Anaerobic Metabolism of Aromatic Compounds

Abstract

Aromatic compounds comprise a wide variety of natural and synthetic compounds that can serve as substrates for bacterial growth. So far, four types of aromatic metabolism are known. (1) The aerobic aromatic metabolism is characterized by the extensive use of molecular oxygen as cosubstrate for oxygenases that introduce hydroxyl groups and cleave the aromatic ring. (2) In the presence of oxygen, facultative aerobes use another so-called hybrid type of aerobic metabolism of benzoate, phenylacetate, and anthranilate (2-aminobenzoate). These pathways use coenzyme A thioesters of the substrates and do not require oxygen for ring cleavage; rather they use an oxygenase/reductase to dearomatize the ring. (3) In the absence of oxygen, facultative aerobes and phototrophs use a reductive aromatic metabolism. Reduction of the aromatic ring of benzoyl-coenzyme A is catalyzed by benzoyl-coenzyme A reductase. This Birch-like reduction is driven by the hydrolysis of 2 ATP molecules. (4) A completely different, still little characterized benzoyl-coenzyme A reductase operates in strict anaerobes, which cannot afford the costly ATP-dependent ring reduction.

Bacterial Degradation of Benzoate

“…In the aerobic catabolism, molecular oxygen is not only the final electron acceptor but also an essential cosubstrate of oxygenases involved in the hydroxylation (activation) and cleavage (dearomatization) of the aromatic ring (5,6). The anaerobic catabolism, however, relies on a completely different strategy based on coenzyme A (CoA)-dependent activation of the aromatic ring followed by reductive dearomatization and then hydrolytic ring cleavage (3,4). A third degradation strategy has been described for the aerobic mineralization of some aromatic compounds (e.g.…”

Identification and characterization of the tungsten-containing class of benzoyl-coenzyme A reductases

“…Under aerobic conditions, bacteria use oxygenases to insert oxygen atoms into the ring to relieve ring resonance. In the absence of oxygen, this biochemical strategy is not an option, and anaerobic bacteria use completely different enzymes that relieve ring resonance by a reductive mechanism (1)(2)(3)(4).…”

Structures of benzylsuccinate synthase elucidate roles of accessory subunits in glycyl radical enzyme activation and activity

“…The persistence of these molecules is due to their solubility and inertness: compounds such as toluene, phenol, and ethylbenzene contain no easily oxidized carbons, and dearomatization of these compounds requires strong oxidants or reductants. Nevertheless, microorganisms have developed diverse strategies to activate and degrade these compounds under both aerobic and anaerobic conditions (1).…”