Enzymatic reductions in the anaerobic aromatic at extremely low redox potentials

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In recent years, a number of unprecedented enzymatic reactions have been discovered in the aromatic metabolism of anaerobic bacteria. Two key enzymes of this metabolism, benzoyl-CoA reductase (BCR, dearomatizing) and 4-hydroxybenzoyl-CoA reductase (HCR, dehydroxylating), catalyze unique reductions at extremely low redox potentials in mechanisms similar to the Birch reduction in organic synthesis. BCR overcomes the high redox barrier by coupling the reductive dearomatization of benzoyl-CoA to a stoichiometrical ATP-hydrolysis. In contrast, the enzymatic dehydroxylation catalyzed by HCR is not coupled to an exergonic reaction. BCR contains three [4Fe-4S] clusters, one of which is supposed to be in special coordination with a disulfide species. HCR is an Fe-S/flavo/molybdo-protein belonging to the xanthine oxidase family. Structure/function relationships of both enzymes will be elucidated by protein crystallization/- X-Ray analysis and by a number of spectroscopic techniques. BCR and HCR will be studied in different facultative and strictly anaerobic bacteria degrading aromatic compounds. The results obtained should provide general insights into the mechanism of enzymatic redox processes at extremely low potentials.