Thermodynamic and spectroscopic characteristics of the cytochrome bc₁ complex. Role of quinone in the behavior of cytochrome b₅₆₂

Cytochrome b₅₆₂ does not behave as a single independent thermodynamic component in preparations of purified quinol cytochrome c reductase. This effect is much more pronounced in quinone sufficient preparations; in such preparations, the epr spectrum of the cytochrome is E(h) sensitive, with a peak shift from g = 3.42 to 3.48 occurring as the potential is lowered from 100 mV to 0 mV. The peak shift is dependent on the presence of quinone and can be restored to quinone-depleted preparations by supplementation with ubiquinol 2 if phospholipid depletion is not too severe. The results suggest that cytochrome b₅₆₂ is strongly interacting with the Qc quinone binding site.