Plastocyanin redox kinetics in spinach chloroplasts: evidence for disequilibrium in the high potential chain

AbstractReduction kinetics of cytochrome f, plastocyanin (PC) and P700 (‘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 P700. 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 P700. 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