Journal
BIOPHYSICAL JOURNAL
Volume 88, Issue 2, Pages 1238-1249Publisher
CELL PRESS
DOI: 10.1529/biophysj.104.050963
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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.
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