Abstract

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. The major principle of aromatic hydrocarbon biodegradation is that a broad range of peripheral reactions are transformed to a restricted range of central intermediates, which are subject to ring-cleavage and funneling into the Krebs cycle. Key enzymes in aerobic aromatic degradation are oxygenases, preparing aromatics for ring-cleavage by the introduction of hydroxyl functions and catalyzing cleavage of the aromatic ring. The diverse monooxygenases and dioxygenases involved in hydroxylations, a significant proportion of them possessing relaxed substrate specificity, are discussed as well as the broad diversity of side chain processing transformations involved in the formation of ring-cleavage central intermediates. Ring cleavage dioxygenases, covering intradiol ring cleavage of ortho dihydroxylated intermediates, and a large number of diverse but mechanistically related extradiol dioxygenases participating in ring cleavage of ortho and para dihydroxylated intermediates are also discussed. Novel CoA dependent aerobic routes to allow ring-cleavage of aromatic hydrocarbons without involvement of dihydroxylated aromatic intermediates have been described in the last years and are also reviewed. The degradation of heteroarenes will not be described in this chapter