Dioxygen pathway

FIGURE 3.23 Alternative dioxygenation pathways for 3-substituted catechols (a) extradiol, (b) intradiol, and (c) distal. 

Lipoxygenation is the major pathway of dioxygenation of arachidonic acid in blood platelets and leads to the 12-5-hydroperoxy acid 12-HPETE and the corresponding 12-hydroxy acid 12-HETE. Several pathways for the synthesis of 12-HETE have been developed. However, despite the availability of this substance, its biological role remains undetermined. 

FIGURE 3.27 Dioxygenation of enediol to formate + CO in the methionine salvage pathway. 

Strains may be able to regulate the pathway of degradation of toluene to the availability of oxygen aerobic degradation by dioxygenation and under denitrifying conditions in the absence of oxygen by the benzylsuccinate pathway. 

The degradation of resorcinol can take place by several pathways that are initiated by hydroxylation. 1,2,4-Trihydroxybenzene is degraded by (a) 1 2 dioxygenation and... 

Chlorobenzoates may be formed during the initial steps in the aerobic degradation of PCBs, and their further metabolism illustrates a number of pathways. There are several reactions that carry out dehalogenation including dioxygenation, hydrolysis, and reduction. 

For 3-chlorobenzoate, an alternative pathway in Alcaligenes sp. strain BR60 may involve 3,4- or 4,5-dioxygenation. Ring fission of the catechols resulted in the production of pyruvate and oxalacetate (Nakatsu and Wyndham 1993). 

Dioxygenation with decarboxylation bnt withont loss of halogen to prodnce chlorocatechols that may be degraded by several ring-fission pathways (3- and 4-chlorobenzoates) Dioxygenation with loss of only halogen to prodnce a dihydroxybenzoate (2-chlorobenzoate)... 

Seeger M, M Zielinski, KN Timmis, B Hofer (1999) Regiospecificity of dioxygenation of di- to pentachlorobi-phenyls and the degradation to chlorobenzoates by the f>p/j-encoded catabolic pathway of Burkholderia sp. strain LB400. Appl Environ Microbiol 65 3614-3621. 

Hydroxylation and dioxygenation are not, however, mntnally exclusive, because the toluene dioxygenase from Pseudomonasputida FI hydroxylates both phenol and 2,5-dichloro-phenol with the introduction of only one atom of oxygen (Spain et al. 1989). Snbsequent degradation by dioxygenation follows established pathways in which chloride is eliminated from muconic acids after ring fission. 

The metabolism of fluorobenzoates has been examined over many years. Early studies using Nocardia erythropoUs (Cain et al. 1968) and Pseudomonas fluorescens (Hughes 1965) showed that although the rates of whole-cell oxidation of fluorobenzoates were less than for benzoate, they were comparable to, and greater than for, the chlorinated analogs. As for their chlorinated analogs, both dioxygenation and hydrolytic pathways may be involved, and studies have revealed that the different pathways depended on the positions of the fluorine substituents. 

Compared with monocyclic aromatic hydrocarbons and the five-membered azaarenes, the pathways used for the degradation of pyridines are less uniform, and this is consistent with the differences in electronic structure and thereby their chemical reactivity. For pyridines, both hydroxylation and dioxygenation that is typical of aromatic compounds have been observed, although these are often accompanied by reduction of one or more of the double bonds in the pyridine ring. Examples are used to illustrate the metabolic possibilities. 

---