Enzymes of anaerobic metabolism of phenolic compounds

Abstract

The reductive removal of aromatic hydroxyl functions plays an important role in the anaerobic metabolism of many phenolic compounds. We describe a new enzyme from a denitrifying Pseudornonas sp., 4-hydroxybenzoyl-CoA reductase (dehydroxylating), which reductively dehydroxylates 4-hydroxybenzoyl-CoA to benzoyl-CoA. The enzyme plays a role in the anaerobic degradation of phenol, 4-hydroxybenzoate, p-cresol, 4-hydroxyphenylacetateate, and other aromatic compounds of which 4-hydroxybenzoyl-CoA is an intermediate. The enzyme is therefore induced only under anoxic conditions with these aromatic substrates, but not with benzoate or under aerobic conditions. A similar enzyme which reductively dehydroxylates 3-hydroxybenzoyl-CoA is induced during anaerobic growth with 3-hydroxybenzoate. The soluble enzyme 4-hydroxybenzoyl-CoA reductase was purified. It has a molecular mass of 260 kDa and consists of three subunits of 75, 35, and 17 kDa. The subunit composition is likely to be a2b2c2. The enzyme contains 12 mol irodmol and 12 mol acid-labile sulfur/mol and exhibits a typical ultraviolethisible spectrum of an iron-sulfur protein.The reaction requires a reduced electron donor such as reduced viologen dyes; no other co-catalysts are required, the product is benzoyl-CoA and oxidized dye. The reductase is rapidly inactivated by oxygen. The inactivation by low concentrations of cyanide or azide in a pseudo-first-order time course suggests that it may contain a transition metal in an oxidation state which reacts with these ligands. 4-Hydroxybenzoyl-CoA reductase represents a type of enzyme which is common in anaerobic aromatic metabolism of phenolic compounds. A similar enzyme is demonstrated in Rhodopseudornonas palustris anaerobically grown with 4-hydroxybenzoate. The biological significance of reductive dehydroxylation of aromatics and a possible reaction mechanism similar to the Birch reduction are discussed.Aromatic compounds comprise the second largest group of natural products. Most of the naturally occurring aromatics carry one or several hydroxyl functions which are often protected against undesired oxidation and other side reactions by methyl ether formation. They are metabolized by microorganisms following two fundamentally different strategies. Under aerobic conditions aromatic compounds are transformed by monooxygenases and dioxygenases into dihydroxylated intermediates such as catechol, protocatechuate, and gentisate. These dihydroxylated compounds are suitable for an oxidative cleavage of the aromatic ring (reviewed recently by [l]).Hydroxylation and oxidative ring cleavage by oxygenases cannot exist under anoxic conditions and therefore a different strategy is required for the anaerobic aromatic metabolism (for recent reviews see 22-61). Notably, in contrast to aerobic metabolism, hydroxylated aromatics are

Initial steps in the fermentation of 3-hydroxybenzoate by Sporotomaculum hydroxybenzoicum

“…The nucleophile could attack the benzene ring with subsequent removal of the hydroxy group. This concept is different from reduction of 4-hydroxybenzoyl-CoA to benzoylCoA by nitrate-reducing bacteria, which proceeds through two successive one-electron transfers with transient formation of an organic radical (Brackmann and Fuchs 1993;Buckel and Keese 1995;El Kasmi et al 1995).…”

“…Little is known so far about anaerobic degradation of 3-hydroxybenzoate. Although for this isomer, CoA-thioesterification (Heising et al 1991;Biegert et al 1993) and reductive dehydroxylation (Tschech and Schink 1986;Brackmann and Fuchs 1993) were assumed to be the first steps in anaerobic degradation, detailed studies on this pathway are still necessary.…”

Shedding light on anaerobic benzene ring degradation: a process unique to prokaryotes?

“…This enzymatic activity has been measured in extracts of R. palustris and the denitrifying species A. evansii, and 4-OHbenzoyl-CoA reductase (dehydroxylating) has been purified from T. aromatica (10). The enzyme is inactivated by oxygen and catalyzes the dehydroxylation of 4-OHbenzoylCoA by two successive one-electron transfer reactions (41).…”

“…The native enzyme has 12 Fe and 12 S atoms, suggesting that iron-sulfur centers participate in electron transfer. Investigators tested for molybdenum but did not find significant amounts of this transition metal in the purified enzyme (10).…”

Nonaromatic Products from Anoxic Conversion of Benzoyl-CoA with Benzoyl-CoA Reductase and Cyclohexa-1,5-diene-1-carbonyl-CoA Hydratase

“…Cells were harvested by centrifugation under anaerobic conditions and were stored in liquid nitrogen. Cell extract was prepared as described (17).…”