A new hypothesis on the simultaneous direct and indirect proton pump mechanisms in NADH-quinone oxidoreductase (complex I)

Recently, Sazanov’s group reported the X-ray structure of whole complex I [Nature, 465, 441 (2010)], which presented a strong clue for a “piston-like” structure as a key element in an “indirect” proton pump. We have studied the NuoL subunit which has a high sequence similarity to Na+/H+ antiporters, as do the NuoM and N subunits. We constructed 27 site-directed NuoL mutants. Our data suggest that the H+/e− stoichiometry seems to have decreased from (4H+/2e−) in the wild-type to approximately (3H+/2e−) in NuoL mutants. We propose a revised hypothesis that each of the “direct” and the “indirect” proton pumps transports 2H+ per 2e−.