Bacterial phenylalanine and phenylacetate catabolic pathway revealed

Abstract

Aromatic compounds constitute the second most abundant class of organic substrates and environmental pollutants, a substantial part of which (e.g., phenylalanine or styrene) is metabolized by bacteria via phenylacetate. Surprisingly, the bacterial catabolism of phenylalanine and phenylacetate remained an unsolved problem. Although a phenylacetate metabolic gene cluster had been identified, the underlying biochemistry remained largely unknown. Here we elucidate the catabolic pathway functioning in 16% of all bacteria whose genome has been sequenced, including Escherichia coli and Pseudomonas putida. This strategy is exceptional in several aspects. Intermediates are processed as CoA thioesters, and the aromatic ring of phenylacetyl-CoA becomes activated to a ring 1,2-epoxide by a distinct multicomponent oxygenase. The reactive nonaromatic epoxide is isomerized to a seven-member O-heterocyclic enol ether, an oxepin. This isomerization is followed by hydrolytic ring cleavage and β-oxidation steps, leading to acetyl-CoA and succinyl-CoA. This widespread paradigm differs significantly from the established chemistry of aerobic aromatic catabolism, thus widening our view of how organisms exploit such inert substrates. It provides insight into the natural remediation of man-made environmental contaminants such as styrene. Furthermore, this pathway occurs in various pathogens, where its reactive early intermediates may contribute to virulence.enoyl-CoA hydratase | epoxide | oxepin | oxygenase | phenylacetic acid T he biggest challenge for organisms using aromatic compounds as growth substrates is to overcome the stabilizing resonance energy of the aromatic ring system. This aromatic structure makes the substrates unreactive toward oxidation or reduction and thus requires elaborate degradation strategies. How microorganisms cope with this problem depends primarily on the availability of oxygen (1). Aerobic pathways use oxygen both for hydroxylation and for cleavage of the ring (2, 3). In contrast, under anaerobic conditions the common strategy consists of activation by CoA-thioester formation, shortening of the side chain, and energy-driven ring reduction, which also applies to phenylacetate catabolism (ref. 4 and literature cited therein).The aerobic strategy is illustrated by the metabolism of phenylacetate and phenylacetyl-CoA, which are derived from a variety of substrates such as phenylalanine, lignin-related phenylpropane units, 2-phenylethylamine, phenylalkanoic acids with an even number of carbon atoms, or even environmental contaminants such as styrene and ethylbenzene (5-7). Rarely, phenylalanine is hydroxylated to tyrosine, which can be converted into 4-hydroxyphenylpyruvate, followed by hydroxylation to homogentisate (2,5-dihydroxyphenylacetate) as the central intermediate. The aromatic ring of homogentisate then is split by a ring-cleaving homogentisate dioxygenase, and finally fumarate and acetoacetate are produced (8). In most cases, however, phenylalanine is converted into phenylacetate. A conventional aero...

Phaeobacter gallaeciensis genomes from globally opposite locations reveal high similarity of adaptation to surface life

“…A proposed model for the biosynthesis of TDA in DSM 17395 is presented in Figure 6, combining the results of this study with those of other publications (Thiel et al, 2010;Teufel et al, 2011). The Paa enzymes of the phenylacetyl-CoA pathway produce compound 8 (Figure 6), which has been proposed as P. gallaeciensis genomes from globally opposite locations S Thole et al P. gallaeciensis genomes from globally opposite locations S Thole et al a precursor of TDA (Cane et al, 1992;Teufel et al, 2010;Teufel et al, 2011). Disruption of the genes paaACDE led to complete loss of TDA production, which corresponds to data obtained for Silicibacter spp.…”

Genetic and chemical characterization of ibuprofen degradation by Sphingomonas Ibu-2

“…Ibu-2 performs similar reactions with other arylacetic acids, including PAA, 2-phenylpropionic acid, 3-and 4-tolylacetic acids, and 2-(4-tolyl)propionic acid, converting them to the corresponding catechol (or methylcatechol). Although this is somewhat reminiscent of the removal of the carboxyl moiety from benzoate (Eaton, 1996;Fetzner et al, 1992;Jeffrey et al, 1992;Reiner, 1971), it differs from the previously characterized PAA pathways of other bacteria, which do not involve catecholic intermediates (Fernández et al, 2006;Ismail et al, 2003;Martínez-Blanco et al, 1990;El-Said Mohamed, 2000;Rost et al, 2002;Teufel et al, 2010).…”

Bacterial Degradation of Benzoate

“…benzoate, phenylacetate, and 2-aminobenzoate) that incorporates features of both the classical aerobic and anaerobic pathways. These aerobic hybrid pathways start with the CoA-dependent activation of the aromatic acids, but then the dearomatization step requires molecular oxygen and the mechanism of ring cleavage is hydrolytic rather than oxygenolytic (3,(7)(8)(9).…”