4-Hydroxybenzoyl-CoA reductase (dehydroxylating) from the denitrifying bacterium Thauera aromatica--prosthetic groups, electron donor, and genes of a member of the molybdenum-flavin-iron-sulfur proteins

Abstract

4-Hydroxybenzoyl-CoA reductase catalyzes an important reaction in the anaerobic metabolism of phenolic compounds, i.e. the reductive removal of an aromatic hydroxyl group. The prosthetic groups and the natural electron donor of the enzyme were investigated and the genes were cloned and sequenced. The enzyme is a molybdenum-flavin-iron-sulfur protein of subunit composition of alpha2beta2gamma2. It contains approximately 1.3 flavin nucleotide, probably FAD, 1.9 Mo, 15 Fe, and 12.5 acid-labile sulfur. Sequence interpretation suggests that the native enzyme contains two [4Fe-4S] and four [2Fe-2S] clusters. A 9.8-kDa ferredoxin with two [4Fe-4S] clusters functions as the natural electron donor. The genes coding for the three subunits, hcrABC, show high similarities to other molybdenum-flavin-iron-sulfur proteins of the xanthine oxidase family, notably to the three putative 4-hydroxybenzoyl-CoA reductase genes in Rhodopseudomonas palustris. In addition, there are close similarities to three open reading frames (orf) in E. coli. A major difference is the presence of an additional domain in the beta-subunit (HcrB, 35 kDa) probably carrying an additional iron-sulfur cluster. The 82-kDa alpha-subunit (HcrA) contains a Mo-cofactor-binding site. The 17-kDa gamma-subunit (HcrC) harbors two [2Fe-2S] clusters. Upstream of the hcrCAB region, an ORF was found coding for a regulatory protein of the MarR family. Downstream of the hcrCAB region lies an ORF presumably coding for a hydrophobic permease.