Is Initiated by Dioxygenation

Whole-cell studies using oxygen uptake by Burkholderia (Pseudomonas) sp. strain LB400 had clearly shown the versatility of this organism in the degradation of PCB congeners containing up 

These results are consistent with the results accumulated over many years, and with the most frequent operation of a single dioxygenase enzyme. 

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The degradation of 3-nitrobenzoate by Pseudomonas sp. strain JS51 and Comamonas sp. strain JS46 is initiated by dioxygenation to 3,4-dihydroxybenzoate with loss of nitrite (Nadeau and Spain 1995) (Figure 9.53b). 

Benzothiophene is isoelectronic with naphthalene, dibenzothiophene with anthracene, and benzothiazole with quinoline, and this is reflected in their aerobic degradation that is initiated by dioxygenation. The diversity of pathways for the degradation of dibenzothiophene is illustrated by the following examples ... 

Whereas the degradation of the carboxylates of the monocyclic furan, thiophene, and pyrrole is initiated by hydroxylation, degradation of their benzo analogs is generally carried out by dioxygenation. The degradation of the analogs dibenzofuran and dibenzo-[l,4]-dioxin is discussed in Part 2 of this chapter. 

Both 2-hydroxy- and 3-hydroxypyridine are hydroxylated to 2,5-dihydroxypyridine by strains of Achromobacter sp. (Houghton and Cain 1972). These metabolites are probably, however, formed by different reactions whereas 3-hydroxypyridine behaves as a true pyridine, addition of H2O across the Cg Nj bond would produce the 2,5-dihydroxy compound 2-hydroxypyridine is a cyclic amide and hydroxylation apparently occurs at the diagonal position. The degradation of 4-hydroxypyridine is also initiated by hydroxylation and is followed by dioxygenation before ring fission (Figure 10.12) (Watson et al. 1974). 

However, benzil [PhC(O)C(O)Ph] cannot enolize and is dioxygenated by Oz - to give two benzoate ions. Scheme 7-11 outlines a proposed mechanism that is initiated by nucleophilic attack. An alternative pathway has been proposed i in which the initial step is electron transfer from 02 - to the carbonyl, followed by coupling of the benzil radical with dioxygen to give the cyclic dioxetanelike intermediate. However, the outer-sphere reduction potential for electron transfer from O2 to a carbonyl carbon is insufficient (-0.60 V versus NHE, Chapter 2). 

Hydroxylation occurs as the initial step during the degradation of 4-hydroxypyridine and is followed by dioxygenation before ring cleavage (Figure 6.44) (Watson et al. 1974). 

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