The membrane-extrinsic domain of cytochrome b_558/566 from the Archaeon Sulfolobus acidocaldarius performs pivoting movements with respect to the membrane surface

The orientation of the membrane-attached cytochrome b_558/566-haem with respect to the membrane was determined by electron paramagnetic resonance spectroscopy on two-dimensionally ordered oxidised membrane fragments from Sulfolobus acidocaldarius. Unlike the other redox centres in the membrane, the cytochrome b_558/566-haem was found to cover a range of orientations between 25° and 90°. The described results are reminiscent of those obtained on the Rieske cluster of bc complexes and indicate that the membrane-extrinsic domain of cytochrome b_558/566 can perform pivoting motion between two extreme positions. Such a conformational flexibility is likely to play a role in electron transfer with its redox partners.