Quinone interactions with the chloroplast cytochrome b6-f complex.
The requirements for reconstitution of electron transfer activity with a plastoquinone (PQ)-depleted cytochrome b6-f complex from spinach have been considered. Full restoration of activity measured as plastocyanin reduction with either duroquinol in the dark or Photosystem II (PSII) in the light requires both PQ-9 and phospholipid. However, a substantial dark activity can be observed with duroquinol and phospholipid in the absence of any added PQ-9. PSII, with its associated PQ molecules, can also donate electrons in the light to the cytochrome complex which has been depleted of plastoquinone. Electron donation by duroquinol in the dark to the PQ-depleted cytochrome complex is stimulated by PSII, and this stimulation is dependent on the presence of the two PQ molecules in the PSII preparation. Measurements of proton translocation with the PQ-depleted complex indicate this quinone is not required for the observed H+/e- ratio of 2. Studies of cytochrome b6 kinetics with the free and liposome-incorporated PQ-depleted complex show this cytochrome undergoes redox reactions similar to those of a control complex which contains PQ. These results indicate the PQ that copurifies with the cytochrome complex is not essential for any of the measured activities. These findings are considered in relation to a quinone binding site(s) in the cytochrome complex which is not specific to PQ but can bind other quinones, such as duroquinol, in a lipid-dependent process.
Willms
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I
Division of Molecular Plant Biology, University of California, Berkeley 94720.
Malkin
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Chain
R K
RK
D001665:Binding Sites; D002734:Chlorophyll / Q000378:metabolism; D002736:Chloroplasts / Q000201:enzymology*; D003580:Cytochromes / Q000378:metabolism*; D045348:Cytochromes f; D004579:Electron Transport; D006912:Hydroxyquinolines / Q000378:metabolism; D045342:Light-Harvesting Protein Complexes; D010084:Oxidation-Reduction; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D010940:Plant Proteins / Q000378:metabolism; D010970:Plastocyanin / Q000378:metabolism; D011809:Quinones / Q000378:metabolism*
D003580:Cytochromes; D006912:Hydroxyquinolines; D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D010940:Plant Proteins; D011809:Quinones; D002734:Chlorophyll; C001333:2-(n-heptyl)-4-hydroxyquinoline N-oxide; D010970:Plastocyanin; D045348:Cytochromes f