Design and use of photoactive ruthenium complexes to study electron transfer within cytochrome bc1 and from cytochrome bc1 to cytochrome c.

The cytochrome bc1 complex (ubiquinone:cytochrome c oxidoreductase) is the central integral membrane protein in the mitochondrial respiratory chain as well as the electron-transfer chains of many respiratory and photosynthetic prokaryotes. Based on X-ray crystallographic studies of cytochrome bc1, a mechanism has been proposed in which the extrinsic domain of the iron-sulfur protein first binds to cytochrome b where it accepts an electron from ubiquinol in the Qo site, and then rotates by 57° to a position close to cytochrome c1 where it transfers an electron to cytochrome c1. This review describes the development of a ruthenium photooxidation technique to measure key electron transfer steps in cytochrome bc1, including rapid electron transfer from the iron-sulfur protein to cytochrome c1. It was discovered that this reaction is rate-limited by the rotational dynamics of the iron-sulfur protein rather than true electron transfer. A conformational linkage between the occupant of the Qo ubiquinol binding site and the rotational dynamics of the iron-sulfur protein was discovered which could play a role in the bifurcated oxidation of ubiquinol. A ruthenium photoexcitation method is also described for the measurement of electron transfer from cytochrome c1 to cytochrome c. This article is part of a Special Issue entitled: Respiratory Complex III and related bc complexes.