Uncovering the [2Fe2S] domain movement in cytochrome bc_1 and its implications for energy conversion

摘要

In crystals of the key respiratory and photosynthetic electron transfer protein called ubihydroquinone:cytochrome (cyt) c oxi- doreductase or cyt bc_1, 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 bc_l complex (and possibly the homologous b_6f complex in chloro- plasts) employs the [2Fe2S] cluster domain as a device to shuttle electrons from ubihydroquinone to cyt c_1 (or cyt f). We demon- strate that this domain movement is essential for cyt bc_1 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 fre- quency slow enough to assure productive Q_o site charge separa- tion 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 bc_1 antibiotics.