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Review
, 78 (15), 5043-51

Epoxy Coenzyme A Thioester Pathways for Degradation of Aromatic Compounds

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Review

Epoxy Coenzyme A Thioester Pathways for Degradation of Aromatic Compounds

Wael Ismail et al. Appl Environ Microbiol.

Abstract

Aromatic compounds (biogenic and anthropogenic) are abundant in the biosphere. Some of them are well-known environmental pollutants. Although the aromatic nucleus is relatively recalcitrant, microorganisms have developed various catabolic routes that enable complete biodegradation of aromatic compounds. The adopted degradation pathways depend on the availability of oxygen. Under oxic conditions, microorganisms utilize oxygen as a cosubstrate to activate and cleave the aromatic ring. In contrast, under anoxic conditions, the aromatic compounds are transformed to coenzyme A (CoA) thioesters followed by energy-consuming reduction of the ring. Eventually, the dearomatized ring is opened via a hydrolytic mechanism. Recently, novel catabolic pathways for the aerobic degradation of aromatic compounds were elucidated that differ significantly from the established catabolic routes. The new pathways were investigated in detail for the aerobic bacterial degradation of benzoate and phenylacetate. In both cases, the pathway is initiated by transforming the substrate to a CoA thioester and all the intermediates are bound by CoA. The subsequent reactions involve epoxidation of the aromatic ring followed by hydrolytic ring cleavage. Here we discuss the novel pathways, with a particular focus on their unique features and occurrence as well as ecological significance.

Figures

Fig 1
Fig 1
Overview of established degradation routes for benzoate that operate under oxic (A) or anoxic (B) conditions. (A) Aerobic degradation strategy based on hydroxylation of the aromatic ring and subsequent cleavage between hydroxyl groups (intradiol) or adjacent to the hydroxyl groups (extradiol). Intradiol cleavage of protocatechuate or catechol leads to β-ketoadipate as a characteristic intermediate. (B) Anaerobic degradation of aromatic compounds proceeds via benzoyl-CoA, a characteristic intermediate. The dearomatization step is strictly endergonic. As an example, ATP usage as it occurs in Thauera aromatica is shown. Dashed arrows indicate that two or more enzymatic steps occur between the two depicted intermediates. Compound 1, benzoate; 2, 4-hydroxybenzoate; 3, protocatechuate; 4, 3-carboxy-cis,cis-muconate; 5, β-ketoadipate; 6, 1,2-dihydroxycyclohexa-3,5-diene-1-carboxylate; 7, catechol; 8, cis,cis-muconate; 9, 2-hydroxymuconic semialdehyde; 10, 2-hydroxy-4-carboxymuconic semialdehyde; 11, benzoyl-CoA; 12, 1,5-dienoyl-CoA; 13, 3-hydroxypimelyl-CoA.
Fig 2
Fig 2
The aerobic phenylacetate (PA) catabolic pathway. Arrows pointing toward PA indicate some aromatic compounds that are degraded via PA. The thick arrow points toward a hypothetical intermediate. Dashed arrows indicate the dead-end products that were detected in studies on aerobic PA degradation. EB, ethylbenzene; STY, styrene; Phe, phenylalanine; CA, cinnamic acid; 2HPA, 2-hydroxyphenylacetate (dead-end product); HYP, hypothetical intermediate (cis-dihydrodiol derivative of PACoA); LACT, 1,2-dihydroxy-1,2-dihydrophenylacetyl lactone (dead-end product); HS-CoA, coenzyme A; PACoA, phenylacetyl-CoA; Ep-PACoA, ring-1,2-epoxy-PACoA; Oxepin-CoA, 2-oxepin-2 (3H)-ylideneacetyl-CoA; PaaK, PA-CoA ligase; PaaABCE, ring-1,2-PACoA epoxidase; PaaG, ring-1,2-epoxy-PACoA isomerase; PaaZ, oxepin-CoA hydrolase/3-oxo-5,6-dehydrosuberyl-CoA semialdehyde dehydrogenase; PaaZ-ECHD, enoyl-CoA hydratase domain of the ring cleavage enzyme PaaZ; PaaZ-ALDH, aldehyde dehydrogenase domain of the ring cleavage enzyme PaaZ; PaaFGHJ, enzymes catalyzing β-oxidation-like reactions.
Fig 3
Fig 3
The epoxybenzoyl-CoA pathway and the putative catalytic steps of the key reactions. Reactions catalyzed by BoxAB, BoxC, and BoxD are indicated. Compound 1, benzoate; 2, benzoyl-CoA; 3, epoxybenzoyl-CoA; 4, 3,4-dehydroadipyl-CoA-semialdehyde; 5, 3,4-dehydroadipyl-CoA; 6, 2,3-dehydroadipyl-CoA; 7, β-hydroxyadipyl-CoA; 8, β-ketoadipyl-CoA.

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