Electron bifurcation: progress and grand challenges.

Electron bifurcation moves electrons from a two-electron donor to reduce two spatially separated one-electron acceptors. If one of the electrons reduces a high-potential (lower energy) acceptor, then the other electron may proceed "uphill" to reduce a low-potential (higher energy) acceptor. This mechanism is now considered the third mode of energy transduction in biology, and offers promise for the development of novel bioinspired energy conversion strategies. Nature uses electron bifurcation to realize highly sought-after reactions: reversible CO2 reduction, nitrogen fixation, and hydrogen production. In this review, we summarize the current understanding of electron bifurcation, including both recent progress and outstanding questions in understanding and developing artificial electron bifurcation systems.