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 Pseudomonas 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‐hydroxyphenylacetate, 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 a 2 b 2 c 2 . The enzyme contains 12 mol iron/mol and 12 mol acid‐labile sulfur/mol and exhibits a typical ultraviolet/visible 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 Rhodopseudomonas 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.