A High-Potential Redox Landscape Bump Enhances Electron Bifurcation Efficiency

Electron bifurcation (EB) is an elegant mechanism that underpins biological energy transduction and catalysis. EB proteins separate electron pairs from a common source, delivering them to high- and low-potential acceptor pools, typically at low thermodynamic cost. The EB enzyme NADH-dependent ferredoxin-NADP+ oxidoreductase I (NfnI) contains a conspicuously high-potential iron-sulfur cluster cofactor (H1) proximal to the bifurcating flavin. We explore how this bump in the redox landscape influences energy transduction in the enzyme by modeling the multielectron flux in NfnI using a fully correlated, three-reservoir master equation approach. We find that the reduction potential bump at the H1 cofactor enhances EB efficiency by suppressing energy-dissipating electron short-circuiting (by enhancing the occupancy effect, which is known to suppress short-circuiting). While the reduction potential bump at H1 increases EB efficiency, it also decreases EB flux. Our findings underscore a structure-function principle in natural electron bifurcating proteins and provide insight into strategies to design and control charge flow in de novo proteins.