Energy transduction by reversible electron bifurcation

Abstract Electron bifurcation is a biological energy conversion process that oxidizes a two-electron donor at medium potential, coupled to the reduction of a high- and a low-potential acceptor species. This process is often fully reversible ( Δ G ≈ 0 ), allowing the creation of strong reductants with minimal free energy cost, using compounds with higher reduction potentials. For many years, the internal workings of electron bifurcating enzymes were poorly understood, especially regarding how short-circuit reactions are prevented. Recently, a conserved energy landscape was proposed to naturally insulate against short-circuit reactions, enabling efficient and reversible electron bifurcation (the EB scheme). We review the physical principles that underpin the EB scheme, describe how the EB scheme is distinct from previous views, and outline questions that remain open.