Dynamics and energetics of cyanobacterial photosystem I:ferredoxin complexes in different redox states

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 (F_AF_B). Newly obtained values were derived firstly from the fact that the dissociation constant between PSI and redox-inactive gallium-substituted Fd increases upon (F_AF_B) 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 (F_AF_B) single reduction and by about one order of magnitude after electron transfer from (F_AF_B)^- 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 (F_AF_B)Fd_oxidized, (F_AF_B)^-Fd_oxidized and (F_AF_B)Fd_reduced, 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 (F_AF_B)^-.