Dehydroxylation, phenol anaerobic degradation

The anaerobic degradation of some hydroxybenzoates and phenols involves reductive removal of the phenolic hydroxyl group. The enzyme that dehydroxylates 4-hydroxybenzoyl-CoA in Thauera aromatica is a molybdenum-flavin-iron-sulfur protein (Breese and Fuchs 1998), and is similar to the enzyme from the nonsulfur phototroph Rhodopseudomonas palustris that carries out the same reaction (Gibson et al. 1997). 

It has become clear that benzoate occupies a central position in the anaerobic degradation of both phenols and alkylated arenes such as toluene and xylenes, and that carboxylation, hydroxylation, and reductive dehydroxylation are important reactions for phenols that are discussed in Part 4 of this chapter. The simplest examples include alkylated benzenes, products from the carboxylation of napthalene and phenanthrene (Zhang and Young 1997), the decarboxylation of o-, m-, and p-phthalate under denitrifying conditions (Nozawa and Maruyama 1988), and the metabolism of phenols and anilines by carboxylation. Further illustrative examples include the following ... 

The anaerobic degradation of phenol proceeds by carboxylation of phenyl phosphate, followed by dehydroxylation, and fission of the ring after partial reduction (Brackmann and Fuchs 1993). 

It seems clear that benzoate occupies a central position in the anaerobic degradation of both phenols and alkaryl hydrocarbons, and that carboxylation, hydroxylation, and reductive dehydroxylation are important—and less expected—reactions. 

A biotechnological synthesis has also been demonstrated to be possible [58]. Thauera aromatica bacteria can use phenol as the only source of carbon under anaerobic conditions phenol is eventually converted into CO2 and water. The first step of the degradation path is the carboxylation of phenolphosphate to 4-hydroxybenzoic add which is then dehydroxylated to benzoic add [59] (Scheme 1.4). The carboxylation of phenol is carried out by a phenolcarboxylase enzyme, a new type of lyase [60]. The isolation of the enzyme from the cytoplasmic portion of the cell allows its use in vitro. In order to extend the lifetime of the enzyme, its supported form on low melting agar can be used [61]. Cut-off membranes (that allow the passage of macromolecules of a given size) [58] can be... 

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