In many energy transducing systems which couple electron and proton transport, for example, bacterial photosynthetic reaction center, cytochrome bc1-complex (complex III) and E. coli quinol oxidase (cytochrome bo3 complex), two protein-associated quinone molecules are known to work together. T. Ohnishi and her collaborators reported that two distinct semiquinone species also play important roles in NADH-ubiquinone oxidoreductase (complex I). They were called SQNf (fast relaxing semiquinone) and SQNs (slow relaxing semiquinone). It was proposed that QNf serves as a "direct" proton carrier in the semiquinone-gated proton pump (Ohnishi and Salerno, FEBS Letters 579 (2005) 4555), while QNs works as a converter between one-electron and two-electron transport processes. This communication presents a revised hypothesis in which QNf plays a role in a "direct" redox-driven proton pump, while QNs triggers an "indirect" conformation-driven proton pump. QNf and QNs together serve as (1e-/2e-) converter, for the transfer of reducing equivalent to the Q-pool. © 2010 Elsevier B.V.
Ohnishi, S. T., Salerno, J. C., & Ohnishi, T. (2010). Possible roles of two quinone molecules in direct and indirect proton pumps of bovine heart NADH-quinone oxidoreductase (complex I). Biochimica et Biophysica Acta - Bioenergetics, 1797(12), 1891–1893. https://doi.org/10.1016/j.bbabio.2010.06.010