Photoreactions of cytochrome b6 and cytochrome f in chloroplast photosystem I fragments.
Photosystem I fragments were prepared by digitonin treatment of spinach chloroplasts. The midpoint potential of cytochrome b6 in the fragments is close to 0V, showing a one electron transition. No cytochrome b559 was detectable, neither in difference absorption spectra nor in light-induced absorbance changes. In the absence of added cofactors only cytochrome b6 photoreduction can be observed. This photoreduction is stimulated by ferredoxin. Ferredoxin-NADP+ reductase appears not to be involved in cytochrome b6 reduction. Photooxidation of cytochrome b6 is dependent on plastocyanin addition and inhibited by DBMIB, a plastoquinone antagonist. Addition of plastocyanin restores cytochrome f photooxidation as well, reacting quite specifically in about equimolar concentrations to bound cytochrome f. The stimulation of cytochrome f oxidation is abolished by an antibody prepared against plastocyanin, indicating a surface location of plastocyanin in digitonin treated membranes. Biphasic kinetics of dark-reduction of cytochrome f by ascorbate indicate that part of this cytochrome f is relatively inaccessible in the membrane. After preillumination a monophasic reduction is observed and the slowly oxidized component is absent. Illumination in the presence of plastocyanin causes a fast and complete reduction of cytochrome f, suggesting equilibration of cytochrome f with added plastocyanin, residing in the membrane surface. It appears that actinic light causes conformation and/or structural changes in the membrane of these digitonin fragments, influencing cytochrome f asseccibility.
D015086:2,6-Dichloroindophenol / Q000494:pharmacology; D002736:Chloroplasts / Q000187:drug effects / Q000378:metabolism*; D003580:Cytochromes / Q000378:metabolism*; D003991:Dibromothymoquinone / Q000494:pharmacology; D004072:Digitonin; D004160:Diphenylcarbazide; D004579:Electron Transport; D005287:Ferredoxin-NADP Reductase / Q000378:metabolism; D005288:Ferredoxins / Q000494:pharmacology; D009249:NADP / Q000378:metabolism; D010084:Oxidation-Reduction; D010269:Paraquat; D010788:Photosynthesis / Q000187:drug effects; D010944:Plants / Q000378:metabolism; D010970:Plastocyanin / Q000494:pharmacology; D012965:Sodium Chloride / Q000494:pharmacology
D003580:Cytochromes; D005288:Ferredoxins; D004160:Diphenylcarbazide; D003991:Dibromothymoquinone; D012965:Sodium Chloride; D009249:NADP; D010970:Plastocyanin; D015086:2,6-Dichloroindophenol; D005287:Ferredoxin-NADP Reductase; D004072:Digitonin; D010269:Paraquat