Photoreactions of cytochrome b6 have been studied using resolved chloroplast electron-transfer complexes. In the presence of Photosystem (PS) II and the cytochrome b6-f complex, photoreduction of the cytochrome can be observed. No soluble components are required for this reaction. Cytochrome b6 photoreduction was found to be inhibited by quinone analogs, which inhibit at the Rieske iron-sulfur center of the cytochrome complex, by the addition of ascorbate and by depletion of the Rieske center and bound plastoquinone from the cytochrome complex. Photoreduction of cytochrome b6 can also be demonstrated in the presence of the cytochrome complex and PS I. This photoreduction requires plastocyanin and a low-potential electron donor, such as durohydroquinone. Cytochrome b6 photoreduction in the presence of PS I is inhibited by quinone analogs which interact with the Rieske iron-sulfur center. These results are discussed in terms of a Q-cycle mechanism in which plastosemiquinone serves as the reductant for cytochrome b6 via an oxidant-induced reductive pathway.
All Science Journal Classification (ASJC) codes
- Cell Biology
- Cytochrome b
- Electron transport