Crystallographic location of two Zn2+-binding sites in the avian cytochrome bc(1) complex

The chicken mitochondrial ubiquinol cytochrome c oxidoreductase (bc(1) comp lex) is inhibited by Zn2+ ions, but with higher K-i (similar to 3 mu M) tha n the corresponding bovine enzyme. When equilibrated with mother liquor con taining 200 mu M ZnCl2 for 7 days, the crystalline chicken bet complex spec ifically binds Zn2+ at 4 sites representing two sites on each monomer in th e dimer. These two sites are close to the stigmatellin-binding site, taken to be center Q(o) of the Q-cycle mechanism, and are candidates for the inhi bitory site. One binding site is actually in the hydrophobic channel betwee n the Q(o) site and the bulk lipid phase, and may interfere with quinone bi nding. The other is in a hydrophilic area between cytochromes b and c(1), a nd might interfere with the egress of protons from the Q(o) site to the int ermembrane aqueous medium. No zinc was bound near the putative proteolytic active site of subunits 1 and 2 (homologous to mitochondrial processing pep tidase) under these conditions. (C) 2000 Elsevier Science B.V. All rights r eserved.