Conformation-driven and semiquinone-gated proton-pump mechanism in the NADH-ubiquinone oxidoreductase (complex I)

A novel mechanism for proton/electron transfer is proposed for NADH-quinone oxidoreductase (complex I) based on the following findings: (1) EPR signals of the protein-bound fast-relaxing semiquinone anion radicals (abbreviated as QNf-) are observable only in the presence of proton-transmembrane electrochemical potential; (2) Iron–sulfur cluster N2 and QNf- are directly spin-coupled; and (3) The projection of the interspin vector extends only 5 Å along the membrane normal [Yano, T., Dunham, W.R. and Ohnishi, T. (2005) Biochemistry, 44, 1744–1754].We propose that the proton pump is operated by redox-driven conformational changes of the quinone binding protein. In the input state, semiquinone is reduced to quinol, acquiring two protons from the N (matrix) side of the mitochondrial inner membrane and an electron from the low potential (NADH) side of the respiratory chain. A conformational change brings the protons into position for release at the P (inter-membrane space) side of the membrane via a proton-well. Concomitantly, an electron is donated to the quinone pool at the high potential side of the coupling site. The system then returns to the original state to repeat the cycle. This hypothesis provides a useful frame work for further investigation of the mechanism of proton translocation in complex I.