Effect of ethoxyformic anhydride on the Rieske iron-sulfur protein of bovine heart ubiquinol: cytochrome c oxidoreductase.

Treatment of bovine heart ubiquinol-cytochrome c oxidoreductase (complex III, bc1 complex) with ethoxyformic anhydride (EFA) inhibits electron transfer between cytochromes b and c1 [Yagi et al., Biochemistry 21 (1982) 4777-4782]. This paper shows that EFA alters the EPR lineshape of the Rieske iron-sulfur cluster in complex III and in the isolated Rieske protein without a significant decrease of spin concentration. The effect of EFA on the Rieske iron-sulfur cluster is competitive with that of Qo site inhibitors, such as stigmatellin, and is completely reversed by hydroxylamine. These results are consistent with the possible ethoxyformylation by EFA of histidine ligands of the Rieske iron-sulfur cluster at the non-iron binding imidazole nitrogens.