The [2Fe-2S] cluster Em as an indicator of the iron-sulfur subunit position in the ubihydroquinone oxidation site of the cytochrome bc1 complex

Recent crystallographic and kinetic data have revealed the crucial role of the large scale domain movement of the iron-sulfur subunit [2Fe-2S] cluster domain during the ubihydroquinone oxidation reaction catalyzed by the cytochrome bc(1) complex. Previously, the electron paramagnetic resonance signature of the [2Fe-2S] cluster and its redox midpoint potential (E-m) value have been used extensively to characterize the interactions of the [2Fe-2S] cluster with the occupants of the ubihydroquinone oxidation (Q(o)) catalytic site. In this work we analyze these interactions in various iron-sulfur subunit mutants that carry mutations in its flexible hinge region. We show that the E-m increases of the iron-sulfur subunit [2Fe-2S] cluster induced either by these mutations or by the addition of stigmatellin do not act synergistically. Moreover, the E-m increases disappear in the presence of class I inhibitors like myxothiazol. Because various inhibitors are known to affect the location of the iron-sulfur subunit cluster domain, the measured E-m value of the [2Fe-2S] cluster therefore reflects its equilibrium position in the Q(o) site. We also demonstrate the existence in this site of a location where the E-m of the cluster is increased by about 150 mV and discuss its possible implications in term of Q(o) site catalysis and energetics.