Plastocyanin redox kinetics in spinach chloroplasts: evidence for disequilibrium in the high potential chain.
Reduction kinetics of cytochrome f, plastocyanin (PC) and P(700) ('high-potential chain') in thylakoids from spinach were followed after pre-oxidation by a saturating light pulse. We describe a novel approach to follow PC redox kinetics from deconvolution of 810-860 nm absorption changes. The equilibration between the redox-components was analyzed by plotting the redox state of cytochrome f and PC against that of P(700). In thylakoids with (1) diminished electron transport rate (adjusted with a cytochrome bf inhibitor) or (2) de-stacked grana, cytochrome f and PC relaxed close to their thermodynamic equilibriums with P(700). In stacked thylakoids with non-inhibited electron transport, the equilibration plots were complex and non-hyperbolic, suggesting that during fast electron flux, the 'high-potential chain' does not homogeneously equilibrate throughout the membrane. Apparent equilibrium constants <5 were calculated, which are below the thermodynamic equilibrium known for the 'high potential chain'. The disequilibrium found in stacked thylakoids with high electron fluxes is explained by restricted long-range PC diffusion. We develop a model assuming that about 30% of Photosystem I mainly located in grana end-membranes and margins rapidly equilibrate with cytochrome f via short-distance transluminal PC diffusion, while long-range lateral PC migration between grana cores and distant stroma lamellae is restricted. Implications for the electron flux control are discussed.
Kirchhoff
Helmut
H
Institut für Botanik, Schlossgarten 3, D-48149 Münster, Germany. kirchhh@uni-muenster.de
Schöttler
Mark Aurel
MA
Maurer
Julia
J
Weis
Engelbert
E
D002734:Chlorophyll / Q000378:metabolism*; D002736:Chloroplasts / Q000502:physiology / Q000528:radiation effects; D003198:Computer Simulation; D045348:Cytochromes f / Q000378:metabolism*; D006706:Homeostasis / Q000502:physiology; D008027:Light; D008954:Models, Biological; D010084:Oxidation-Reduction; D045331:Photosystem I Protein Complex / Q000378:metabolism / Q000528:radiation effects; D018515:Plant Leaves / Q000502:physiology* / Q000528:radiation effects; D010970:Plastocyanin / Q000378:metabolism*; D018724:Spinacia oleracea / Q000378:metabolism* / Q000528:radiation effects; D020524:Thylakoids / Q000502:physiology* / Q000528:radiation effects
D045331:Photosystem I Protein Complex; D002734:Chlorophyll; C001785:chlorophyll P 700; D010970:Plastocyanin; D045348:Cytochromes f