Possible Roles of Two Quinone Molecules in Direct and Indirect Proton Pumps of Bovine Heart NADH-Quinone Oxidoreductase (Complex I)
Molecular and Cellular Biology
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.
Biochimica et Biophysica Acta (BBA) - Bioenergetics
Digital Object Identifier (DOI)