4‐Hydroxybenzoyl‐CoenzymeAReductase

Abstract

The dehydroxylating 4‐hydroxybenzoyl‐CoA reductase (HBCR) plays an important role in the degradation of para ‐hydroxylated aromatic compounds in anaerobic bacteria. The enzyme catalyzes the removal of the phenolic hydroxyl group from 4‐hydroxybenzoyl‐CoA, yielding benzoyl‐CoA and water; a reduced ferredoxin serves as an electron donor. The enzyme belongs to the xanthine oxidase (XO) family of molybdenum cofactor containing enzymes and contains a molybdopterin‐cytosine dinucleotide cofactor, two [2Fe–2S] clusters, FAD and a [4Fe–4S] cluster per functional unit. HBCR is unique among XO family members in catalyzing the irreversible reduction of the substrate. A mechanism that differs from all other XO members via highly reactive radical intermediates has been suggested. The structural, electrochemical, spectroscopic, and kinetic properties of HBCR from Thauera aromatica are highlighted and compared with other members of the xanthine oxidase family.