Crystallographic location of two Zn2+-binding sites in the avian cytochrome bc1 complex

AbstractThe chicken mitochondrial ubiquinol cytochrome c oxidoreductase (bc1 complex) is inhibited by Zn2+ ions, but with higher Ki (∼3 μM) than the corresponding bovine enzyme. When equilibrated with mother liquor containing 200 μM ZnCl2 for 7 days, the crystalline chicken bc1 complex specifically binds Zn2+ at 4 sites representing two sites on each monomer in the dimer. These two sites are close to the stigmatellin-binding site, taken to be center Qo of the Q-cycle mechanism, and are candidates for the inhibitory site. One binding site is actually in the hydrophobic channel between the Qo site and the bulk lipid phase, and may interfere with quinone binding. The other is in a hydrophilic area between cytochromes b and c1, and might interfere with the egress of protons from the Qo site to the intermembrane aqueous medium. No zinc was bound near the putative proteolytic active site of subunits 1 and 2 (homologous to mitochondrial processing peptidase) under these conditions