Asymmetric electron transfer in cyanobacterial photosystem I: Charge separation and secondary electron transfer dynamics of mutations near the primary electron acceptor A₀

Point mutations were introduced near the primary electron acceptor sites assigned to A₀ in both the PsaA and PsaB branches of Photosystem I in the cyanobacterium Synechocystis sp. PCC 6803. The residues Met688 _PsaA and Met668_PsaB, which provide the axial ligands to the Mg²⁺ of the eC-A3 and eC-B3 chlorophylls, were changed to leucine and asparagine (chlorophyll notation follows Jordan et al., 2001). The removal of the ligand is expected to alter the midpoint potential of the A ₀/A₀⁺ redox pair and result in a change in the intrinsic charge separation rate and secondary electron transfer kinetics from A₀^- to A₁. The dynamics of primary charge separation and secondary electron transfer were studied at 690 nm and 390 nm in these mutants by ultrafast optical pump-probe spectroscopy. The data reveal that mutations in the PsaB branch do not alter electron transfer dynamics, whereas mutations in the PsaA branch have a distinct effect on electron transfer, slowing down both the primary charge separation and the secondary electron transfer step (the latter by a factor of 3-10). These results suggest that electron transfer in cyanobacterial Photosystem I is asymmetric and occurs primarily along the PsaA branch of cofactors.