Microbes and the mechanism of degradation of organic pollutants

The microbial degradation of aromatic compounds has tremendous practical significance. Both aerobic and anaerobic microorganisms have been isolated that degrade aromatic compounds, but much more is known about aerobic pathways. In general, degradation proceeds in two phases. First, an aromatic compound is prepared for ring cleavage by a variety of ring modification reactions. Of the many diverse pathways that have been identified, all have in common a monoor dioxygenation step that results in the formation of a dihydroxylated benzene ring. The second phase of degradation includes ring fission and subsequent reactions leading to the generation of tricarboxylic acid cycle intermediates. Ring fission is catalyzed by dioxygenases and is termed ortho-cleavage when it occurs between the hydroxyl groups (intradiol cleavage) and metacleavage when it occurs adjacent to one of the hydroxyls (extradiol cleavage). A third aerobic ring cleavage pathway, the gentisate pathway, is followed when the two hydroxyl groups on the aromatic ring are para to each other, and cleavage occurs between the carboxyl-substituted carbon and the adjacent hydroxylated carbon. Of the three pathways, the best studied are the meta-cleavage pathway and the ortho-cleavage or, as it is commonly known, the [Beta]ketoadipate pathway. The latter name derives from the fact that [Beta]-ketoadipate is a key intermediate of the ortho-cleavage pathway.