Anaerobic metabolism of l -phenylalanine via benzoyl-CoA in the denitrifying bacterium Thauera aromatica

Abstract

The anaerobic metabolism of phenylalanine was studied in the denitrifying bacterium Thauera aromatica, a member of the beta-subclass of the Proteobacteria. Phenylalanine was completely oxidized and served as the sole source of cell carbon. Evidence is presented that degradation proceeds via benzoyl-CoA as the central aromatic intermediate; the aromatic ring-reducing enzyme benzoyl-CoA reductase was present in cells grown on phenylalanine. Intermediates in phenylalanine oxidation to benzoyl-CoA were phenylpyruvate, phenylacetaldehyde, phenylacetate, phenylacetyl-CoA, and phenylglyoxylate. The required enzymes were detected in extracts of cells grown with phenylalanine and nitrate. Oxidation of phenylalanine to benzoyl-CoA was catalyzed by phenylalanine transaminase, phenylpyruvate decarboxylase, phenylacetaldehyde dehydrogenase (NAD+), phenylacetate-CoA ligase (AMP-forming), enzyme(s) oxidizing phenylacetyl-CoA to phenylglyoxylate with nitrate, and phenylglyoxylate:acceptor oxidoreductase. The capacity for phenylalanine oxidation to phenylacetate was induced during growth with phenylalanine. Evidence is provided that alpha-oxidation of phenylacetyl-CoA is catalyzed by a membrane-bound enzyme. This is the first report on the complete anaerobic degradation of an aromatic amino acid and the regulation of this process.

Anaerobic Catabolism of Aromatic Compounds: a Genetic and Genomic View

“…Some of the genes that are likely to be involved in the fermentation of phenylalanine to phenylacetate via phenyllactate and cinnamate have been identified for some Clostridium strains (87). In contrast to fermenting bacteria, phototrophs and anaerobic respirers can further oxidize the aromatic ring of the phenylacetate and 4-hydroxyphenylacetate generated during the anaerobic catabolism of phenylalanine and tyrosine, respectively (324).…”

“…Anaerobic phenylalanine degradation was suggested to proceed in T. aromatica via transamination and decarboxylation to phenylacetaldehyde and then via dehydrogenation to phenylacetate (324) (Fig. 8).…”

“…The initial transamination of phenylalanine to phenylpyruvate is catalyzed by the phenylalanine aminotransferase (Pat). The transaminase uses 2-oxoglutarate as a preferential cosubstrate (324). The second step in phenylalanine degradation is performed by phenylpyruvate decarboxylase (Pdc), which generates phenylacetaldehyde from phenylpyruvate.…”

“…The second step in phenylalanine degradation is performed by phenylpyruvate decarboxylase (Pdc), which generates phenylacetaldehyde from phenylpyruvate. Phenylacetaldehyde is then oxidized to phenylacetate (324). The predicted proteins of the anaerobic phenylalanine degradation pathway were identified by in silico analysis of the Azoarcus sp.…”

Possibility of Bacterial Recruitment of Plant Genes Associated with the Biosynthesis of Secondary Metabolites

“…However, there is evidence for two PAL independent pathways to benzoate: the anaerobic degradation of l-Phe (Schneider et al, 1997;Breese et al, 1998) and, similar to plants, a benzoate biosynthesis directly from shikimate (Grond et al, 2000).…”