Superoxide production has been clearly demonstrated in vitro for the mitochon-drial cyt bc1 complex. Superoxide production during ubiquinol oxidase activity of the bc1 complex is observed under inhibited conditions where processing of electrons by the low-potential chain is hindered and oxidation of ubiquinone results in accumulation of the semiquinone (SQ) intermediate at the Q- site [58-62]. Unstable SQ species at the Qp site can reduce O2, forming superoxide. Models of Q-cycle bypass reactions of mitochondrial bc1 complex and possible mechanisms preventing O2 reduction at the Oo site have been reviewed [48, 63]. To explain the significance of Q-cycle inhibition for the superoxide producing reaction, it has been suggested that this site may act as a sealed "reaction chamber" that effectively shields the unstable semiquinone intermediate under uninhibited conditions . However, SQ can escape from the reaction chamber, or oxygen can diffuse into the chamber, when the low-potential electron transport chain is blocked. A similar mechanism resulting in superoxide production as a consequence of low-potential chain inhibition may exist at the Q- site of the cyt b6 f complex. Semiquinone stabilization at Qn site of the cyt bc1 complex is believed to prevent reduction of O2  .
Photosynthetic oxygen evolution creates local hyperoxic conditions, resulting in the production of reactive oxygen species (ROS) . If lumenal p-side generation of semiquinone is the precursor to production of superoxide in the bc1 complex, then it must be generated in the b6 fcomplex. This likelihood is indicated in Figure 7.3, although there is presently a lack ofdocumentation (only ) for lumenal p-side superoxide production in the b6 f complex, and none for the stromal n- side. Structural differences at the Qn site of the cyt b6f complex, involving heme c^ located at the site homologous to that of Qn in the bc1 complex, suggest a different (2-electron) reaction for PQ reduction. Such a 2-electron reduction of PQ would provide a mechanism to prevent superoxide formation by cyt b6 f in spite of the hyperoxic conditions in oxygenic photosynthetic membranes.
164 I 7 Heme cn and n-Side Electron and Proton Transfer Reactions 7.3
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