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 A(0) 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)/A(0)(-) redox pair and result in a change in the intrinsic charge separation rate and secondary electron transfer kinetics from A(0)(-) to A(1). 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.