Structural basis of enzymatic benzene ring reduction.

In chemical synthesis, the widely used Birch reduction of aromatic compounds to cyclic dienes requires alkali metals in ammonia as extremely low-potential electron donors. An analogous reaction is catalyzed by benzoyl-coenzyme A reductases (BCRs) that have a key role in the globally important bacterial degradation of aromatic compounds at anoxic sites. Because of the lack of structural information, the catalytic mechanism of enzymatic benzene ring reduction remained obscure. Here, we present the structural characterization of a dearomatizing BCR containing an unprecedented tungsten cofactor that transfers electrons to the benzene ring in an aprotic cavity. Substrate binding induces proton transfer from the bulk solvent to the active site by expelling a Zn(2+) that is crucial for active site encapsulation. Our results shed light on the structural basis of an electron transfer process at the negative redox potential limit in biology. They open the door for biological or biomimetic alternatives to a basic chemical synthetic tool.

Reversible biological Birch reduction at an extremely low redox potential.

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.