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The motion control of the iron‐sulfur‐protein in complex III

Bifurcated electron flow is the basis of the Q‐cycle hypothesis which describes the double efficiency of the redox chemistry of ubiquinol/ubiquinone coupled with vectorial proton transport by complex III (bc1). The separation of the two electrons in ubiquinol oxidation into two distinct paths is achieved by bridging the quinol oxidation site and the high‐potential oxidant (cytochrome c1, f) by a controlled swivel motion of the one‐electron carrier the iron‐sulfur‐ protein's extrinsic domain (ISP‐ED). While the basic mobility of the ISP‐ED has received much experimental support it is still controversial whether its motion is under a controlling influence of the substrate or a purely statistical event. Here we present structural and biochemical evidence from mitochondrial and bacterial forms of bc1 that suggests that the capture of the ISP‐ED at the quinol oxidation site is part of the mechanism and not an accidental consequence of any set of hydrogen bonds that might have formed. This evidence involves structural information from inhibited mitochondrial and bacterial bc1 with different types of inhibitors that show clear and consistent patterns of mobile or bound ISP‐ED with or without forming direct ISP‐ED‐inhibitor hydrogen bonds. Furthermore mutational data on Rhodobacter spheroides bc1 shows a variation in specific activity and kinetics that likewise points towards direct motion control of the ISP‐ED. This work was supported by the NIH intramural research program.