The dichotomy of complex I: A sodium ion pump or a proton pump

Complex I (NADH/quinone oxidoreductase) is the only enzyme of the membrane-bound respiratory chain to remain a “black box.” Atomic resolution structural models are available for three of the four energy-transducing respiratory complexes: the cytochrome bc ₁ complex, cytochrome oxidase, and the F₁ domain of ATP synthase. Detailed mechanisms that integrate these structural models with the results of spectroscopic and functional studies are therefore being constructed (1). In comparison, due to its large size and subunit complexity, and because its cofactors are difficult to distinguish spectroscopically, complex I has proved intractable. The complex is L shaped (2) and, although the exact cofactor content and ligation remain unconfirmed, complex I is known to contain a noncovalently bound flavin mononucleotide at the active site for NADH oxidation and a number of iron–sulfur clusters (3). Complex I plays a pivotal role in energy transduction. It is the entry point for electrons into the respiratory chain, and it contributes to the proton-motive force (Δμ_H+) across the inner-mitochondrial membrane, which is harnessed for the synthesis of ATP (1). In mitochondria, it is well established that complex I uses the free energy from transferring two electrons from NADH to ubiquinone to pump four protons across the inner-mitochondrial membrane (4, 5), and it has been widely assumed that all complexes I operate in the same way. In this issue of PNAS, Gemperli et al. (6) challenge this central tenet and present evidence that complex I from the enterobacterium Klebsiella pneumoniae pumps sodium ions rather than protons across the bacterial cytosolic membrane. In a reconstituted system, the sodium ion motive force (Δμ_Na⁺) supports ATP synthesis by the sodium ion driven ATP synthase from Ilyobacter tartaricus. Complex I from Escherichia coli, closely related to the K. pneumoniae enzyme, has also been proposed to …