Shedding light on anaerobic benzene ring degradation: a process unique to prokaryotes?

نویسندگان

  • C S Harwood
  • J Gibson
چکیده

Studies of anaerobic benzene ring degradation date back 50 years, but research on this topic proceeded only sporadically until about 10 years ago. At that time, concerns about toxic industrial wastes converged with a realization that many polluted environments are anaerobic to stimulate a renewed and sustained interest in the anaerobic metabolism of aromatic compounds. It is now clear that a wide variety of aromatic compounds can be completely degraded by bacteria in the complete absence of oxygen (14, 21, 24, 31, 32, 55). Increasing numbers of bacterial strains, representing most known modes of anaerobic energy metabolism, including phototrophy, denitrification, and sulfate or iron reduction, as well as fermentation, are now being isolated and studied in pure culture for their abilities to degrade a range of aromatic hydrocarbons, phenols, and halogenated aromatics. The global recycling of naturally occurring aromatic compounds derived from lignin, an aromatic polymer that makes up 25% or more of the woody tissues of plants, is also catalyzed by anaerobic bacteria. As far as is known, the anaerobic degradation of aromatic compounds is a talent restricted to bacteria. It must be remembered, however, that the metabolic capabilities of anaerobic fungi have only recently begun to attract the interest they deserve and are little known. Anaerobic aromatic ring degradation has not yet been reported in members of the archaea. A major area of interest has been reactions that modify benzene ring substituents, particularly dehalogenation (49, 58) and methyl group modification of, for example, toluene and cresols (7, 13, 14) as these effectively convert toxic compounds to nontoxic derivatives. Other modifications including removal of hydroxyl, methoxyl, amino, and acyl groups are also being studied (24, 55, 57) (Fig. 1). Although in most cases, the enzymes involved in these conversions have been examined only superficially, it is evident that many catalyze reactions unique to anaerobic aromatic degradation (24). Studies with members of the range of different bacteria involved have shown that preliminary modifications of substituted aromatic rings lead to the production of benzoate, usually as its coenzyme A (CoA) thioester, and often via 4-hydroxybenzoyl CoA (4-OHbenzoylCoA) (Fig. 1). Benzoyl-CoA is also a hypothesized intermediate in the degradation of ethylbenzenes and xylene (53). The benzoate pathway is then the principal route of aromatic ring cleavage leading to the complete oxidation of diverse aromatic substrates. Anaerobic attack on the extremely stable structure of the benzene ring proceeds by a ring reduction mechanism, a biochemical process that is radically different from the oxygenrequiring strategies used by microbes to catalyze aromatic compound degradation under aerobic conditions (35). The anaerobic degradation of aromatic compounds is a major unsolved problem in microbial physiology. Here we present a status report on one aspect of this problem, focusing on enzymatic and molecular studies of benzoate and 4-hydroxybenzoate (4-OHBen) degradation.

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عنوان ژورنال:
  • Journal of bacteriology

دوره 179 2  شماره 

صفحات  -

تاریخ انتشار 1997