Coupling of electron-bifurcation modules powers aromatic ring reduction beyond the biological redox window
Microbial degradation of ubiquitous aromatic compounds is central to the global carbon cycle and bioremediation, yet the intrinsic stability of aromatic rings poses a major barrier to their breakdown. Under strictly anaerobic conditions, class II benzoyl-CoA reductases (BCRII) catalyse the key step of this process, a Birch-like reduction of the aromatic ring to a cyclic diene at a tungsten cofactor. This reaction operates beyond the redox limits of conventional biological electron transfer, yet the mechanism by which BCRII generates such extreme reducing power has remained unclear. Here, high-resolution cryo-electron microscopy, in situ cryo-electron tomography, and enzymatic analyses reveal that the one-MDa BCRII complex from Geobacter metallireducens links two distinct flavin-based electron-bifurcation modules, previously characterised in hydrogenases and heterodisulfide reductases, to drive aromatic ring reduction. Reduced ferredoxin and NADH deliver electrons through sequential confurcation and bifurcation to the catalytic site, while cryo-electron tomography of native cells identifies electron-transferring flavoprotein as the second acceptor directing high-potential electrons into the respiratory chain. These results show that modular FBEB units can be hierarchically assembled to extend metabolic redox capacity, highlighting their versatility as adaptable components for electron transfer pathways.