THE BACTERIAL-DEGRADATION OF BENZOIC-ACID AND BENZENOID COMPOUNDS UNDER ANAEROBIC CONDITIONS - UNIFYING TRENDS AND NEW PERSPECTIVES

Simple homocyclic aromatic compounds are extremely abundant in the env ironment and are derived largely from lignin. Such compounds may enter anaerobic environments and several groups of bacteria, exhibiting div erse energy-yielding mechanisms, have evolved the capacity to overcome the thermodynamic stability of the benzene nucleus and degrade aromat ic compounds under these conditions. Over the last few years considera ble advances have been made in our understanding of the biochemical st rategies underlying the bacterial degradation of aromatic compounds in anoxic environments. The study of the biochemistry, and more recently the molecular genetics of the photosynthetic bacterium Rhodopseudomon as palustris and several strains of denitrifying pseudomonads, has pro vided the greatest insight into the mechanism and regulation of aromat ic degradation under anaerobic conditions. Research has centred around the anaerobic degradation of benzoic acid. This involves the initial activation to form benzoyl-Coenzyme A, reduction of the aromatic nucle us - a reaction that has only recently been demonstrated in vitro - an d the subsequent degradation of the alicyclic intermediates. Recently, much information regarding the exact nature of these intermediates ha s been obtained. Also through recent studies, it has become increasing ly clear that benzoyl-CoA is a central metabolic intermediate during t he anaerobic degradation of structurally diverse aromatic compounds. T he initial metabolism of these compounds involves the formation of a c arboxyl group on the aromatic nucleus (if necessary) and the synthesis of the respective Coenzyme A thioester; this results in the direct fo rmation of benzoyl-Coenzyme A rather than benzoate. In many cases of a naerobic aromatic degradation studied in batch culture, aromatic inter mediates are transiently excreted into the medium. It is argued that t he study of this phenomenon may facilitate the understanding of the re gulation and kinetics of the aromatic degradative pathways.