Molecular strategies for flux control in an electron bifurcation protein

Electron-bifurcating flavoproteins split electron pairs and deposit them in spatially separated electron-acceptor pools. Just how electron-transfer networks regulate steady-state electron-bifurcation (EB) fluxes and efficiencies remains unclear. We model the steady-state kinetics of electron-bifurcation in NADH-dependent ferredoxin-NADP[Formula: see text] oxidoreductase I (Nfn1), based on the protein's structure and electrochemistry. We use a many-particle master-equation to simulate the steady-state kinetics. The analysis identifies control points that are defined by electronic couplings between cofactors, and we explore how the interactions between cofactors influence EB function. We find that couplings on the low-potential branch dictate EB efficiency and the onset of short-circuiting, while the couplings on the high-potential branch determine the magnitude of the steady-state bifurcation flux in Nfn1. Analysis of interactions using the tunneling pathway model indicates that pathway connectivity between the bifurcating donor and its high-potential acceptor is weak for its distance; this decoupling may serve to further suppress short-circuiting and favor productive electron flow.