Fast turnover of ferredoxin/Fd reduction by photosystem-I/PSI requires that it dissociates rapidly after it has been reduced by PSI:Fd intracomplex electron transfer. The rate constants of Fd dissociation from PSI have been determined by flash-absorption spectroscopy with different combinations of cyanobacterial PSIs and Fds, and different redox states of Fd and of the terminal PSI acceptor (FAFB). Newly obtained values were derived firstly from the fact that the dissociation constant between PSI and redox-inactive gallium-substituted Fd increases upon (FAFB) reduction and secondly from the characterization and elucidation of a kinetic phase following intracomplex Fd reduction to binding of oxidized Fd to PSI, a process which is rate-limited by the foregoing dissociation of reduced Fd from PSI. By reference to the complex with oxidized partners, dissociation rate constants were found to increase moderately with (FAFB) single reduction and by about one order of magnitude after electron transfer from (FAFB)- to Fd, therefore favoring turnover of Fd reduction by PSI. With Thermosynechococcus elongatus partners, values of 270, 730 and >10000s-1 were thus determined for (FAFB)Fdoxidized, (FAFB)-Fdoxidized and (FAFB)Fdreduced, respectively. Moreover, assuming a conservative upper limit for the association rate constant between reduced Fd and PSI, a significant negative shift of the Fd midpoint potential upon binding to PSI has been calculated (< -60mV for Thermosynechococcus elongatus). From the present state of knowledge, the question is still open whether this redox shift is compatible with a large (>10) equilibrium constant for intracomplex reduction of Fd from (FAFB)-.
Keywords: Association and dissociation kinetics; Binding-induced shift of midpoint potential; Electron transfer; Ferredoxin binding; Gallium-substituted ferredoxin; Photosynthesis; Redox potential.
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