Inhibitor effects on redox-linked protonations of the b haems of the mitochondrial bc1 complex

The effects of pH and inhibitors on the spectra and redox properties of the haems b of the bc1 complex of beef heart submitochondrial particles were investigated. The major findings were: (1) both haems have a weakly redox-linked protonatable group with pKox and pKred of around 6 and 8; (2) at pH values above 7, haem bH becomes heterogeneous in its redox behaviour. This heterogeneity is removed by the Qi site inhibitors antimycin A, funiculosin and HQNO, but not by the Qo site inhibitors myxothiazol or stigmatellin; (3) of all inhibitors tested only funiculosin had a large effect on the Em/pH profile of either haem b. In all cases where definite effects were found, the haem most affected was that thought to be closest to the site of inhibitor binding; (4) spectral shifts of haem groups caused by inhibitor binding were usually, but not always, of the haem group closest to the binding site; (5) titrations with succinate/fumarate were in reasonable agreement with redox-mediated data provided that strict anaerobiosis was maintained. Apparent large shifts of haem midpoint potentials with antimycin A and myxothiazol could be produced in aerobic succinate/fumarate titrations in the presence of cyanide, as already reported in the literature, but these were artefactual; (6) the heterogeneous haem bH titration behaviour can be simulated with a model similar to that proposed by Salerno et al. (J. Biol. Chem. (1989) 264, 15398-15403) in which there is redox interaction between haem bH and ubiquinone species bound at the Qi site. Simulations closely fit both the haem bH data and known semiquinone data only if it is assumed that semiquinone bound to oxidised haem bH is EPR-silent.