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Cytochrome  bc   1  (mitochondrial complex III) catalyzes electron transfer from quinols to cytochrome  c  and couples this reaction with proton translocation across lipid membrane; thus, it contributes to the generation of protonmotive force used for the synthesis of ATP. The energetic efficiency of the enzyme relies on a bifurcation reaction taking place at the Q o  site which upon oxidation of ubiquinol directs one electron to the Rieske 2Fe2S cluster and the other to heme  b   L . The molecular mechanism of this reaction remains unclear. A semiquinone spin-coupled to the reduced 2Fe2S cluster (SQ o -2Fe2S) was identified as a state associated with the operation of the Q o  site. To get insights into the mechanism of the formation of this state, we first constructed a mutant in which one of the histidine ligands of the iron ion of heme  b   L   Rhodobacter capsulatus  cytochrome  bc   1  was replaced by asparagine (H198N). This converted the low-spin, low-potential heme into the high-spin, high-potential species which is unable to support enzymatic turnover. We performed a comparative analysis of redox titrations of antimycin-supplemented bacterial photosynthetic membranes containing native enzyme and the mutant. The titrations revealed that H198N failed to generate detectable amounts of SQ o -2Fe2S under neither equilibrium (in dark) nor nonequilibrium (in light), whereas the native enzyme generated clearly detectable SQ o -2Fe2S in light. This provided further support for the mechanism in which the back electron transfer from heme  b   L  to a ubiquinone bound at the Q o  site is mainly responsible for the formation of semiquinone trapped in the SQ o -2Fe2S state in  R. capusulatus  cytochrome  bc   1 .

frontiers in chemistry

The High-Spin Heme bL Mutant Exposes Dominant Reaction Leading to the Formation of the Semiquinone Spin-Coupled to the [2Fe-2S]+ Cluster at the Qo Site of Rhodobacter capsulatus Cytochrome bc1