Zinc ions inhibit the QP center of bovine heart mitochondrial bc1 complex by blocking a protonatable group

Bovine heart bc1 complex is reversibly inhibited by zinc ions with an inhibition constant KI of 10(-7) M at pH > or = 7.0. Binding of zinc is at least a factor of 10 tighter than binding of any other metal ion tested. Essentially complete inhibition of ubihydroquinone:cytochrome c oxidoreductase activity is observed at concentrations of [Zn2+] > 5 microM. Zinc does not affect the Km for the substrates, ubihydroquinone or cytochrome c, but zinc inhibits reduction of the cytochromes by ubihydroquinone through the QP center. A radioactive binding assay using 65Zn revealed one high affinity binding site per bc1 complex with KD < or = 10(-7) M at pH = 7.0 and 3-4 additional low affinity binding sites (KD > 2 x 10(-6) M). Zinc binding does not depend on the redox state of the high potential chain (iron-sulfur protein and cytochrome c1). Zinc binds 3 times tighter to Fe-S-depleted bc1 complex indicating that the zinc binding site is not on the "Rieske" iron-sulfur protein in contrast to a recent report by Lorusso et al. (Lorusso, M., Cocco, T., Sardanella, A.M., Minuto, M., Bonomi, F., and Papa, S. (1991) Eur. J. Biochem. 197, 555-561). Zinc binds to a site which has the same affinity for zinc as for protons. We conclude that the zinc binding site is close to a protonatable group of the bc1 complex with pKa = 7.2 which has not been identified previously. We propose that this group is part of the proton channel at the hydroquinone oxidation center of the bc1 complex.