Aerobic Styrene Degradation

Under aerobic conditions, side-chain oxygenation of styrene seems to be the favored mechanism, since most microorganisms investigated in that respect were found to follow this degradation pathway (Hartmans et al. 1990; Cox et al. 1996; Itoh et al. 1996; Beltrametti et al. 1997; Panke et al. 1998; Velasco et al. 1998; Park et al. 2006b). In the first reaction step, styrene is oxygenated into styrene oxide by the action of a monooxygenase (Fig. 3.1). Styrene monooxygenases (SMOs) of bacteria are flavin-dependent, whereas this reaction is typically catalyzed by heme-containing cytochrome P450 monooxygenases in fungi (Cox et al. 1996). Bacterial styrene monooxygenases were shown to be highly enantioselective leading in almost all

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Fig. 3.1 Microbial pathways of aerobic styrene degradation. Characterized enzymes are indicated by their abbreviation

cases to the formation of the (S)-enantiomer of styrene oxide (Otto et al. 2004; van Hellemond et al. 2007; Tischler et al. 2009). Most commonly, the reactive oxirane is then isomerized to phenylacetaldehyde by a styrene oxide isomerase (SOI). In contrast to cytosolic SMOs, several studies indicated a membrane-bound localization of SOIs in Pseudomonas, Corynebacterium (Itoh et al. 1997a), and Rhodococcus (unpublished). The formed phenylacetaldehyde is then converted to phenylacetic acid by a phenylacetaldehyde dehydrogenase (PAD) (Fig. 3.1). It must be mentioned that E. coli strains are able to convert phenylacetaldehyde into 2-phenylethanol, too, which might interfere with studies using whole-cell assays (Beltrametti et al. 1997).

A slight modification of the above pathway comprises the additional action of both a styrene oxide reductase (SOR) and a phenylacetaldehyde reductase (PAR). Styrene oxide is reduced to 2-phenylethanol by SOR and oxidized to phenyl-acetaldehyde by PAR or another dehydrogenase. In Pseudomonas fluorescens ST, this variant was shown to be a side reaction (Marconi et al. 1996), whereas in Pseudomonas sp. 305-STR-1-4, Pseudomonas sp. Y2, and Xanthobacter sp. strain 124X, 2-phenylethanol was identified as one major metabolite (Shirai and Hisatsuka 1979; Hartmans et al. 1989; Utkin et al. 1991). According a current hypothesis, this route might belong to ethylbenzene degradation via 2-phenyl-ethanol and thus reflects an unspecific conversion of styrene by enzymes of this pathway.

As mentioned above, styrene metabolism by bacteria and fungi shares the initial step of monooxygenation. However, all following metabolic reactions in fungi differ (Braun-Lullemann et al. 1997) and are quite similar to styrene detoxification route in human (Warhurst and Fewson 1994; Rueff et al. 2009). Styrene oxide is hydrolyzed to phenylethan-1,2-diol by the action of an epoxide hydrolase (EH) and oxidized to mandelic acid by a dehydrogenase. Enzymatic decarboxylation then yields benzoic acid. Further, metabolites, like 2-phenylethanol, were detected and might be side products as observed for other organisms.

Initial dioxygenation of the aromatic nucleus and ring cleavage is another type of mechanism through which various bacteria degrade styrene (Bestetti et al. 1989; Hartmans et al. 1989; Warhurst et al. 1994; Patrauchan et al. 2008). In this case, a styrene 2,3-dioxygenase (SDO) introduces two oxygen atoms adjacent to the vinyl group and a styrene c/s-glycol is formed. Subsequently, a styrene-2,3-dihydrodiol dehydrogenase (SDD) catalyzes re-aromatization to 3-vinylcatechol. These steps are consistent with the peripheral pathways of benzene-, toluene- and ethylbenzene degradation (Smith 1990; Warhurst et al. 1994; Mars et al. 1997) which yield catechol, 3-methylcatechol, and 3-ethylcatechol, respectively, as the central intermediates. Since the involved 2,3-dioxygenases usually show a relatively high substrate tolerance, conversion of styrene may be the result of fortuitous metabolism. The central intermediate 3-vinylcatechol then may undergo ortho- or meta-cleavage by the action of a vinylcatechol 1,2-dioxygenase (VC12O) or a vinylcatechol 2,3-dioxygenase (VC23O) yielding

2-vinyl-c/s,c/s-muconic acid and 2-hydroxy-6-oxoocta-2,4,7-trienoic acid, respectively. Further degradation of 2-vinyl-c/s,c/s-muconate by ortho-pathway fails and as a result this compound accumulates as a dead-end metabolite (Warhurst et al. 1994). A similar observation was made for methylaromatics which are mineralized by most bacteria through the meta-cleavage pathway (Marin et al. 2010). For example, if 4-methylcatechol undergoes ortho-cleavage, 4-methylmuconolactone accumulates in the growth medium (Knackmuss et al. 1976). Therefore, the ortho-cleavage pathway is usually unsuited for the degradation of alkylcatechols.

In contrast, the intensively yellow-colored semialdehyde from meta-cleavage of

3-vinylcatechol seems to be subject of further turnover. First indication for the presence of a styrene-catabolic route by meta-cleavage may be drawn from the preliminary occurrence of a yellow-colored intermediate from a styrene-growing

Styrene Degradation
Fig. 3.2 Proposed steps of anaerobic styrene degradation pathways (adapted from Grbic-Galic et al. 1990). The herein shown routes for anaerobic styrene breakdown are based on identified metabolites from pure or mixed cultures. No data about involved enzymes are available so far

culture. However, co-existence of styrene catabolic pathways of side-chain oxygenation and direct ring attack might be possible (Hartmans et al. 1989).

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