A novel electron transfer mechanism suggested by crystallographic studies of mitochondrial cytochrome bc1 complex

The crystal structure of bovine mitochondrial cytochrome bc1 complex, an in tegral membrane protein complex of 11 different subunits with a total molec ular mass of 242 kDa, demonstrated a tightly associated dimer consisting of three major regions: a matrix region primarily made of subunits core1, cor e2, 6, and 9; a transmembrane-helix region of 26 helices in the dimer contr ibuted by cytochrome b, cytochrome c1, the Rieske iron-sulfur protein (ISP) , subunits 7, 10, and 11; and an intermembrane-space region composed of ext ramembrane domains of ISP, cytochrome c1, and subunit 8. The structure also revealed the positions of and distances between irons of prosthetic groups , and two symmetry related cavities in the transmembrane-helix region upon dimerization of the bc1 complex. Extensive crystallographic studies on crys tals of bc1 complexed with inhibitors of electron transfer identified bindi ng pockets for both Q(o) and Q(i) site inhibitors. Discrete binding sites f or subtypes of Q(o) site inhibitors have been mapped onto the Q(o) binding pocket, and bindings of different subtypes of Q(o) site inhibitors are capa ble of inducing dramatic conformational changes in the extramembrane domain of ISP. A novel electron transfer mechanism for the bc1 complex consistent with crystallographic observations is discussed.