Unusual reactions involved in anaerobic metabolism of phenolic compounds

Abstract

Aerobic bacteria use molecular oxygen as a common co-substrate for key enzymes of aromatic metabolism. In contrast, in anaerobes all oxygen-dependent reactions are replaced by a set of alternative enzymatic processes. The anaerobic degradation of phenol to a non-aromatic product involves enzymatic processes that are uniquely found in the aromatic metabolism of anaerobic bacteria: (i) ATP-dependent phenol carboxylation to 4-hydroxybenzoate via a phenylphosphate intermediate (biological Kolbe-Schmitt carboxylation); (ii) reductive dehydroxylation of 4-hydroxybenzoyl-CoA to benzoyl-CoA; and (iii) ATP-dependent reductive dearomatization of the key intermediate benzoyl-CoA in a 'Birch-like' reduction mechanism. This review summarizes the results of recent mechanistic studies of the enzymes involved in these three key reactions.

Heterologous Expression and Identification of the Genes Involved in Anaerobic Degradation of 1,3-Dihydroxybenzene (Resorcinol) in Azoarcus anaerobius

“…In these experiments, nitrate was used as the electron acceptor for two reasons. In a previous study, it was shown that (i) oxidation of resorcinol and HHQ was dependent on electron acceptors with a positive redox potential such as K 3 Fe(CN) 6 and nitrate (32) and (ii) nitrate was the only electron acceptor that did not react chemically with HHQ (32). Nitrate was used in excess to provide an electron acceptor for the first two reactions, namely, resorcinol hydroxylation and HHQ dehydrogenation, and to provide an electron acceptor for potential oxidations involved in the further degradation of HBQ.…”

“…Resorcinol hydroxylase and hydroxyhydroquinone dehydrogenase enzyme activities were measured at 30°C under anoxic conditions as previously described (32), except that potassium phosphate buffer (50 mM, pH 7.0) was used as the buffer in all of the assays. A standard reaction mixture used to assay resorcinol hydroxylase contained 0.2 to 0.3 mg of protein, 1 mM K 3 Fe(CN) 6 , and 1 mM resorcinol. A standard reaction mixture used to assay hydroxyhydroquinone dehydrogenase contained 0.2 to 0.3 mg of protein, 1 mM NaNO 3 , and 1 mM resorcinol.…”

“…While aerobes dearomatize the benzene ring by using molecular oxygen as a cosubstrate for monooxygenases and dioxygenases, anaerobes have to replace all of the oxygen-dependent steps with alternative sets of reactions (6,16,21,36). Notably, in anaerobic pathways, the aromatic ring is reduced rather than oxidized and benzoyl coenzyme A (CoA) emerges as the most common intermediate in the degradation of a large diversity of aromatic substrates (4,5,7,21,36,46).…”

“…uses reductive biochemistry to convert resorcinol to cyclohexanedione (26,44), which is further hydrolyzed to 5-oxohexanoate (36), Anaerobius anaerobius, a nitrate-reducing bacterium, uses two oxidative reactions (32). Resorcinol is hydroxylated at position 4 of the aromatic ring with K 3 Fe(CN) 6 or nitrate as an electron acceptor to form hydroxyhydroquinone (HHQ) in a reaction catalyzed by resorcinol hydroxylase. HHQ is subsequently oxidized with nitrate to HBQ (2-hydroxy-1,4-benzoquinone) in a reaction catalyzed by hydroxyhydroquinone dehydrogenase (32).…”

Phenol Degradation in the Strictly Anaerobic Iron-Reducing Bacterium Geobacter metallireducens GS-15

“…In this strain, phenol is initially converted to phenylphosphate by phenylphosphate synthase (Pps) with concomitant hydrolysis of ATP (5,16,28) (Fig. 1A).…”