Anaerobic Metabolism of Aromatic Compounds

Aromatic compounds comprise a wide variety of low‐molecular‐mass natural compounds (amino acids, quinones, flavonoids, etc.) and biopolymers (lignin, melanin). They are almost exclusively degraded by microorganisms. Aerobic aromatic metabolism is characterised by the extensive use of molecular oxygen. Monoxygenases and dioxygenases are essential for the hydroxylation and cleavage of aromatic ring structures. Accordingly, the characteristic central intermediates of the aerobic pathways (e.g. catechol) are readily attacked oxidatively. Anaerobic aromatic catabolism requires, of necessity, a quite different strategy. The basic features of this metabolism have emerged from studies on bacteria that degrade soluble aromatic substrates to CO2, in the complete absence of molecular oxygen.Essential to anaerobic aromatic metabolism is the replacement of all the oxygen‐dependent steps by an alternative set of novel reactions and the formation of different central intermediates (e.g. benzoyl‐CoA) for breaking the aromaticity and cleaving the ring; notably, in anaerobic pathways, the aromatic ring is reduced rather than oxidised. The two‐electron reduction of benzoyl‐CoA to a cyclic diene requi