain processing transformations. The broad range of peripheral reactions results in a restricted range of central intermediates, which are subject to ringcleavage and funneling into the Krebs cycle. In addition to the intradiol ring cleaving reactions, a number of different extradiol dioxygenases participate in meta ring cleavage central pathways. Novel CoA dependent aerobic routes to allow ring-cleavage of aromatic hydrocarbons without involvement of dihydroxylated intermediates have been described and are also discussed. The degradation of heteroarenes will not be described in this chapter.