Uncovering the [2Fe2S] domain movement in cytochrome bc 1 and its implications for energy conversion

Abstract

In crystals of the key respiratory and photosynthetic electron transfer protein called ubihydroquinone:cytochrome (cyt) c oxidoreductase or cyt bc₁, the extrinsic [2Fe2S] cluster domain of its Fe-S subunit assumes several conformations, suggesting that it may move during catalysis. Herein, using Rhodobacter capsulatus mutants that have modifications in the hinge region of this subunit, we were able to reveal this motion kinetically. Thus, the bc₁ complex (and possibly the homologous b₆f complex in chloroplasts) employs the [2Fe2S] cluster domain as a device to shuttle electrons from ubihydroquinone to cyt c₁ (or cyt f). We demonstrate that this domain movement is essential for cyt bc₁ function, because a mutant enzyme with a nonmoving Fe-S subunit has no catalytic activity, and one with a slower movement has lower activity. This motion is apparently designed with a natural frequency slow enough to assure productive Q_o site charge separation but fast enough not to be rate limiting. These findings add the unprecedented function of intracomplex electron shuttling to large-scale domain motions in proteins and may well provide a target for cyt bc₁ antibiotics.