title: Reversible biological Birch reduction at an extremely low redox potential.
abstract: The Birch reduction of aromatic rings to cyclohexadiene compounds is widely used in chemical synthesis and requires solvated electrons, the most potent reductants known in organic chemistry. Benzoyl-coenzyme A (CoA) reductases (BCR) are key enzymes in the anaerobic bacterial degradation of aromatic compounds and catalyze an analogous reaction under physiological conditions. Class I BCRs are FeS enzymes and couple the reductive dearomatization of benzoyl-CoA to cyclohexa-1,5-diene-1-carboxyl-CoA (dienoyl-CoA) to a stoichiometric ATP hydrolysis. Here, we report on a tungsten-containing class II BCR from Geobacter metallireducens that catalyzed the fully reversible, ATP-independent dearomatization of benzoyl-CoA to dienoyl-CoA. BCR additionally catalyzed the disproportionation of dienoyl-CoA to benzoyl-CoA/monoenoyl-CoA and the four- and six-electron reduction of benzoyl-CoA in the presence of a reduced low-potential bridged 2,2'-bipyridyl redox dye. Reversible redox titration experiments in the presence of this redox dye revealed a midpoint potential of E(0)' = -622 mV for the benzoyl-CoA/dienoyl-CoA couple, which is far below the values of other known reversible substrate/product redox couples in enzymology. This work demonstrates the efficiency of reversible metalloenzyme catalysis, which in chemical synthesis can only be achieved under essentially irreversible conditions.
authors.affiliation: Institute of Biochemistry, University of Leipzig, 04103 Leipzig, Germany.
authors.forename: Johannes W
authors.initials: JW
authors.lastname: Kung
authors.forename: Sven
authors.initials: S
authors.lastname: Baumann
authors.forename: Martin
authors.initials: M
authors.lastname: von Bergen
authors.forename: Michael
authors.initials: M
authors.lastname: Müller
authors.forename: Peter-Leon
authors.initials: PL
authors.lastname: Hagedoorn
authors.forename: Wilfred R
authors.initials: WR
authors.lastname: Hagen
authors.forename: Matthias
authors.initials: M
authors.lastname: Boll
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D002384:Catalysis; D004798:Enzymes / Q000378:metabolism*; D006868:Hydrolysis; D010084:Oxidation-Reduction; D013056:Spectrophotometry, Ultraviolet
chemical_list: D004798:Enzymes; D000255:Adenosine Triphosphate