Electron transfer through center o of the cytochrome b-c1 complex of yeast mitochondria involves subunit VII, the ubiquinone-binding protein

The role of subunit VII, the ubiquinone-binding protein of the cytochrome b-c1 complex, in electron transfer reactions was investigated in yeast mitochondria. Preincubation of submitochondrial particles with specific antibody against subunit VII prior to addition of either succinate, NADH, or the reduced form of the decyl analogue of ubiquinol resulted in an approximately 40% increase in the extent of cytochrome c1 reduction compared with controls containing preimmune serum. Addition of antimycin, an inhibitor of center i, to submitochondrial particles resulted in a 21% decrease in the rate and a 36% decrease in the extent of cytochrome c1 reduction by succinate. Preincubation of submitochondrial particles with the antibody against subunit VII prior to addition of antimycin resulted in an increase in both the rate and extent of cytochrome c1 reduction to the levels observed in the control without inhibitor. The addition of myxothiazol (an inhibitor of center o), myxothiazol plus antimycin, or alkyl hydroxynaphthoquinone (an inhibitor analogue of ubiquinone) resulted in an almost complete inhibition in both the rate and extent of cytochrome c1 reduction; however, preincubation with the antibody against subunit VII prior to addition of these inhibitors resulted in a significant increase in cytochrome c1 reduction. These results confirm our previous report (Japa, S., Zhu, Q. S., and Beattie, D. S. (1987) J. Biol. Chem. 262, 5441-5444) that subunit VII is involved in electron transfer reactions at center o of the b-c1 complex. We suggest that the binding of antibody to subunit VII inhibits the transfer of electrons to cytochrome b-566. Consequently, two electrons are transferred to the iron-sulfur protein and cytochrome c1 through an antimycin-insensitive pathway. Moreover, the antibody may change the conformation of subunit VII, such that the myxothiazol and hydroxynaphthoquinone binding sites are partially blocked thus permitting electron flow to cytochrome c1.