EPR detection of two protein-associated ubiquinone components (SQNf and SQNs) in the membrane in situ and in proteoliposomes of isolated bovine heart complex I

Abstract

The success of Sazanov's group in determining the X-ray structure of the whole bacterial complex I is a great contribution to the progress of complex I research. In this mini-review of 35years' history of my laboratory and collaborators, we characterized the function of protein-associated semiquinone molecules in the proton-pumping mechanism in complex I (NADH-quinone oxidoreductase). We have constructed most of the frame work of our hypothesis, utilizing EPR techniques before the X-ray structures of complex I were reported by Sazanov's and Brandt's groups. One of the semiquinones (SQNf) is extremely sensitive to a proton motive force imposed on the energy-transducing membrane, while the other (SQNs) is insensitive. Their sensitivity to rotenone inhibition also differs. These differences were exploited using tightly coupled bovine heart submitochondrial particles with a high respiratory control ratio (>8). We determined the distance between SQNf and iron–sulfur cluster N2 on the basis of their direct spin–spin interaction. We are extending this line of work using reconstituted bovine heart complex I proteoliposomes which shows a respiratory control ratio >5. Two frontier research groups support our view point based on their mutagenesis studies. High frequency (33.9GHz; Q-band) EPR experiments appear to favor our two-semiquinone model. This article is part of a Special Issue entitled: 17th European Bioenergetics Conference (EBEC 2012).