Possible roles of two quinone molecules in direct and indirect proton pumps of bovine heart NADH-quinone oxidoreductase (complex I)

Abstract

In many energy transducing systems which couple electron and proton transport, for example, bacterial photosynthetic reaction center, cytochrome bc 1-complex (complex III) and E. coli quinol oxidase (cytochrome bo 3 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 SQ Nf (fast relaxing semiquinone) and SQ Ns (slow relaxing semiquinone). It was proposed that Q Nf serves as a “direct” proton carrier in the semiquinone-gated proton pump (Ohnishi and Salerno, FEBS Letters 579 (2005) 4555), while Q Ns works as a converter between one-electron and two-electron transport processes. This communication presents a revised hypothesis in which Q Nf plays a role in a “direct” redox-driven proton pump, while Q Ns triggers an “indirect” conformation-driven proton pump. Q Nf and Q Ns together serve as (1e −/2e −) converter, for the transfer of reducing equivalent to the Q-pool.